To investigate the fracture initiation and propagation behavior of fractures in tight sandstone under the supercritical CO_(2)(SCCO_(2))shock fracturing,laboratory fracturing experiments were conducted using a true-tr...To investigate the fracture initiation and propagation behavior of fractures in tight sandstone under the supercritical CO_(2)(SCCO_(2))shock fracturing,laboratory fracturing experiments were conducted using a true-triaxial-like SCCO_(2)shock fracturing system.Computed tomography(CT)scanning and three-dimensional fracture reconstruction were employed to elucidate the effects of shock pressure,pore pressure,and in-situ stress on fracture characteristics.In addition,nuclear magnetic resonance(NMR)transverse relaxation time spectra were used to assess the internal damage induced by SCCO_(2)shock fracturing.The results indicate that,compared with conventional hydraulic fracturing and SCCO_(2)quasi-static fracturing,SCCO_(2)shock fracturing facilitates multidirectional fracture initiation and the formation of complex fracture networks.Increasing shock pressure more readily activates bedding-plane weaknesses,with main and subsidiary fractures interweaving into a dense fracture network.Under the same impulse intensity,elevated pore pressure reduces the effective normal stress and alters stress-wave scattering paths,thereby inducing more branch fractures and enhancing fracture complexity.An increase in differential in-situ stress promotes fracture propagation along the direction of the maximum principal stress,reduces branching,and simplifies fracture morphology.With increasing SCCO_(2)shock pressure,pore volume and connectivity generally increase:small-to-medium pores primarily respond through increased number and enhanced connectivity;when the shock pressure rises to 40-45 MPa,crack coalescence generates larger pores and fissures,which play a dominant role in improving flow pathways and effective storage space,ultimately forming a multiscale pore-fracture network.展开更多
The increase in CO_(2)injectivity and shifting of CO_(2)-absorbing layers in multilayered geological CO_(2)sequestration(GCS)reservoirs in Ordos,China indicate significantpermeability variations in certain layers.To c...The increase in CO_(2)injectivity and shifting of CO_(2)-absorbing layers in multilayered geological CO_(2)sequestration(GCS)reservoirs in Ordos,China indicate significantpermeability variations in certain layers.To capture these system changes,a numerical model incorporating all 21 aquifers and internal aquitards was developed.The monitored pressure was well matched through multiphase and thermalhydraulic-mechanical(THM)coupling numerical simulations by introducing permeability variations.The results revealed that the permeability in the second layer increased on approximately day 13 due to the abrupt pressure buildup and temperature decrease.Even such a low rate of CO_(2)(2.8 kg/s)injected into the low permeability system initiated some fractures and the permeability in the second layer around the wellbore increased by 722 times.The second critical system change occurred on approximately day 386.As demonstrated in the numerical simulation,the substantial injection of cold CO_(2)induced strong thermal stress,leading to rock contraction and the initiation of several cracks.The permeability of the firstlayer around the wellbore unexpectedly increased by 4 orders of magnitude.Since no additional pressure could drive the CO_(2)into the remaining 17 layers,the total storage capability of the multilayered system was reduced.A whole picture of the system variation is fully presented and the underlying mechanisms are analyzed.It is believed that the phenomenon of thermal-hydraulic fracturing observed in this fieldand the simulation procedures will benefitother fluidinjection and production works in various geotechnical settings.展开更多
To address the key scientific challenge of monitoring the dynamic fracturing of surrounding rock in deep roadways,this study systematically investigates the quantitative relationship between stress and charge signals ...To address the key scientific challenge of monitoring the dynamic fracturing of surrounding rock in deep roadways,this study systematically investigates the quantitative relationship between stress and charge signals during coal mass loading.By integrating innovative analytical approaches,introducing quantitative evaluation indices,and developing a charge–stress inversion model,and incorporating underground monitoring practices,significant progress has been achieved in elucidating the correlation between stress variations and charge signals throughout the entire coal mass fracturing process.First,in the field of stress–charge correlation analysis,empirical mode decomposition(EMD)was combined with wavelet coherence analysis for the first time,enabling the removal of slow-varying stress trends while retaining high-frequency fluctuations.This approach allowed for the quantitative characterization of the evolution of coherence between stress variations and charge fluctuations across multiple time scales.Second,coherence skewness and the proportion of high-coherence intervals were innovatively introduced to examine the influence of time scale selection on correlation results.On this basis,a criterion for determining the near-optimal observation scale of charge signals was proposed,providing a quantitative reference for time scale selection in similar signal analyses.Finally,by correlating charge signals with coal damage factors and stress states,a charge-based damage evolution equation was established to achieve effective stress inversion.Combined with in situ monitoring of stress and charge in roadway surrounding rock,this approach revealed the correlation characteristics of stress and charge intensity responses during the dynamic fracturing process.The results indicate,first,that charge signals are not significantly correlated with the absolute stress level of coal but are directly associated with stress variations following coal damage and failure,with the amplitude of charge fluctuations increasing alongside stress fluctuations.Second,coherence between stress and charge signals varies markedly across time scales,with excessively small or large scales leading to distortion,and the scale corresponding to the peak proportion of intervals with coherence>0.8 was identified as the near-optimal observation scale.Third,charge signals can effectively characterize coal damage factors,and the established damage evolution equation can effectively invert stress variation trends.Fourth,in underground roadways,zones of dynamic fracturing in surrounding rock are commonly located in areas where stress concentration overlaps with regions of high charge intensity,further confirming the strong consistency between charge and stress variations.These findings improve the theoretical framework of charge signal responses in loaded coal and provide a scientific basis for precise“stress-charge”monitoring of dynamic disasters,offering practical potential for engineering applications.展开更多
Abstract:Microwave-based destressing is regarded as a promising approach for proactively preventing and controlling rockbursts in deep hard rock.As the fracturing degree of microwave-induced boreholes is affected by b...Abstract:Microwave-based destressing is regarded as a promising approach for proactively preventing and controlling rockbursts in deep hard rock.As the fracturing degree of microwave-induced boreholes is affected by borehole diameter,water content,mineral content,etc.,it is difficult to establish relationships between them.The research aims to unify various factors with heating rate and temperature,and establish a microwave parameter design method based thereon.Tests on microwave-induced borehole fracturing in hard rock with different or similar heating rates and temperatures under true triaxial stress were conducted.The test results show that both heating rate and temperature promote radial fracture of the rock,but have little effect on the development of axial fractures.Compared with heating rate,temperature is a more critical factor influencing microwave-induced fracturing.The effects of the heating rate on rock fracturing become noticeable only at higher temperatures.When the heating rate and temperature are similar but the diameter of the boreholes is different,the crack distribution,total length,wave velocity attenuation,and fracture process are similar.It is feasible to reverse-design microwave parameters under different borehole diameters based on the heating rate and temperature.Thermal fracturing of basalt shows a distinct threshold effect between 150℃ and 195℃(with an average of about 175℃),and the heating rate and borehole diameter exert minor influences thereon.The results provide guidance for the design of microwave parameters in practice.展开更多
This paper proposes an approach to determing the optimal cluster spacing for volume fracturing in shale oil reservoirs based on three scales,i.e.microscopic capillary displacement,large-scale core imbibition,and macro...This paper proposes an approach to determing the optimal cluster spacing for volume fracturing in shale oil reservoirs based on three scales,i.e.microscopic capillary displacement,large-scale core imbibition,and macroscopic reservoir nuclear magnetic resonance(NMR)logging.Through flow experiments using capillary with different diameters and lengths,and large-scale core counter-current and dynamic imbibition tests,and combing with the NMR logging data of single wells,a graded optimization criterion for cluster spacing is established.The proposed approach was tested in the shale oil reservoir in the seventh member of the Triassic Yanchang Formation(Change 7 Member),the Ordos Basin.The following findings are obtained.First,in the Chang 7 reservoir,oil in pores smaller than 8μm requires a threshold pressure,and for 2-8μm pores,the movable drainage distance ranges from 0.7 m to 4.6 m under a pressure difference of 27 mPa.Second,the large-scale core imbibition tests show a counter-current imbibition distance of only 10 cm,but a dynamic imbibition distance up to 30 cm.Third,in-situ NMR logging results verified that the post-fracturing matrix drainage radius around fractures is 0-4 m,which is consistent with those of capillary flow experiments and large-scale core imbibition tests.The main pore-size range(2-8μm)of the Chang 7 reservoir corresponds to a permeability interval of(0.1-0.4)×10^(-3)μm^(2).Accordingly,a graded optimization criterion for cluster spacing is proposed as follows:for reservoirs with permeability less than 0.20×10^(-3)μm^(2),the cluster spacing should be reduced to smaller than 4.2 m;for reservoirs with permeability of(0.2-0.4)×10^(-3)μm^(2),the cluster spacing should be designed as 4.2-9.2 m.Field application on a pilot platform,where the cluster spacing was reduced to 4.0-6.0 m,yielded an increased initial oil production by approximately 36.6%over a 100-m horizontal reservoir section as compared with untested similar platforms.展开更多
Fractures are typically characterized by roughness that significantlyaffects the mechanical and hydraulic characteristics of reservoirs.However,hydraulic fracturing mechanisms under the influenceof fracture morphology...Fractures are typically characterized by roughness that significantlyaffects the mechanical and hydraulic characteristics of reservoirs.However,hydraulic fracturing mechanisms under the influenceof fracture morphology remain largely unexplored.Leveraging the advantages of the finite-discrete element method(FDEM)for explicitly simulating fracture propagation and the strengths of the unifiedpipe model(UPM)for efficientlymodeling dual-permeability seepage,we propose a new hydromechanical(HM)coupling approach for modeling hydraulic fracturing.Validated against benchmark examples,the proposed FDEM-UPM model is further augmented by incorporating a Fourier-based methodology for reconstructing non-planar fractures,enabling quantitative analysis of hydraulic fracturing behavior within rough discrete fracture networks(DFNs).The FDEM-UPM model demonstrates computational advantages in accurately capturing transient hydraulic seepage phenomena,while the asynchronous time-stepping schemes between hydraulic and mechanical analyses substantially enhanced computational efficiencywithout compromising computational accuracy.Our results show that fracture morphology can affect both macroscopic fracture networks and microscopic interaction types between hydraulic fractures(HFs)and natural fractures(NFs).In an isotropic stress field,the initiation azimuth,propagation direction and microcracking mechanism are significantly influencedby fracture roughness.In an anisotropic stress field,HFs invariably propagate parallel to the direction of the maximum principal stress,reducing the overall complexity of the stimulated fracture networks.Additionally,stress concentration and perturbation attributed to fracture morphology tend to be compromised as the leak-off increases,while the breakdown and propagation pressures remain insensitive to fracture morphology.These findingsprovide new insights into the hydraulic fracturing mechanisms of fractured reservoirs containing complex rough DFNs.展开更多
Re-fracturing horizontal wells is a critical strategy for enhancing recovery from tight oil reservoirs,but its success depends on the evaluation of candidate wells and locations.This process is complicated by producti...Re-fracturing horizontal wells is a critical strategy for enhancing recovery from tight oil reservoirs,but its success depends on the evaluation of candidate wells and locations.This process is complicated by production-induced alterations in reservoir pressure and geomechanical responses.This study introduces a workflow to evaluate re-fracturing potential by integrating coupled fluid flow and geomechanical modeling for the production of initial hydraulic fractures.We developed a numerical model that simulates the poroelastic response of a tight oil reservoir to depletion from an initial set of hydraulic fractures.To quantify the re-fracturing potential along the horizontal wellbore,a novel composite re-fracturing potential index is proposed where fracture shape,stress,and pressure are considered.This index considers four key physical factors:current reservoir pressure,fracture initiation ease,fracture geometry favorability,and fracture propagation efficiency considering tortuosity.Numerical simulations were conducted for scenarios with both uniform and non-uniform initial hydraulic fractures.The results consistently demonstrate that the optimal locations for re-fracturing are the midpoints between existing fractures,where a favorable balance of high reservoir pressure and altered stress conditions exists.The analysis reveals that the overall re-fracturing potential tends to increase with production time,suggesting that a period of depletion can enhance the geomechanical conditions for subsequent stimulation.Furthermore,a sensitivity analysis on the index weighting factors shows that the optimum re-fracturing strategy is highly dependent on the primary field objective,whether it is maximizing resource contact,ensuring geomechanical feasibility,or avoiding operational complexity.The study concludes that heterogeneity in the initial fracture network creates complex and asymmetric potential profiles,which implies the necessity of case-specific and integrated analysis over simplified assumptions.The proposed methodology provides a framework for optimizing re-fracturing designs in tight oil reservoirs.展开更多
In the development of coalbed methane(CBM)reservoirs using multistage fractured horizontal wells,there often exist areas that are either repeatedly stimulated or completely unstimulated between fracturing stages,leadi...In the development of coalbed methane(CBM)reservoirs using multistage fractured horizontal wells,there often exist areas that are either repeatedly stimulated or completely unstimulated between fracturing stages,leading to suboptimal reservoir performance.Currently,there is no well-established method for accurately evaluating the effectiveness of such stimulation.This study introduces,for the first time,the concept of the Fracture Network Bridging Coefficient(FNBC)as a novel metric to assess stimulation performance.By quantitatively coupling the proportions of unstimulated and overstimulated volumes,the FNBC effectively characterizes the connectivity and efficiency of the fracture network.A background grid calibration method is developed to quantify the stage-controlled volume,effectively stimulated volume,unstimulated volume,and repeatedly stimulated volume among different stages of horizontal wells.Furthermore,an optimization model is constructed by taking the FNBC as the objective function and the fracturing injection rate and fluid volume as optimization variables.The Simultaneous Perturbation Stochastic Approximation(SPSA)algorithm is employed to iteratively perturb and optimize these variables,progressively improving the FNBC until the optimal displacement rate and fluid volume corresponding to the maximum FNBC are obtained.Field application in a typical CBM multistage fractured horizontal well in China demonstrates that the FNBC increased from 0.358 to 0.539(a 50.6% improvement),with the injection rate rising from 16 m^(3)/min to 24 m^(3)/min and the average fluid volume per stage increasing from 2490 m^(3) to 3192 m^(3),significantly enhancing the stimulation effectiveness.This research provides theoretical support for designing high-efficiency stimulation strategies in unconventional reservoirs under dynamic limits.展开更多
In the Southern Sichuan Basin,China(SSBC),some moderate-sized seismic events(local magnitude M_(L)ranging between 4 and 5)have affected the safe production of shale gas.In this study,we used the recorded seismic data ...In the Southern Sichuan Basin,China(SSBC),some moderate-sized seismic events(local magnitude M_(L)ranging between 4 and 5)have affected the safe production of shale gas.In this study,we used the recorded seismic data from China national and temporary networks within the SSBC to obtain the relocated seismic hypocenter distribution between January 2016 and May 2017 based on the hypocenter double-difference(HypoDD)method.The statistical characteristics of microseismicity resulting from water injection in SSBC were analyzed,and the potential correlation between the event rate and statistical parameters,such as Gutenberg-Richter b-value,spatial correlation length,and fractal dimension,was quantified.Based on spatial variations of b-value and fractal dimension of event distribution,we identified two potential risk areas in the East and West of the Zhaotong shale gas block(YS108),respectively.The focal mechanism solutions(FMSs)of the observed seismic events(M_(L)>2.5)near the H7 well pad were calculated utilizing the generalized cut-and-paste(gCAP)technique combined with P-wave polarity.The FMSs’results show reverse faults,and some of them have fault planes oriented in the N-S direction,causing oblique slip movement.In addition,we also inverted the regional stress field using high-quality FMSs,revealing that the maximum principal stress(σ1)trends NW–SE and lies nearly horizontal,in agreement with the World Stress Map and borehole breakout records.Considering geological structures and regional stress distribution,the reasons for induced seismicity were mainly linked to pore pressure diffusion.Our obtained findings may provide insights for future seismic risk assessment and mitigation strategies.展开更多
High-voltage electric pulse(HVEP)rock fragmentation has demonstrated substantial potential for sustainable fracturing of hard rocks owing to its energy efficiency.The transient nature and highly disruptive characteris...High-voltage electric pulse(HVEP)rock fragmentation has demonstrated substantial potential for sustainable fracturing of hard rocks owing to its energy efficiency.The transient nature and highly disruptive characteristics of its physical fracturing process render experimental investigation of the underlying rock-breaking mechanisms challenging.However,existing numerical studies lack comprehensive models that precisely link electrical breakdown phenomena with mechanical disintegration processes.This study combines COMSOL electrical breakdown simulations with four-dimension lattice spring model(4D-LSM)mechanical analysis to establish a coupled HVEP rock fragmentation model.The core concept of the model construction is to import the temperature field of the plasma channel obtained from the electrical breakdown into the mechanical solver to realize the precise connection between the two stages.The validated numerical model elucidates the full process of HVEP-induced fragmentation under varying electrical parameters.Furthermore,the effects of confining pressure and mineral grain size on fragmentation behavior have been investigated.Finally,parametric simulations across 25 electrical parameter combinations demonstrate the critical role of electrode spacing optimization in achieving energy-efficient rock fragmentation.These findings provide a predictive tool for designing efficient HVEP systems in deep resource extraction and mineral processing engineering.展开更多
Methane in-situ deflagration fracturing in shale is a revolutionary anhydrous technology.This paper selects shale samples from the Longmaxi Formation in Southern Sichuan to conduct deflagration fracturing tests with p...Methane in-situ deflagration fracturing in shale is a revolutionary anhydrous technology.This paper selects shale samples from the Longmaxi Formation in Southern Sichuan to conduct deflagration fracturing tests with pressures ranging from 25 MPa to 91 MPa.Pore structure changes were experimentally measured to explore the modification differences of nanoscale pore characteristics under varying deflagration pressures.The results show that within the deflagration pressure range examined in this study,(1)Deflagration fracturing can alter the pore volume and specific surface area but does not affect the distribution characteristics of the pore size's peak position.The maximum increments of total pore volume occur at a pressure of 45 MPa.(2)When the deflagration pressure is less than 45 MPa,porosity gradually increases with rising deflagration pressure.When it is greater than 45 MPa,the porosity does not change significantly.With increasing deflagration pressure,it gradually increases:from nanopores,such as mesopores and macropores,to large pores and microcracks.(3)At the low deflagration pressure stage,under the influence of high temperature,slippage pores gradually increase,which is conducive to gas desorption and diffusion migration.As pressure increases,the impact of the explosion shock wave gradually increases,the volume of seepage pores increases significantly,and seepage dominates the migration mode.展开更多
As an emerging rock-breaking technology,microwave irradiation has demonstrated significant potential as an auxiliary technique for volume stimulation in hydraulic fracturing.This study focuses on tight sandstone gas e...As an emerging rock-breaking technology,microwave irradiation has demonstrated significant potential as an auxiliary technique for volume stimulation in hydraulic fracturing.This study focuses on tight sandstone gas extraction,introducing a hollow double-wing crack(HDWC)configuration into the research on tight sandstone.Laboratory experiments were conducted to investigate microwave-induced fracturing mechanisms and the mechanical behavior of HDWC-containing sandstone,aiming to elucidate the thermal cracking patterns and underlying mechanisms under microwave irradiation conditions.To further explore the electromagnetic-thermal-mechanical(E-T-M)interactions in tight sandstone under microwave treatment,a coupled finite element method(FEM)-discrete element method(DEM)numerical model was developed.This model enabled a detailed analysis of force chain evolution and microcrack propagation within HDWC-containing sandstone.Additionally,preliminary hydraulic fracturing simulations were performed to investigate fracture initiation pressure and fracture evolution following microwave exposure.The main findings of this study are as follows:(1)Microwave heating induces thermal cracks at both the tips and midsections of the HDWC.Microwave irradiation degrades the mechanical properties of HDWC-containing sandstone.(2)Simulation results reveal that significant stress concentration and tensile-compressive zoning occur near the HDWC under microwave irradiation.Microcrack development exhibits an avalanche effect.(3)Hydraulic fracturing simulations indicate that microwave heating generally promotes hydraulic fracture generation.Microwave irradiation reduces the fracture initiation pressure and enhances the complexity and connectivity of the fracture network.These findings provide valuable insights into the application of microwave-assisted volume stimulation as a supporting technology for hydraulic fracturing in deep reservoirs.展开更多
To elucidate the dynamic characteristics of in-situ methane deflagration in coalbed methane wellbores and its mechanisms for fracturing coal rock,this study first developed a simulation experimental system specificall...To elucidate the dynamic characteristics of in-situ methane deflagration in coalbed methane wellbores and its mechanisms for fracturing coal rock,this study first developed a simulation experimental system specifically designed for methane in-situ deflagration fracturing.This experimental system,which is capable of withstanding pressures up to 150 MPa and meanwhile applying axial and confining pressures of up to 50 MPa to rock cores,enables the coupled simulation on methane deflagration and rock core fracturing processes.With the aid of this experimental system,physical simulation experiments on in-situ methane deflagration fracturing were conducted,and the following findings were obtained.Methane deflagration loads in enclosed wellbores exhibit characteristics of multi-level pulsed oscillation.With the rise of initial gas pressure,the peak deflagration load increases approximately linearly,with the pressure amplification factor spanning from 23.14 to 31.10,and its peak loading rate grows exponentially.Accordingly,the fracture volume and fracture porosity augment.To be specific,when the initial gas pressure rises from 0.6 to 2.4 MPa,the fracture volume and fracture porosity augment by factors of 14.0 and 8.73,respectively.The fractal dimension of spatial distribution of fractures also increases with the rise of deflagration load,indicating that a higher deflagration load conduces to the development of a larger and more complex fracture network.Methane deflagration fracturing is characterized as a composite fracture mode that involves the impact of strong stress waves and the driving force of high-pressure fluids.The primary factors influencing damage to coal-rock include the high-stress impact in the initial stage of deflagration,the fluid pressure driving effect in the middle stage,and the thermal shock resulting from high temperatures in the later stage.展开更多
With the increasing demand for energy,traditional oil resources are facing depletion and insufficient supply.Many countries are rapidly turning to the development of unconventional oil and gas resources.Among them,sha...With the increasing demand for energy,traditional oil resources are facing depletion and insufficient supply.Many countries are rapidly turning to the development of unconventional oil and gas resources.Among them,shale oil and gas reservoirs have become the focus of unconventional oil and gas resources exploration and development.Based on the characteristics of shale oil and gas reservoirs,supercritical CO_(2) fracturing is more conducive to improving oil recovery than other fracturing technologies.In this paper,the mechanism of fracture initiation and propagation of supercritical CO_(2) in shale is analyzed,including viscosity effect,surface tension effect,permeation diffusion effect of supercritical CO_(2),and dissolution-adsorption effect between CO_(2) and shale.The effects of natural factors,such as shale properties,bedding plane and natural fractures,and controllable factors,proppant,temperature,pressure,CO_(2) concentration and injection rate on fracture initiation and propagation are clarified.The methods of supercritical CO_(2) fracturing process,thickener and proppant optimization to improve the efficiency of supercritical CO_(2) fracturing are discussed.In addition,some new technologies of supercritical CO_(2) fracturing are introduced.The challenges and prospects in the current research are also summarized.For example,supercritical CO_(2) is prone to filtration when passing through porous media,and it is difficult to form a stable flow state.Therefore,in order to achieve stable fracturing fluid suspension and effectively support fractu res,it is urge nt to explo re new fracturing fluid additives or improve fracturing fluid formulations combined with the research of new proppants.This paper is of great significance for understanding the behavior mechanism of supercritical CO_(2) in shale and optimizing fracturing technology.展开更多
Medium-high maturity continental shale oil is one of the hydrocarbon resources with the most potential for successful development in China.Nevertheless,the unique geological conditions of a multi-lithologic superposit...Medium-high maturity continental shale oil is one of the hydrocarbon resources with the most potential for successful development in China.Nevertheless,the unique geological conditions of a multi-lithologic superposition shield the vertical propagation of hydraulic fractures and limit the longitudinal reconstruction in reservoirs,posing a great challenge for large-scale volumetric fracturing.Radial wellbore crosslayer fracturing,which transforms the interaction between the hydraulic fractures and lithologic interface into longitudinal multilayer competitive initiation,could provide a potential solution for this engineering challenge.To determine the longitudinal propagation behaviors of fractures guided by radial wellbores,true triaxial fracturing experiments were performed on multilayer shale-sandstone samples,with a focus on the injection pressure response,fracture morphology,and cross-layer pattern.The effects of the radial borehole length L,vertical stress difference K_(v),injection rate Q,and viscosity m of the fracturing fluid were analyzed.The results indicate that radial wellbores can greatly facilitate fracture initiation and cross-layer propagation.Unlike conventional hydraulic fracturing,there are two distinct fracture propagation patterns in radial wellbore fracturing:cross-layering and skip-layering.The fracture height guided by a radial wellbore is positively correlated with K_(v),Q,and m.Increasing these parameters causes a shift in the fracture initiation from a single root to an asynchronous root/toe end and can improve the cross-layer propagation capacity.Critical parameter thresholds exist for fracture propagation through and across interlayers under the guidance of radial boreholes.A parameter combination of critical cross-layering/skip-layering or alternating displacement/viscosity is recommended to simultaneously improve the fracture height and degree of lateral activation.The degree of correlation of different parameters with the vertical fracture height can be written as L>Q/m>K_(v).Increasing the radial wellbore length can effectively facilitate fracture cross-/skip-layer propagation and reduce the critical threshold of injection parameters,which is conducive to maximizing the stimulated reservoir volume.展开更多
The effectiveness of horizontal well multi-stage and multi-cluster fracturing in the fractured soft coal seam roof for coalbed methane(CBM) extraction has been demonstrated.This study focuses on the geological charact...The effectiveness of horizontal well multi-stage and multi-cluster fracturing in the fractured soft coal seam roof for coalbed methane(CBM) extraction has been demonstrated.This study focuses on the geological characteristics of the No.5 and No.11 coal seams in the Hancheng Block,Ordos Basin,China.A multi-functional,variable-size rock sample mold capable of securing the wellbore was developed to simulate layered formations comprising strata of varying lithology and thicknesses.A novel segmented fracturing simulation method based on an expandable pipe plugging technique is proposed.Large-scale true triaxial experiments were conducted to investigate the effects of horizontal wellbore location,perforation strategy,roof lithology,and vertical stress difference on fracture propagation,hydraulic energy variation,and the stimulated reservoir volume in horizontal wells targeting the soft coal seam roof.The results indicate that bilateral downward perforation with a phase angle of 120° optimizes hydraulic energy conservation,reduces operational costs,enhances fracture formation,and prevents fracturing failure caused by coal powder generation and migration.This perforation mode is thus considered optimal for coal seam roof fracturing.When the roof consists of sandstone,each perforation cluster tends to initiate a single dominant fracture with a regular geometry.In contrast,hydraulic fractures formed in mudstone roofs display diverse morphology.Due to its high strength,the sandstone roof requires significantly higher pressure for crack initiation and propagation,whereas the mudstone roof,with its strong water sensitivity,exhibits lower fracturing pressures.To mitigate inter-cluster interference,cluster spacing in mudstone roofs should be greater than that in sandstone roofs.Horizontal wellbore placement critically influences fracturing effectiveness.For indirect fracturing in sandstone roofs,an optimal position is 25 mm away from the lithological interface.In contrast,the optimal location for indirect fracturing in mudstone roofs is directly at the lithological interface with the coal seam.Higher vertical stress coefficients lead to increased fractu ring pressures and promote vertical,layer-penetrating fractures.A coefficient of 0.5 is identified as optimal for achieving effective indirect fracturing.This study provides valuable insights for the design and optimization of staged fracturing in horizontal wells targeting crushed soft coal seam roofs.展开更多
Low-salinity fracturing fluids tend to induce ion migration,alter wettability,and cause fluctuations in gas desorption efficiency when penetrating deep coal seams.Taking the No.8 coal from the Daning-Jixian area in th...Low-salinity fracturing fluids tend to induce ion migration,alter wettability,and cause fluctuations in gas desorption efficiency when penetrating deep coal seams.Taking the No.8 coal from the Daning-Jixian area in the Ordos Basin,NW China,as a representative example,this study employs physical simulation experiments to reveal the coupled control mechanism of salinity gradient on the ion-coal matrix-gas/water interfacial system and its key role in the imbibition-desorption process.The increasing ionic concentration improves the hydrophobicity of coal,with multivalent ions exhibiting particularly significant effects.The imbibition and ion diffusion occur in opposite directions,with imbibition equilibrium being achieved earlier than ionic equilibrium.Water-coal interactions induce both mineral dissolution and secondary precipitation.When a low-salinity fracturing fluid is injected into a high-salinity reservoir,the osmotic-pressure difference drives imbibition,promotes CH4 desorption,but results in higher fluid loss.Conversely,injecting high-salinity fracturing fluid into a low-salinity reservoir creates a reverse osmotic gradient that suppresses leak-off while improving flowback efficiency.Based on these findings,a high-low salinity sequential injection strategy is proposed for deep coal seams:high-salinity fluid is first injected to form stable fracture networks,followed by low-salinity fluid to enlarge the imbibition zone and enhance CH4 desorption and diffusion.Moderate well soaking is recommended to increase the imbibition volume,thereby achieving multiple positive effects such as maintaining reservoir pressure,preserving formation energy,and promoting imbibition-driven displacement.展开更多
Multistage fracturing technology has been used to enhance tight hydrocarbon resource recovery.Determining the proper well spacing and fracturing strategy is crucial for generating a complex fracture network that facil...Multistage fracturing technology has been used to enhance tight hydrocarbon resource recovery.Determining the proper well spacing and fracturing strategy is crucial for generating a complex fracture network that facilitates oil and gas flow in reservoirs.The stress-shadow effect that occurs between multiple wells significantly affects the development of fracture networks in reservoirs.However,the quantification of the stress-shadow effect and its influence on fracture networks has not been satisfactorily resolved because of the difficulties in detecting and identifying fracture propagation and reorientation in reservoirs.In this study,based on the geological information from the Shengli oilfield,we applied a hybrid finite element-discrete element method to analyze engineering-scale three-dimensional fracture propagation and reorientation by altering well spacings and fracturing strategies.The results indicate that the fracturing area generated by the synchronous fracturing scheme is much smaller than those generated by the sequential and alternative schemes.An alternative hydrofracturing scheme is optimal with respect to fracturing area.The stress-blind area was defined to quantify the mechanical disturbance between adjacent wells.Our study improves the understanding of the effect of fracturing schemes on fracture networks and the impact of independent factors contributing to stress-shadow effects.展开更多
Multi-stage and multi-cluster fracturing(MMF)is a crucial technology in unconventional oil and gas development,aiming to enhance production by creating extensive fracture networks.However,achieving uniform expansion o...Multi-stage and multi-cluster fracturing(MMF)is a crucial technology in unconventional oil and gas development,aiming to enhance production by creating extensive fracture networks.However,achieving uniform expansion of multi-cluster hydraulic fractures(HFs)in MMF remains a significant challenge.Field practice has shown that the use of temporary plugging and diversion fracturing(TPDF)can promote the balanced expansion of multi-cluster HFs.This study conducted TPDF experiments using a true triaxial fracturing simulation system setting a horizontal well completion with multi-cluster jetting perforations to investigate the equilibrium initiation and extension of multi-cluster fractures.The influence of key parameters,including cluster spacing,fracturing fluid viscosity,differential stress,and fracturing fluid injection rate,on fracture initiation and propagation was systematically examined.The results indicate that while close-spaced multi-cluster fracturing significantly increases the number of HFs,it also leads to uneven extension of HFs in their propagation.In contrast,TPDF demonstrates effectiveness in mitigating uneven HF extension,increasing the number of HFs,and creating a larger stimulated reservoir volume,ultimately leading to improved oil and gas well productivity.Moreover,under conditions of high differential stress,the differential stress within the formation exerts a stronger guiding effect in HFs,which are more closely aligned with the minimum principal stress.Low-viscosity fluids facilitate rapid and extensive fracture propagation within the rock formation.High-volume fluid injection,on the other hand,more comprehensively fills the formation.Therefore,employing lowviscosity and high-volume fracturing is advantageous for the initiation and extension of multi-cluster HFs.展开更多
Based on continuum-discontinuum element method,the numerical simulation of fracture propagation during deflagration-hydraulic composite fracturing was constructed by considering deflagration stress impact induced frac...Based on continuum-discontinuum element method,the numerical simulation of fracture propagation during deflagration-hydraulic composite fracturing was constructed by considering deflagration stress impact induced fracture creation,deflagrating gas driven fracture propagation,and hydraulic fracture propagation,exploring the effects of in-situ stress difference,deflagration peak pressure,deflagration pressurization rate,hydraulic fracturing displacement and hydraulic fracturing fluid viscosity on fracture propagation in deflagration-hydraulic composite fracturing.The deflagration-hydraulic composite fracturing combines the advantages of deflagration fracturing in creating complex fractures near wells and the deep penetration of hydraulic fracturing at the far-field region,which can form multiple deep penetrating long fractures with better stimulation effects.With the increase of in-situ stress difference,the stimulated area of deflagration-hydraulic composite fracturing is reduced,and the deflagration-hydraulic composite fracturing is more suitable for reservoirs with small in-situ stress difference.Higher peak pressure and pressurization rate are conducive to increasing the maximum fracture length and burst degree of the deflagration fractures,which in turn increases the stimulated area of deflagration-hydraulic composite fracturing and improves the stimulation effect.Increasing the displacement and viscosity of hydraulic fracturing fluid can enhance the net pressure within the fractures,activate the deflagration fractures,increase the turning radius of the fractures,generate more long fractures,and effectively increase the stimulated reservoir area.The stimulated reservoir area is not completely positively correlated with the hydraulic fracturing displacement and fracturing fluid viscosity,and there is a critical value.When the critical value is exceeded,the stimulated area decreases.展开更多
基金Supported by the National Natural Science Foundation for Outstanding Young Scholars(52425402)National Natural Science Foundation of China(52341401)International(Regional)Cooperation and Exchange Program of the National Natural Science Foundation of China(W2412078)。
文摘To investigate the fracture initiation and propagation behavior of fractures in tight sandstone under the supercritical CO_(2)(SCCO_(2))shock fracturing,laboratory fracturing experiments were conducted using a true-triaxial-like SCCO_(2)shock fracturing system.Computed tomography(CT)scanning and three-dimensional fracture reconstruction were employed to elucidate the effects of shock pressure,pore pressure,and in-situ stress on fracture characteristics.In addition,nuclear magnetic resonance(NMR)transverse relaxation time spectra were used to assess the internal damage induced by SCCO_(2)shock fracturing.The results indicate that,compared with conventional hydraulic fracturing and SCCO_(2)quasi-static fracturing,SCCO_(2)shock fracturing facilitates multidirectional fracture initiation and the formation of complex fracture networks.Increasing shock pressure more readily activates bedding-plane weaknesses,with main and subsidiary fractures interweaving into a dense fracture network.Under the same impulse intensity,elevated pore pressure reduces the effective normal stress and alters stress-wave scattering paths,thereby inducing more branch fractures and enhancing fracture complexity.An increase in differential in-situ stress promotes fracture propagation along the direction of the maximum principal stress,reduces branching,and simplifies fracture morphology.With increasing SCCO_(2)shock pressure,pore volume and connectivity generally increase:small-to-medium pores primarily respond through increased number and enhanced connectivity;when the shock pressure rises to 40-45 MPa,crack coalescence generates larger pores and fissures,which play a dominant role in improving flow pathways and effective storage space,ultimately forming a multiscale pore-fracture network.
基金supports from the National Natural Science Foundation of China(Grant Nos.52179095,52378323,and 42407216)are gratefully acknowledged.
文摘The increase in CO_(2)injectivity and shifting of CO_(2)-absorbing layers in multilayered geological CO_(2)sequestration(GCS)reservoirs in Ordos,China indicate significantpermeability variations in certain layers.To capture these system changes,a numerical model incorporating all 21 aquifers and internal aquitards was developed.The monitored pressure was well matched through multiphase and thermalhydraulic-mechanical(THM)coupling numerical simulations by introducing permeability variations.The results revealed that the permeability in the second layer increased on approximately day 13 due to the abrupt pressure buildup and temperature decrease.Even such a low rate of CO_(2)(2.8 kg/s)injected into the low permeability system initiated some fractures and the permeability in the second layer around the wellbore increased by 722 times.The second critical system change occurred on approximately day 386.As demonstrated in the numerical simulation,the substantial injection of cold CO_(2)induced strong thermal stress,leading to rock contraction and the initiation of several cracks.The permeability of the firstlayer around the wellbore unexpectedly increased by 4 orders of magnitude.Since no additional pressure could drive the CO_(2)into the remaining 17 layers,the total storage capability of the multilayered system was reduced.A whole picture of the system variation is fully presented and the underlying mechanisms are analyzed.It is believed that the phenomenon of thermal-hydraulic fracturing observed in this fieldand the simulation procedures will benefitother fluidinjection and production works in various geotechnical settings.
基金supported by the Research Fund of the National Natural Science Foundation of China(No.52374205)the Fundamental Research Project of the Educational Department of Liaoning Province(No.JYTMS20230793)the Research Fund of the State Key Laboratory of Coal Resources and Safe Mining,CUMT(No.YJY-XD-2024-A-016).
文摘To address the key scientific challenge of monitoring the dynamic fracturing of surrounding rock in deep roadways,this study systematically investigates the quantitative relationship between stress and charge signals during coal mass loading.By integrating innovative analytical approaches,introducing quantitative evaluation indices,and developing a charge–stress inversion model,and incorporating underground monitoring practices,significant progress has been achieved in elucidating the correlation between stress variations and charge signals throughout the entire coal mass fracturing process.First,in the field of stress–charge correlation analysis,empirical mode decomposition(EMD)was combined with wavelet coherence analysis for the first time,enabling the removal of slow-varying stress trends while retaining high-frequency fluctuations.This approach allowed for the quantitative characterization of the evolution of coherence between stress variations and charge fluctuations across multiple time scales.Second,coherence skewness and the proportion of high-coherence intervals were innovatively introduced to examine the influence of time scale selection on correlation results.On this basis,a criterion for determining the near-optimal observation scale of charge signals was proposed,providing a quantitative reference for time scale selection in similar signal analyses.Finally,by correlating charge signals with coal damage factors and stress states,a charge-based damage evolution equation was established to achieve effective stress inversion.Combined with in situ monitoring of stress and charge in roadway surrounding rock,this approach revealed the correlation characteristics of stress and charge intensity responses during the dynamic fracturing process.The results indicate,first,that charge signals are not significantly correlated with the absolute stress level of coal but are directly associated with stress variations following coal damage and failure,with the amplitude of charge fluctuations increasing alongside stress fluctuations.Second,coherence between stress and charge signals varies markedly across time scales,with excessively small or large scales leading to distortion,and the scale corresponding to the peak proportion of intervals with coherence>0.8 was identified as the near-optimal observation scale.Third,charge signals can effectively characterize coal damage factors,and the established damage evolution equation can effectively invert stress variation trends.Fourth,in underground roadways,zones of dynamic fracturing in surrounding rock are commonly located in areas where stress concentration overlaps with regions of high charge intensity,further confirming the strong consistency between charge and stress variations.These findings improve the theoretical framework of charge signal responses in loaded coal and provide a scientific basis for precise“stress-charge”monitoring of dynamic disasters,offering practical potential for engineering applications.
基金the financial support from the Na-tional Key Research and Development Program of China(Grant No.2023YFC2907202)the Postdoctoral Fellowship Program of CPSF(Grant No.GZB20240129).
文摘Abstract:Microwave-based destressing is regarded as a promising approach for proactively preventing and controlling rockbursts in deep hard rock.As the fracturing degree of microwave-induced boreholes is affected by borehole diameter,water content,mineral content,etc.,it is difficult to establish relationships between them.The research aims to unify various factors with heating rate and temperature,and establish a microwave parameter design method based thereon.Tests on microwave-induced borehole fracturing in hard rock with different or similar heating rates and temperatures under true triaxial stress were conducted.The test results show that both heating rate and temperature promote radial fracture of the rock,but have little effect on the development of axial fractures.Compared with heating rate,temperature is a more critical factor influencing microwave-induced fracturing.The effects of the heating rate on rock fracturing become noticeable only at higher temperatures.When the heating rate and temperature are similar but the diameter of the boreholes is different,the crack distribution,total length,wave velocity attenuation,and fracture process are similar.It is feasible to reverse-design microwave parameters under different borehole diameters based on the heating rate and temperature.Thermal fracturing of basalt shows a distinct threshold effect between 150℃ and 195℃(with an average of about 175℃),and the heating rate and borehole diameter exert minor influences thereon.The results provide guidance for the design of microwave parameters in practice.
基金Supported by the China National Oil and Gas Major Project(2025ZD1404800)PetroChina Science and Technology Major Project(2023ZZ15YJ03)CNPC Changqing Oilfield Company Major Special Project(2023DZZ04)。
文摘This paper proposes an approach to determing the optimal cluster spacing for volume fracturing in shale oil reservoirs based on three scales,i.e.microscopic capillary displacement,large-scale core imbibition,and macroscopic reservoir nuclear magnetic resonance(NMR)logging.Through flow experiments using capillary with different diameters and lengths,and large-scale core counter-current and dynamic imbibition tests,and combing with the NMR logging data of single wells,a graded optimization criterion for cluster spacing is established.The proposed approach was tested in the shale oil reservoir in the seventh member of the Triassic Yanchang Formation(Change 7 Member),the Ordos Basin.The following findings are obtained.First,in the Chang 7 reservoir,oil in pores smaller than 8μm requires a threshold pressure,and for 2-8μm pores,the movable drainage distance ranges from 0.7 m to 4.6 m under a pressure difference of 27 mPa.Second,the large-scale core imbibition tests show a counter-current imbibition distance of only 10 cm,but a dynamic imbibition distance up to 30 cm.Third,in-situ NMR logging results verified that the post-fracturing matrix drainage radius around fractures is 0-4 m,which is consistent with those of capillary flow experiments and large-scale core imbibition tests.The main pore-size range(2-8μm)of the Chang 7 reservoir corresponds to a permeability interval of(0.1-0.4)×10^(-3)μm^(2).Accordingly,a graded optimization criterion for cluster spacing is proposed as follows:for reservoirs with permeability less than 0.20×10^(-3)μm^(2),the cluster spacing should be reduced to smaller than 4.2 m;for reservoirs with permeability of(0.2-0.4)×10^(-3)μm^(2),the cluster spacing should be designed as 4.2-9.2 m.Field application on a pilot platform,where the cluster spacing was reduced to 4.0-6.0 m,yielded an increased initial oil production by approximately 36.6%over a 100-m horizontal reservoir section as compared with untested similar platforms.
基金supported by the National Natural Science Foundation of China(Grant Nos.52574103 and 42277150).
文摘Fractures are typically characterized by roughness that significantlyaffects the mechanical and hydraulic characteristics of reservoirs.However,hydraulic fracturing mechanisms under the influenceof fracture morphology remain largely unexplored.Leveraging the advantages of the finite-discrete element method(FDEM)for explicitly simulating fracture propagation and the strengths of the unifiedpipe model(UPM)for efficientlymodeling dual-permeability seepage,we propose a new hydromechanical(HM)coupling approach for modeling hydraulic fracturing.Validated against benchmark examples,the proposed FDEM-UPM model is further augmented by incorporating a Fourier-based methodology for reconstructing non-planar fractures,enabling quantitative analysis of hydraulic fracturing behavior within rough discrete fracture networks(DFNs).The FDEM-UPM model demonstrates computational advantages in accurately capturing transient hydraulic seepage phenomena,while the asynchronous time-stepping schemes between hydraulic and mechanical analyses substantially enhanced computational efficiencywithout compromising computational accuracy.Our results show that fracture morphology can affect both macroscopic fracture networks and microscopic interaction types between hydraulic fractures(HFs)and natural fractures(NFs).In an isotropic stress field,the initiation azimuth,propagation direction and microcracking mechanism are significantly influencedby fracture roughness.In an anisotropic stress field,HFs invariably propagate parallel to the direction of the maximum principal stress,reducing the overall complexity of the stimulated fracture networks.Additionally,stress concentration and perturbation attributed to fracture morphology tend to be compromised as the leak-off increases,while the breakdown and propagation pressures remain insensitive to fracture morphology.These findingsprovide new insights into the hydraulic fracturing mechanisms of fractured reservoirs containing complex rough DFNs.
基金funding from the National Natural Science Foundation of China(No.U24B6001)the CNPC Innovation Fund(No.2021DQ02-0502).
文摘Re-fracturing horizontal wells is a critical strategy for enhancing recovery from tight oil reservoirs,but its success depends on the evaluation of candidate wells and locations.This process is complicated by production-induced alterations in reservoir pressure and geomechanical responses.This study introduces a workflow to evaluate re-fracturing potential by integrating coupled fluid flow and geomechanical modeling for the production of initial hydraulic fractures.We developed a numerical model that simulates the poroelastic response of a tight oil reservoir to depletion from an initial set of hydraulic fractures.To quantify the re-fracturing potential along the horizontal wellbore,a novel composite re-fracturing potential index is proposed where fracture shape,stress,and pressure are considered.This index considers four key physical factors:current reservoir pressure,fracture initiation ease,fracture geometry favorability,and fracture propagation efficiency considering tortuosity.Numerical simulations were conducted for scenarios with both uniform and non-uniform initial hydraulic fractures.The results consistently demonstrate that the optimal locations for re-fracturing are the midpoints between existing fractures,where a favorable balance of high reservoir pressure and altered stress conditions exists.The analysis reveals that the overall re-fracturing potential tends to increase with production time,suggesting that a period of depletion can enhance the geomechanical conditions for subsequent stimulation.Furthermore,a sensitivity analysis on the index weighting factors shows that the optimum re-fracturing strategy is highly dependent on the primary field objective,whether it is maximizing resource contact,ensuring geomechanical feasibility,or avoiding operational complexity.The study concludes that heterogeneity in the initial fracture network creates complex and asymmetric potential profiles,which implies the necessity of case-specific and integrated analysis over simplified assumptions.The proposed methodology provides a framework for optimizing re-fracturing designs in tight oil reservoirs.
基金the financial support from National Natural Science Foundation of China(No.52474029)Strategic and Applied Scientific Research Project of PetroChina Company Limited(2023ZZ18,2023ZZ18YJ04).
文摘In the development of coalbed methane(CBM)reservoirs using multistage fractured horizontal wells,there often exist areas that are either repeatedly stimulated or completely unstimulated between fracturing stages,leading to suboptimal reservoir performance.Currently,there is no well-established method for accurately evaluating the effectiveness of such stimulation.This study introduces,for the first time,the concept of the Fracture Network Bridging Coefficient(FNBC)as a novel metric to assess stimulation performance.By quantitatively coupling the proportions of unstimulated and overstimulated volumes,the FNBC effectively characterizes the connectivity and efficiency of the fracture network.A background grid calibration method is developed to quantify the stage-controlled volume,effectively stimulated volume,unstimulated volume,and repeatedly stimulated volume among different stages of horizontal wells.Furthermore,an optimization model is constructed by taking the FNBC as the objective function and the fracturing injection rate and fluid volume as optimization variables.The Simultaneous Perturbation Stochastic Approximation(SPSA)algorithm is employed to iteratively perturb and optimize these variables,progressively improving the FNBC until the optimal displacement rate and fluid volume corresponding to the maximum FNBC are obtained.Field application in a typical CBM multistage fractured horizontal well in China demonstrates that the FNBC increased from 0.358 to 0.539(a 50.6% improvement),with the injection rate rising from 16 m^(3)/min to 24 m^(3)/min and the average fluid volume per stage increasing from 2490 m^(3) to 3192 m^(3),significantly enhancing the stimulation effectiveness.This research provides theoretical support for designing high-efficiency stimulation strategies in unconventional reservoirs under dynamic limits.
基金supported by the National Natural Science Foundation of China(Grant No.U24B2038)Scientific and technological research projects in Sichuan province(Grant Nos.2024YFHZ0286and2025NSFTD0012).
文摘In the Southern Sichuan Basin,China(SSBC),some moderate-sized seismic events(local magnitude M_(L)ranging between 4 and 5)have affected the safe production of shale gas.In this study,we used the recorded seismic data from China national and temporary networks within the SSBC to obtain the relocated seismic hypocenter distribution between January 2016 and May 2017 based on the hypocenter double-difference(HypoDD)method.The statistical characteristics of microseismicity resulting from water injection in SSBC were analyzed,and the potential correlation between the event rate and statistical parameters,such as Gutenberg-Richter b-value,spatial correlation length,and fractal dimension,was quantified.Based on spatial variations of b-value and fractal dimension of event distribution,we identified two potential risk areas in the East and West of the Zhaotong shale gas block(YS108),respectively.The focal mechanism solutions(FMSs)of the observed seismic events(M_(L)>2.5)near the H7 well pad were calculated utilizing the generalized cut-and-paste(gCAP)technique combined with P-wave polarity.The FMSs’results show reverse faults,and some of them have fault planes oriented in the N-S direction,causing oblique slip movement.In addition,we also inverted the regional stress field using high-quality FMSs,revealing that the maximum principal stress(σ1)trends NW–SE and lies nearly horizontal,in agreement with the World Stress Map and borehole breakout records.Considering geological structures and regional stress distribution,the reasons for induced seismicity were mainly linked to pore pressure diffusion.Our obtained findings may provide insights for future seismic risk assessment and mitigation strategies.
基金financial support from the National Natural Science Foundation of China(Nos.52209144 and 12472405).
文摘High-voltage electric pulse(HVEP)rock fragmentation has demonstrated substantial potential for sustainable fracturing of hard rocks owing to its energy efficiency.The transient nature and highly disruptive characteristics of its physical fracturing process render experimental investigation of the underlying rock-breaking mechanisms challenging.However,existing numerical studies lack comprehensive models that precisely link electrical breakdown phenomena with mechanical disintegration processes.This study combines COMSOL electrical breakdown simulations with four-dimension lattice spring model(4D-LSM)mechanical analysis to establish a coupled HVEP rock fragmentation model.The core concept of the model construction is to import the temperature field of the plasma channel obtained from the electrical breakdown into the mechanical solver to realize the precise connection between the two stages.The validated numerical model elucidates the full process of HVEP-induced fragmentation under varying electrical parameters.Furthermore,the effects of confining pressure and mineral grain size on fragmentation behavior have been investigated.Finally,parametric simulations across 25 electrical parameter combinations demonstrate the critical role of electrode spacing optimization in achieving energy-efficient rock fragmentation.These findings provide a predictive tool for designing efficient HVEP systems in deep resource extraction and mineral processing engineering.
基金supported by the National Key Research and Development Program of China(Grant No.2020YFA0711800)the National Natural Science Foundation of China(Grant No.42372159).
文摘Methane in-situ deflagration fracturing in shale is a revolutionary anhydrous technology.This paper selects shale samples from the Longmaxi Formation in Southern Sichuan to conduct deflagration fracturing tests with pressures ranging from 25 MPa to 91 MPa.Pore structure changes were experimentally measured to explore the modification differences of nanoscale pore characteristics under varying deflagration pressures.The results show that within the deflagration pressure range examined in this study,(1)Deflagration fracturing can alter the pore volume and specific surface area but does not affect the distribution characteristics of the pore size's peak position.The maximum increments of total pore volume occur at a pressure of 45 MPa.(2)When the deflagration pressure is less than 45 MPa,porosity gradually increases with rising deflagration pressure.When it is greater than 45 MPa,the porosity does not change significantly.With increasing deflagration pressure,it gradually increases:from nanopores,such as mesopores and macropores,to large pores and microcracks.(3)At the low deflagration pressure stage,under the influence of high temperature,slippage pores gradually increase,which is conducive to gas desorption and diffusion migration.As pressure increases,the impact of the explosion shock wave gradually increases,the volume of seepage pores increases significantly,and seepage dominates the migration mode.
基金supported by the National Natural Science Foundation of China(Grant Nos.42377143 and 52225403)Sichuan Natural Science Foundation(Grant No.2024NSFSC0097).
文摘As an emerging rock-breaking technology,microwave irradiation has demonstrated significant potential as an auxiliary technique for volume stimulation in hydraulic fracturing.This study focuses on tight sandstone gas extraction,introducing a hollow double-wing crack(HDWC)configuration into the research on tight sandstone.Laboratory experiments were conducted to investigate microwave-induced fracturing mechanisms and the mechanical behavior of HDWC-containing sandstone,aiming to elucidate the thermal cracking patterns and underlying mechanisms under microwave irradiation conditions.To further explore the electromagnetic-thermal-mechanical(E-T-M)interactions in tight sandstone under microwave treatment,a coupled finite element method(FEM)-discrete element method(DEM)numerical model was developed.This model enabled a detailed analysis of force chain evolution and microcrack propagation within HDWC-containing sandstone.Additionally,preliminary hydraulic fracturing simulations were performed to investigate fracture initiation pressure and fracture evolution following microwave exposure.The main findings of this study are as follows:(1)Microwave heating induces thermal cracks at both the tips and midsections of the HDWC.Microwave irradiation degrades the mechanical properties of HDWC-containing sandstone.(2)Simulation results reveal that significant stress concentration and tensile-compressive zoning occur near the HDWC under microwave irradiation.Microcrack development exhibits an avalanche effect.(3)Hydraulic fracturing simulations indicate that microwave heating generally promotes hydraulic fracture generation.Microwave irradiation reduces the fracture initiation pressure and enhances the complexity and connectivity of the fracture network.These findings provide valuable insights into the application of microwave-assisted volume stimulation as a supporting technology for hydraulic fracturing in deep reservoirs.
基金National Key Research and Development Program of China,2020YFA0711800,Ting LiuNational Natural Science Foundation of China,52274241,Ting Liu,52474261,Ting Liu+2 种基金Basic Research Program of Jiangsu,BK20240207,Ting Liuthe Fundamental Research Funds for the Central Universities(2023KYJD1007)China Postdoctoral Science Foundation(2022M722672).
文摘To elucidate the dynamic characteristics of in-situ methane deflagration in coalbed methane wellbores and its mechanisms for fracturing coal rock,this study first developed a simulation experimental system specifically designed for methane in-situ deflagration fracturing.This experimental system,which is capable of withstanding pressures up to 150 MPa and meanwhile applying axial and confining pressures of up to 50 MPa to rock cores,enables the coupled simulation on methane deflagration and rock core fracturing processes.With the aid of this experimental system,physical simulation experiments on in-situ methane deflagration fracturing were conducted,and the following findings were obtained.Methane deflagration loads in enclosed wellbores exhibit characteristics of multi-level pulsed oscillation.With the rise of initial gas pressure,the peak deflagration load increases approximately linearly,with the pressure amplification factor spanning from 23.14 to 31.10,and its peak loading rate grows exponentially.Accordingly,the fracture volume and fracture porosity augment.To be specific,when the initial gas pressure rises from 0.6 to 2.4 MPa,the fracture volume and fracture porosity augment by factors of 14.0 and 8.73,respectively.The fractal dimension of spatial distribution of fractures also increases with the rise of deflagration load,indicating that a higher deflagration load conduces to the development of a larger and more complex fracture network.Methane deflagration fracturing is characterized as a composite fracture mode that involves the impact of strong stress waves and the driving force of high-pressure fluids.The primary factors influencing damage to coal-rock include the high-stress impact in the initial stage of deflagration,the fluid pressure driving effect in the middle stage,and the thermal shock resulting from high temperatures in the later stage.
文摘With the increasing demand for energy,traditional oil resources are facing depletion and insufficient supply.Many countries are rapidly turning to the development of unconventional oil and gas resources.Among them,shale oil and gas reservoirs have become the focus of unconventional oil and gas resources exploration and development.Based on the characteristics of shale oil and gas reservoirs,supercritical CO_(2) fracturing is more conducive to improving oil recovery than other fracturing technologies.In this paper,the mechanism of fracture initiation and propagation of supercritical CO_(2) in shale is analyzed,including viscosity effect,surface tension effect,permeation diffusion effect of supercritical CO_(2),and dissolution-adsorption effect between CO_(2) and shale.The effects of natural factors,such as shale properties,bedding plane and natural fractures,and controllable factors,proppant,temperature,pressure,CO_(2) concentration and injection rate on fracture initiation and propagation are clarified.The methods of supercritical CO_(2) fracturing process,thickener and proppant optimization to improve the efficiency of supercritical CO_(2) fracturing are discussed.In addition,some new technologies of supercritical CO_(2) fracturing are introduced.The challenges and prospects in the current research are also summarized.For example,supercritical CO_(2) is prone to filtration when passing through porous media,and it is difficult to form a stable flow state.Therefore,in order to achieve stable fracturing fluid suspension and effectively support fractu res,it is urge nt to explo re new fracturing fluid additives or improve fracturing fluid formulations combined with the research of new proppants.This paper is of great significance for understanding the behavior mechanism of supercritical CO_(2) in shale and optimizing fracturing technology.
基金supported by the National Natural Science Foun-dation of China(52421002,U24B6001,52204019,and 52192624)the Open Foundation of the Shanxi Key Laboratory of Carbon Dioxide Sequestration and Enhanced Oil Recovery。
文摘Medium-high maturity continental shale oil is one of the hydrocarbon resources with the most potential for successful development in China.Nevertheless,the unique geological conditions of a multi-lithologic superposition shield the vertical propagation of hydraulic fractures and limit the longitudinal reconstruction in reservoirs,posing a great challenge for large-scale volumetric fracturing.Radial wellbore crosslayer fracturing,which transforms the interaction between the hydraulic fractures and lithologic interface into longitudinal multilayer competitive initiation,could provide a potential solution for this engineering challenge.To determine the longitudinal propagation behaviors of fractures guided by radial wellbores,true triaxial fracturing experiments were performed on multilayer shale-sandstone samples,with a focus on the injection pressure response,fracture morphology,and cross-layer pattern.The effects of the radial borehole length L,vertical stress difference K_(v),injection rate Q,and viscosity m of the fracturing fluid were analyzed.The results indicate that radial wellbores can greatly facilitate fracture initiation and cross-layer propagation.Unlike conventional hydraulic fracturing,there are two distinct fracture propagation patterns in radial wellbore fracturing:cross-layering and skip-layering.The fracture height guided by a radial wellbore is positively correlated with K_(v),Q,and m.Increasing these parameters causes a shift in the fracture initiation from a single root to an asynchronous root/toe end and can improve the cross-layer propagation capacity.Critical parameter thresholds exist for fracture propagation through and across interlayers under the guidance of radial boreholes.A parameter combination of critical cross-layering/skip-layering or alternating displacement/viscosity is recommended to simultaneously improve the fracture height and degree of lateral activation.The degree of correlation of different parameters with the vertical fracture height can be written as L>Q/m>K_(v).Increasing the radial wellbore length can effectively facilitate fracture cross-/skip-layer propagation and reduce the critical threshold of injection parameters,which is conducive to maximizing the stimulated reservoir volume.
基金support from China National Natural Science Foundation (11672333)。
文摘The effectiveness of horizontal well multi-stage and multi-cluster fracturing in the fractured soft coal seam roof for coalbed methane(CBM) extraction has been demonstrated.This study focuses on the geological characteristics of the No.5 and No.11 coal seams in the Hancheng Block,Ordos Basin,China.A multi-functional,variable-size rock sample mold capable of securing the wellbore was developed to simulate layered formations comprising strata of varying lithology and thicknesses.A novel segmented fracturing simulation method based on an expandable pipe plugging technique is proposed.Large-scale true triaxial experiments were conducted to investigate the effects of horizontal wellbore location,perforation strategy,roof lithology,and vertical stress difference on fracture propagation,hydraulic energy variation,and the stimulated reservoir volume in horizontal wells targeting the soft coal seam roof.The results indicate that bilateral downward perforation with a phase angle of 120° optimizes hydraulic energy conservation,reduces operational costs,enhances fracture formation,and prevents fracturing failure caused by coal powder generation and migration.This perforation mode is thus considered optimal for coal seam roof fracturing.When the roof consists of sandstone,each perforation cluster tends to initiate a single dominant fracture with a regular geometry.In contrast,hydraulic fractures formed in mudstone roofs display diverse morphology.Due to its high strength,the sandstone roof requires significantly higher pressure for crack initiation and propagation,whereas the mudstone roof,with its strong water sensitivity,exhibits lower fracturing pressures.To mitigate inter-cluster interference,cluster spacing in mudstone roofs should be greater than that in sandstone roofs.Horizontal wellbore placement critically influences fracturing effectiveness.For indirect fracturing in sandstone roofs,an optimal position is 25 mm away from the lithological interface.In contrast,the optimal location for indirect fracturing in mudstone roofs is directly at the lithological interface with the coal seam.Higher vertical stress coefficients lead to increased fractu ring pressures and promote vertical,layer-penetrating fractures.A coefficient of 0.5 is identified as optimal for achieving effective indirect fracturing.This study provides valuable insights for the design and optimization of staged fracturing in horizontal wells targeting crushed soft coal seam roofs.
基金Supported by the National Natural Science Foundation of China(NSFC)-Enterprise Innovation and Development Joint Fund Key Project(U24B2018)the Distinguished Young Scholars Program of the National Natural Science Foundation of China(42125205).
文摘Low-salinity fracturing fluids tend to induce ion migration,alter wettability,and cause fluctuations in gas desorption efficiency when penetrating deep coal seams.Taking the No.8 coal from the Daning-Jixian area in the Ordos Basin,NW China,as a representative example,this study employs physical simulation experiments to reveal the coupled control mechanism of salinity gradient on the ion-coal matrix-gas/water interfacial system and its key role in the imbibition-desorption process.The increasing ionic concentration improves the hydrophobicity of coal,with multivalent ions exhibiting particularly significant effects.The imbibition and ion diffusion occur in opposite directions,with imbibition equilibrium being achieved earlier than ionic equilibrium.Water-coal interactions induce both mineral dissolution and secondary precipitation.When a low-salinity fracturing fluid is injected into a high-salinity reservoir,the osmotic-pressure difference drives imbibition,promotes CH4 desorption,but results in higher fluid loss.Conversely,injecting high-salinity fracturing fluid into a low-salinity reservoir creates a reverse osmotic gradient that suppresses leak-off while improving flowback efficiency.Based on these findings,a high-low salinity sequential injection strategy is proposed for deep coal seams:high-salinity fluid is first injected to form stable fracture networks,followed by low-salinity fluid to enlarge the imbibition zone and enhance CH4 desorption and diffusion.Moderate well soaking is recommended to increase the imbibition volume,thereby achieving multiple positive effects such as maintaining reservoir pressure,preserving formation energy,and promoting imbibition-driven displacement.
基金supported in part by the National Key Research and Development Project of China(No.2022YFC3004602)in part by the National Natural Science Foundation of China(Nos.52121003 and 52342403).
文摘Multistage fracturing technology has been used to enhance tight hydrocarbon resource recovery.Determining the proper well spacing and fracturing strategy is crucial for generating a complex fracture network that facilitates oil and gas flow in reservoirs.The stress-shadow effect that occurs between multiple wells significantly affects the development of fracture networks in reservoirs.However,the quantification of the stress-shadow effect and its influence on fracture networks has not been satisfactorily resolved because of the difficulties in detecting and identifying fracture propagation and reorientation in reservoirs.In this study,based on the geological information from the Shengli oilfield,we applied a hybrid finite element-discrete element method to analyze engineering-scale three-dimensional fracture propagation and reorientation by altering well spacings and fracturing strategies.The results indicate that the fracturing area generated by the synchronous fracturing scheme is much smaller than those generated by the sequential and alternative schemes.An alternative hydrofracturing scheme is optimal with respect to fracturing area.The stress-blind area was defined to quantify the mechanical disturbance between adjacent wells.Our study improves the understanding of the effect of fracturing schemes on fracture networks and the impact of independent factors contributing to stress-shadow effects.
基金funded by the National Natural Science Foundation of China(52104046).
文摘Multi-stage and multi-cluster fracturing(MMF)is a crucial technology in unconventional oil and gas development,aiming to enhance production by creating extensive fracture networks.However,achieving uniform expansion of multi-cluster hydraulic fractures(HFs)in MMF remains a significant challenge.Field practice has shown that the use of temporary plugging and diversion fracturing(TPDF)can promote the balanced expansion of multi-cluster HFs.This study conducted TPDF experiments using a true triaxial fracturing simulation system setting a horizontal well completion with multi-cluster jetting perforations to investigate the equilibrium initiation and extension of multi-cluster fractures.The influence of key parameters,including cluster spacing,fracturing fluid viscosity,differential stress,and fracturing fluid injection rate,on fracture initiation and propagation was systematically examined.The results indicate that while close-spaced multi-cluster fracturing significantly increases the number of HFs,it also leads to uneven extension of HFs in their propagation.In contrast,TPDF demonstrates effectiveness in mitigating uneven HF extension,increasing the number of HFs,and creating a larger stimulated reservoir volume,ultimately leading to improved oil and gas well productivity.Moreover,under conditions of high differential stress,the differential stress within the formation exerts a stronger guiding effect in HFs,which are more closely aligned with the minimum principal stress.Low-viscosity fluids facilitate rapid and extensive fracture propagation within the rock formation.High-volume fluid injection,on the other hand,more comprehensively fills the formation.Therefore,employing lowviscosity and high-volume fracturing is advantageous for the initiation and extension of multi-cluster HFs.
基金Supported by the Basic Science Center Project of the National Natural Science Foundation of China(52288101).
文摘Based on continuum-discontinuum element method,the numerical simulation of fracture propagation during deflagration-hydraulic composite fracturing was constructed by considering deflagration stress impact induced fracture creation,deflagrating gas driven fracture propagation,and hydraulic fracture propagation,exploring the effects of in-situ stress difference,deflagration peak pressure,deflagration pressurization rate,hydraulic fracturing displacement and hydraulic fracturing fluid viscosity on fracture propagation in deflagration-hydraulic composite fracturing.The deflagration-hydraulic composite fracturing combines the advantages of deflagration fracturing in creating complex fractures near wells and the deep penetration of hydraulic fracturing at the far-field region,which can form multiple deep penetrating long fractures with better stimulation effects.With the increase of in-situ stress difference,the stimulated area of deflagration-hydraulic composite fracturing is reduced,and the deflagration-hydraulic composite fracturing is more suitable for reservoirs with small in-situ stress difference.Higher peak pressure and pressurization rate are conducive to increasing the maximum fracture length and burst degree of the deflagration fractures,which in turn increases the stimulated area of deflagration-hydraulic composite fracturing and improves the stimulation effect.Increasing the displacement and viscosity of hydraulic fracturing fluid can enhance the net pressure within the fractures,activate the deflagration fractures,increase the turning radius of the fractures,generate more long fractures,and effectively increase the stimulated reservoir area.The stimulated reservoir area is not completely positively correlated with the hydraulic fracturing displacement and fracturing fluid viscosity,and there is a critical value.When the critical value is exceeded,the stimulated area decreases.
