The Carter model is used to characterize the dynamic behaviors of fracture growth and fracturing fluid leakoff.A thermo-fluid coupling temperature response forward model is built considering the fluid flow and heat tr...The Carter model is used to characterize the dynamic behaviors of fracture growth and fracturing fluid leakoff.A thermo-fluid coupling temperature response forward model is built considering the fluid flow and heat transfer in wellbore,fracture and reservoir.The influences of fracturing parameters and fracture parameters on the responses of distributed temperature sensing(DTS)are analyzed,and a diagnosis method of fracture parameters is presented based on the simulated annealing algorithm.A field case study is introduced to verify the model’s reliability.Typical V-shaped characteristics can be observed from the DTS responses in the multi-cluster fracturing process,with locations corresponding to the hydraulic fractures.The V-shape depth is shallower for a higher injection rate and longer fracturing and shut-in time.Also,the V-shape is wider for a higher fracture-surface leakoff coefficient,longer fracturing time and smaller fracture width.Additionally,the cooling effect near the wellbore continues to spread into the reservoir during the shut-in period,causing the DTS temperature to decrease instead of rise.Real-time monitoring and interpretation of DTS temperature data can help understand the fracture propagation during fracturing operation,so that immediate measures can be taken to improve the fracturing performance.展开更多
Shale oil reservoir is generally characterized by well-developed bedding planes,and multi-cluster fracturing is the most effective technique to achieve stable shale oil production.In this paper,a multi-cluster fractur...Shale oil reservoir is generally characterized by well-developed bedding planes,and multi-cluster fracturing is the most effective technique to achieve stable shale oil production.In this paper,a multi-cluster fracturing model for a horizontal well in shale with high-density bedding planes is established.The fracture morphology,fracture geometry,fracturing area and multiple fracture propagation mechanism are analyzed under simultaneous fracturing,sequential fracturing,and alternative fracturing.Results show that in the case of small cluster spacing and three clusters,the growth of the middle fracture is inhibited and develops along the bedding planes under both simultaneous fracturing and alternative fracturing.For sequential fracturing,the increase in the interval time between each fracturing advances the post fracturing fracture deflecting to the pre-existing fractures through the bedding planes.The reactivation of the bedding planes can promote the extension of the fracturing area.Increasing the injection rate and the number of clusters promotes the activation of bedding planes.However,it is preferable to reduce the number of clusters to obtain more main fractures.Compared with modified alternating fracturing and cyclic alternating fracturing,alternating shut-in fracturing creates more main fractures towards the direction of the maximum in-situ stress.The fracturing efficiency for high-density layered shale is ranked as simultaneous fracturing>alternative fracturing>sequential fracturing.展开更多
During multi-cluster fracturing of horizontal wells,low-viscosity fracturing fluid permeates extensively along the fracture walls into the rock pores,exerting seepage forces on the rock skeleton.However,the mechanism ...During multi-cluster fracturing of horizontal wells,low-viscosity fracturing fluid permeates extensively along the fracture walls into the rock pores,exerting seepage forces on the rock skeleton.However,the mechanism of how seepage forces influence multi-cluster fracture propagation behavior remains unclear.In this study,we analyze the response relationship between the pore pressure field and the seepage field under transient seepage conditions.We established a numerical simulation model of multi-cluster fracture propagation in horizontal wells considering seepage forces based on the discrete element particle flow method.Using this model,we examined the mechanisms by which seepage forces affect the dynamic evolution of the induced stress field and fracture propagation behavior under various combinations of fracturing fluid viscosity,injection rate,in-situ stress,and cluster spacing.Numerical simulation results indicate that the infiltration of fracturing fluid into rock pores and the resulting seepage forces significantly alter the propagation patterns of inter-cluster fractures and the distribution of the induced stress field.Under the influence of seepage forces,hydraulic fractures suppressed by stress shadows are redirected and captured by adjacent fractures.Selecting appropriate fracturing fluid viscosity and injection rate can effectively utilize seepage forces to mitigate the inhibitory effects of the high compressive stress zones within stress shadows on intermediate fracture propagation,thereby increasing the likelihood of inter-fracture communication and the formation of a complex fracture network.This study not only underscores the importance of considering seepage forces in the design of multi-stage fracturing treatments but also provides critical theoretical insights for optimizing fracturing operation parameters.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
The authors regret Acknowledgements Firstly,the authors wish to acknowledge the academic support from Ruhr University Bochum during the first author's(Xiao Yan)research stay from 2018.11 to 2020.10,including the s...The authors regret Acknowledgements Firstly,the authors wish to acknowledge the academic support from Ruhr University Bochum during the first author's(Xiao Yan)research stay from 2018.11 to 2020.10,including the soft code implement and debug support from Vladislav Gudzulic and academic advising from Günther Meschke.展开更多
The continental shale reservoirs of Jurassic Lianggaoshan Formation in Sichuan Basin contain thin lamina,which is characterized by strong plasticity and developed longitudinal shell limestone interlayer.To improve the...The continental shale reservoirs of Jurassic Lianggaoshan Formation in Sichuan Basin contain thin lamina,which is characterized by strong plasticity and developed longitudinal shell limestone interlayer.To improve the production efficiency of reservoirs by multi-cluster fracturing,it is necessary to consider the unbalanced propagation of hydraulic fractures and the penetration effect of fractures.This paper constructed a numerical model of multi-fracture propagation and penetration based on the finite element coupling cohesive zone method;considering the construction cluster spacing,pump rate,lamina strength and other parameters studied the influencing factors of multi-cluster fracture interaction propagation;combined with AE energy data and fracture mode reconstruction method,quantitatively characterized the comprehensive impact of the strength of thin interlayer rock interfaces on the initiation and propagation of fractures that penetrate layers,and accurately predicted the propagation pattern of hydraulic fractures through laminated shale oil reservoirs.Simulation results revealed that in the process of multi-cluster fracturing,the proportion of shear damage is low,and mainly occurs in bedding fractures activated by outer fractures.Reducing the cluster spacing enhances the fracture system's penetration ability,though it lowers the activation efficiency of lamina.The high plasticity of the limestone interlayer may impact the vertical propagation distance of the main fracture.Improving the interface strength is beneficial to the reconstruction of the fracture height,but the interface communication effect is limited.Reasonable selection of layers with moderate lamina strength for fracturing stimulation,increasing the pump rate during fracturing and setting the cluster spacing reasonably are beneficial to improve the effect of reservoir stimulation.展开更多
Multistage multi-cluster hydraulic fracturing has enabled the economic exploitation of shale reservoirs,but the interpretation of hydraulic fracture parameters is challenging.The pressure signals after pump shutdown a...Multistage multi-cluster hydraulic fracturing has enabled the economic exploitation of shale reservoirs,but the interpretation of hydraulic fracture parameters is challenging.The pressure signals after pump shutdown are influenced by hydraulic fractures,which can reflect the geometric features of hydraulic fracture.The shutdown pressure can be used to interpret the hydraulic fracture parameters in a real-time and cost-effective manner.In this paper,a mathematical model for shutdown pressure evolution is developed considering the effects of wellbore friction,perforation friction and fluid loss in fractures.An efficient numerical simulation method is established by using the method of characteristics.Based on this method,the impacts of fracture half-length,fracture height,opened cluster and perforation number,and filtration coefficient on the evolution of shutdown pressure are analyzed.The results indicate that a larger fracture half-length may hasten the decay of shutdown pressure,while a larger fracture height can slow down the decay of shutdown pressure.A smaller number of opened clusters and perforations can significantly increase the perforation friction and decrease the overall level of shutdown pressure.A larger filtration coefficient may accelerate the fluid filtration in the fracture and hasten the drop of the shutdown pressure.The simulation method of shutdown pressure,as well as the analysis results,has important implications for the interpretation of hydraulic fracture parameters.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
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
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
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
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
This study conducted temporary plugging and diversion fracturing(TPDF)experiments using a true triaxial fracturing simulation system within a laboratory setting that replicated a lab-based horizontal well completion w...This study conducted temporary plugging and diversion fracturing(TPDF)experiments using a true triaxial fracturing simulation system within a laboratory setting that replicated a lab-based horizontal well completion with multi-cluster sand jetting perforation.The effects of temporary plugging agent(TPA)particle size,TPA concentration,single-cluster perforation number and cluster number on plugging pressure,multi-fracture diversion pattern and distribution of TPAs were investigated.A combination of TPAs with small particle sizes within the fracture and large particle sizes within the segment is conducive to increasing the plugging pressure and promoting the diversion of multi-fractures.The addition of fibers can quickly achieve ultra-high pressure,but it may lead to longitudinal fractures extending along the wellbore.The temporary plugging peak pressure increases with an increase in the concentration of the TPA,reaching a peak at a certain concentration,and further increases do not significantly improve the temporary plugging peak pressure.The breaking pressure and temporary plugging peak pressure show a decreasing trend with an increase in single-cluster perforation number.A lower number of single-cluster perforations is beneficial for increasing the breaking pressure and temporary plugging peak pressure,and it has a more significant control on the propagation of multi-cluster fractures.A lower number of clusters is not conducive to increasing the total number and complexity of artificial fractures,while a higher number of clusters makes it difficult to achieve effective plugging.The TPAs within the fracture is mainly concentrated in the complex fracture areas,especially at the intersections of fractures.Meanwhile,the TPAs within the segment are primarily distributed near the perforation cluster apertures which initiated complex fractures.展开更多
Prepulse combined hydraulic fracturing facilitates the development of fracture networks by integrating prepulse hydraulic loading with conventional hydraulic fracturing.The formation mechanisms of fracture networks be...Prepulse combined hydraulic fracturing facilitates the development of fracture networks by integrating prepulse hydraulic loading with conventional hydraulic fracturing.The formation mechanisms of fracture networks between hydraulic and pre-existing fractures under different prepulse loading parameters remain unclear.This research investigates the impact of prepulse loading parameters,including the prepulse loading number ratio(C),prepulse loading stress ratio(S),and prepulse loading frequency(f),on the formation of fracture networks between hydraulic and pre-existing fractures,using both experimental and numerical methods.The results suggest that low prepulse loading stress ratios and high prepulse loading number ratios are advantageous loading modes.Multiple hydraulic fractures are generated in the specimen under the advantageous loading modes,facilitating the development of a complex fracture network.Fatigue damage occurs in the specimen at the prepulse loading stage.The high water pressure at the secondary conventional hydraulic fracturing promotes the growth of hydraulic fractures along the damage zones.This allows the hydraulic fractures to propagate deeply and interact with pre-existing fractures.Under advantageous loading conditions,multiple hydraulic fractures can extend to pre-existing fractures,and these hydraulic fractures penetrate or propagate along pre-existing fractures.Especially when the approach angle is large,the damage range in the specimen during the prepulse loading stage increases,resulting in the formation of more hydraulic fractures.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
Through a case analysis,this study examines the spatiotemporal evolution of microseismic(MS)events,energy characteristics,volumetric features,and fracture network development in surface well hydraulic fracturing.A tot...Through a case analysis,this study examines the spatiotemporal evolution of microseismic(MS)events,energy characteristics,volumetric features,and fracture network development in surface well hydraulic fracturing.A total of 349 MS events were analyzed across different fracturing sections,revealing significant heterogeneity in fracture propagation.Energy scanning results showed that cumulative energy values ranged from 240 to 1060 J across the sections,indicating notable differences.Stimulated reservoir volume(SRV)analysis demonstrated well-developed fracture networks in certain sections,with a total SRV exceeding 1540000 m^(3).The hydraulic fracture network analysis revealed that during the midfracturing stage,the density and spatial extent of MS events significantly increased,indicating rapid fracture propagation and the formation of complex networks.In the later stage,the number of secondary fractures near fracture edges decreased,and the fracture network stabilized.By comparing the branching index,fracture length,width,height,and SRV values across different fracturing sections,Sections No.1 and No.8 showed the best performance,with high MS event densities,extensive fracture networks,and significant energy release.However,Sections No.4 and No.5 exhibited sparse MS activity and poor fracture connectivity,indicating suboptimal stimulation effectiveness.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
Discrete fracture network(DFN)commonly existing in natural rock masses plays an important role in geological complexity which can influence rock fracturing behaviour during fluid injection.This paper simulated the hyd...Discrete fracture network(DFN)commonly existing in natural rock masses plays an important role in geological complexity which can influence rock fracturing behaviour during fluid injection.This paper simulated the hydraulic fracturing process in lab-scale coal samples with DFNs and the induced seismic activities by the discrete element method(DEM).The effects of DFNs on hydraulic fracturing,induced seismicity and elastic property changes have been concluded.Denser DFNs can comprehensively decrease the peak injection pressure and injection duration.The proportion of strong seismic events increases first and then decreases with increasing DFN density.In addition,the relative modulus of the rock mass is derived innovatively from breakdown pressure,breakdown fracture length and the related initiation time.Increasing DFN densities among large(35–60 degrees)and small(0–30 degrees)fracture dip angles show opposite evolution trends in relative modulus.The transitional point(dip angle)for the opposite trends is also proportionally affected by the friction angle of the rock mass.The modelling results have much practical meaning to infer the density and geometry of pre-existing fractures and the elastic property of rock mass in the field,simply based on the hydraulic fracturing and induced seismicity monitoring data.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
Supercritical CO_(2)(SC-CO_(2))fracturing stands out a promising waterless stimulation technique in the development of unconventional resources.While numerous studies have delved into the inducedfracture mechanism of ...Supercritical CO_(2)(SC-CO_(2))fracturing stands out a promising waterless stimulation technique in the development of unconventional resources.While numerous studies have delved into the inducedfracture mechanism of SC-CO_(2),the small scale of rock samples and synthetic materials used in many studies have limited a comprehensive understanding of fracture propagation in unconventional formations.In this study,cubic tight sandstone samples with dimensions of 300 mm were employed to conduct SC-CO_(2)fractu ring experiments under true-triaxial stre ss conditions.The spatial morphology and quantitative attributes of fracture induced by water and SC-CO_(2)fracturing were compared,while the impact of in-situ stress on fracture propagation was also investigated.The results indicate that the SCCO_(2)fracturing takes approximately ten times longer than water fracturing.Furthermore,under identical stress condition,the breakdown pressure(BP)for SC-CO_(2)fracturing is nearly 25%lower than that for water fracturing.A quantitative analysis of fracture morphology reveals that water fracturing typically produces relatively simple fracture pattern,with the primary fracture distribution predominantly controlled by bedding planes.In contrast,SC-CO_(2)fracturing results in a more complex fracture morphology.As the differential of horizontal principal stress increases,the BP for SC-CO_(2)fractured rock exhibits a downward trend,and the induced fracture morphology becomes more simplified.Moreover,the presence of abnormal in-situ stress leads to a further increase in the BP for SC-CO_(2)fracturing,simultaneously enhancing the development of a more conductive fracture network.These findings provide critical insights into the efficiency and behavior of SC-CO_(2)fracturing in comparison to traditional water-based fracturing,offering valuable implication for its potential applications in unconventional reservoirs.展开更多
基金Supported by the National High-Tech Research Project(GJSCB-HFGDY-2024-004)National Natural Science Foundation of China(12402305)+2 种基金Postdoctoral Fellowship Program of CPSF(GZC20232200)China Postdoctoral Science Foundation(2024M762703)Sichuan Science and Technology Program(2025ZNSFSC1352)。
文摘The Carter model is used to characterize the dynamic behaviors of fracture growth and fracturing fluid leakoff.A thermo-fluid coupling temperature response forward model is built considering the fluid flow and heat transfer in wellbore,fracture and reservoir.The influences of fracturing parameters and fracture parameters on the responses of distributed temperature sensing(DTS)are analyzed,and a diagnosis method of fracture parameters is presented based on the simulated annealing algorithm.A field case study is introduced to verify the model’s reliability.Typical V-shaped characteristics can be observed from the DTS responses in the multi-cluster fracturing process,with locations corresponding to the hydraulic fractures.The V-shape depth is shallower for a higher injection rate and longer fracturing and shut-in time.Also,the V-shape is wider for a higher fracture-surface leakoff coefficient,longer fracturing time and smaller fracture width.Additionally,the cooling effect near the wellbore continues to spread into the reservoir during the shut-in period,causing the DTS temperature to decrease instead of rise.Real-time monitoring and interpretation of DTS temperature data can help understand the fracture propagation during fracturing operation,so that immediate measures can be taken to improve the fracturing performance.
基金the financial support from Intergovernmental International Science and Technology Innovation Cooperation Key Project(2022YFE0128400)National Natural Science Foundation of China(42307209)+2 种基金Shanghai Pujiang Program(2022PJD076)State Energy Center for Shale Oil Research and Development(33550000-22-ZC0613-0365)Natural Science Foundation of Qinghai Province(No.2024-ZJ-717).
文摘Shale oil reservoir is generally characterized by well-developed bedding planes,and multi-cluster fracturing is the most effective technique to achieve stable shale oil production.In this paper,a multi-cluster fracturing model for a horizontal well in shale with high-density bedding planes is established.The fracture morphology,fracture geometry,fracturing area and multiple fracture propagation mechanism are analyzed under simultaneous fracturing,sequential fracturing,and alternative fracturing.Results show that in the case of small cluster spacing and three clusters,the growth of the middle fracture is inhibited and develops along the bedding planes under both simultaneous fracturing and alternative fracturing.For sequential fracturing,the increase in the interval time between each fracturing advances the post fracturing fracture deflecting to the pre-existing fractures through the bedding planes.The reactivation of the bedding planes can promote the extension of the fracturing area.Increasing the injection rate and the number of clusters promotes the activation of bedding planes.However,it is preferable to reduce the number of clusters to obtain more main fractures.Compared with modified alternating fracturing and cyclic alternating fracturing,alternating shut-in fracturing creates more main fractures towards the direction of the maximum in-situ stress.The fracturing efficiency for high-density layered shale is ranked as simultaneous fracturing>alternative fracturing>sequential fracturing.
基金supported by the National Natural Science Foundation of China,Grant number U23B2089Shaanxi Provincial Natural Science Basic Research Program Project,Grant number 2024JC-YBQN-0554Scientific Research Program Funded by Shaanxi Provincial Education Department(Program No.24JK0597).
文摘During multi-cluster fracturing of horizontal wells,low-viscosity fracturing fluid permeates extensively along the fracture walls into the rock pores,exerting seepage forces on the rock skeleton.However,the mechanism of how seepage forces influence multi-cluster fracture propagation behavior remains unclear.In this study,we analyze the response relationship between the pore pressure field and the seepage field under transient seepage conditions.We established a numerical simulation model of multi-cluster fracture propagation in horizontal wells considering seepage forces based on the discrete element particle flow method.Using this model,we examined the mechanisms by which seepage forces affect the dynamic evolution of the induced stress field and fracture propagation behavior under various combinations of fracturing fluid viscosity,injection rate,in-situ stress,and cluster spacing.Numerical simulation results indicate that the infiltration of fracturing fluid into rock pores and the resulting seepage forces significantly alter the propagation patterns of inter-cluster fractures and the distribution of the induced stress field.Under the influence of seepage forces,hydraulic fractures suppressed by stress shadows are redirected and captured by adjacent fractures.Selecting appropriate fracturing fluid viscosity and injection rate can effectively utilize seepage forces to mitigate the inhibitory effects of the high compressive stress zones within stress shadows on intermediate fracture propagation,thereby increasing the likelihood of inter-fracture communication and the formation of a complex fracture network.This study not only underscores the importance of considering seepage forces in the design of multi-stage fracturing treatments but also provides critical theoretical insights for optimizing fracturing operation parameters.
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.
文摘The authors regret Acknowledgements Firstly,the authors wish to acknowledge the academic support from Ruhr University Bochum during the first author's(Xiao Yan)research stay from 2018.11 to 2020.10,including the soft code implement and debug support from Vladislav Gudzulic and academic advising from Günther Meschke.
基金financial support by the National Key Research and Development Program of China (No.2022YFE0129800)the National Natural Science Foundation of China (No.52074311)。
文摘The continental shale reservoirs of Jurassic Lianggaoshan Formation in Sichuan Basin contain thin lamina,which is characterized by strong plasticity and developed longitudinal shell limestone interlayer.To improve the production efficiency of reservoirs by multi-cluster fracturing,it is necessary to consider the unbalanced propagation of hydraulic fractures and the penetration effect of fractures.This paper constructed a numerical model of multi-fracture propagation and penetration based on the finite element coupling cohesive zone method;considering the construction cluster spacing,pump rate,lamina strength and other parameters studied the influencing factors of multi-cluster fracture interaction propagation;combined with AE energy data and fracture mode reconstruction method,quantitatively characterized the comprehensive impact of the strength of thin interlayer rock interfaces on the initiation and propagation of fractures that penetrate layers,and accurately predicted the propagation pattern of hydraulic fractures through laminated shale oil reservoirs.Simulation results revealed that in the process of multi-cluster fracturing,the proportion of shear damage is low,and mainly occurs in bedding fractures activated by outer fractures.Reducing the cluster spacing enhances the fracture system's penetration ability,though it lowers the activation efficiency of lamina.The high plasticity of the limestone interlayer may impact the vertical propagation distance of the main fracture.Improving the interface strength is beneficial to the reconstruction of the fracture height,but the interface communication effect is limited.Reasonable selection of layers with moderate lamina strength for fracturing stimulation,increasing the pump rate during fracturing and setting the cluster spacing reasonably are beneficial to improve the effect of reservoir stimulation.
基金The work is supported by the Sub-Project of“Research on Key Technologies and Equipment of Reservoir Stimulation”of China National Petroleum Corporation Post–14th Five-Year Plan Forward-Looking Major Science and Technology Project“Research on New Technology of Monitoring and Diagnosis of Horizontal Well Hydraulic Fracture Network Distribution Pattern”(2021DJ4502).
文摘Multistage multi-cluster hydraulic fracturing has enabled the economic exploitation of shale reservoirs,but the interpretation of hydraulic fracture parameters is challenging.The pressure signals after pump shutdown are influenced by hydraulic fractures,which can reflect the geometric features of hydraulic fracture.The shutdown pressure can be used to interpret the hydraulic fracture parameters in a real-time and cost-effective manner.In this paper,a mathematical model for shutdown pressure evolution is developed considering the effects of wellbore friction,perforation friction and fluid loss in fractures.An efficient numerical simulation method is established by using the method of characteristics.Based on this method,the impacts of fracture half-length,fracture height,opened cluster and perforation number,and filtration coefficient on the evolution of shutdown pressure are analyzed.The results indicate that a larger fracture half-length may hasten the decay of shutdown pressure,while a larger fracture height can slow down the decay of shutdown pressure.A smaller number of opened clusters and perforations can significantly increase the perforation friction and decrease the overall level of shutdown pressure.A larger filtration coefficient may accelerate the fluid filtration in the fracture and hasten the drop of the shutdown pressure.The simulation method of shutdown pressure,as well as the analysis results,has important implications for the interpretation of hydraulic fracture parameters.
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 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 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 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(51974332).
文摘This study conducted temporary plugging and diversion fracturing(TPDF)experiments using a true triaxial fracturing simulation system within a laboratory setting that replicated a lab-based horizontal well completion with multi-cluster sand jetting perforation.The effects of temporary plugging agent(TPA)particle size,TPA concentration,single-cluster perforation number and cluster number on plugging pressure,multi-fracture diversion pattern and distribution of TPAs were investigated.A combination of TPAs with small particle sizes within the fracture and large particle sizes within the segment is conducive to increasing the plugging pressure and promoting the diversion of multi-fractures.The addition of fibers can quickly achieve ultra-high pressure,but it may lead to longitudinal fractures extending along the wellbore.The temporary plugging peak pressure increases with an increase in the concentration of the TPA,reaching a peak at a certain concentration,and further increases do not significantly improve the temporary plugging peak pressure.The breaking pressure and temporary plugging peak pressure show a decreasing trend with an increase in single-cluster perforation number.A lower number of single-cluster perforations is beneficial for increasing the breaking pressure and temporary plugging peak pressure,and it has a more significant control on the propagation of multi-cluster fractures.A lower number of clusters is not conducive to increasing the total number and complexity of artificial fractures,while a higher number of clusters makes it difficult to achieve effective plugging.The TPAs within the fracture is mainly concentrated in the complex fracture areas,especially at the intersections of fractures.Meanwhile,the TPAs within the segment are primarily distributed near the perforation cluster apertures which initiated complex fractures.
基金financially supported by,the Fundamental Research Funds for the Central Universities(Grant No.2023QN1064)the China Postdoctoral Science Foundation(Grant No.2023M733772)Jiangsu Funding Program for Excellent Postdoctoral Talent(Grant No.2023ZB847)。
文摘Prepulse combined hydraulic fracturing facilitates the development of fracture networks by integrating prepulse hydraulic loading with conventional hydraulic fracturing.The formation mechanisms of fracture networks between hydraulic and pre-existing fractures under different prepulse loading parameters remain unclear.This research investigates the impact of prepulse loading parameters,including the prepulse loading number ratio(C),prepulse loading stress ratio(S),and prepulse loading frequency(f),on the formation of fracture networks between hydraulic and pre-existing fractures,using both experimental and numerical methods.The results suggest that low prepulse loading stress ratios and high prepulse loading number ratios are advantageous loading modes.Multiple hydraulic fractures are generated in the specimen under the advantageous loading modes,facilitating the development of a complex fracture network.Fatigue damage occurs in the specimen at the prepulse loading stage.The high water pressure at the secondary conventional hydraulic fracturing promotes the growth of hydraulic fractures along the damage zones.This allows the hydraulic fractures to propagate deeply and interact with pre-existing fractures.Under advantageous loading conditions,multiple hydraulic fractures can extend to pre-existing fractures,and these hydraulic fractures penetrate or propagate along pre-existing fractures.Especially when the approach angle is large,the damage range in the specimen during the prepulse loading stage increases,resulting in the formation of more hydraulic fractures.
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.
基金supported by Yunlong Lake Laboratory of Deep Underground Science and Engineering Project(No.104024008)the National Natural Science Foundation of China(Nos.52274241 and 52474261)the Natural Science Foundation of Jiangsu Province(No.BK20240207).
文摘Through a case analysis,this study examines the spatiotemporal evolution of microseismic(MS)events,energy characteristics,volumetric features,and fracture network development in surface well hydraulic fracturing.A total of 349 MS events were analyzed across different fracturing sections,revealing significant heterogeneity in fracture propagation.Energy scanning results showed that cumulative energy values ranged from 240 to 1060 J across the sections,indicating notable differences.Stimulated reservoir volume(SRV)analysis demonstrated well-developed fracture networks in certain sections,with a total SRV exceeding 1540000 m^(3).The hydraulic fracture network analysis revealed that during the midfracturing stage,the density and spatial extent of MS events significantly increased,indicating rapid fracture propagation and the formation of complex networks.In the later stage,the number of secondary fractures near fracture edges decreased,and the fracture network stabilized.By comparing the branching index,fracture length,width,height,and SRV values across different fracturing sections,Sections No.1 and No.8 showed the best performance,with high MS event densities,extensive fracture networks,and significant energy release.However,Sections No.4 and No.5 exhibited sparse MS activity and poor fracture connectivity,indicating suboptimal stimulation effectiveness.
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Australian Research Council Linkage Program(LP200301404)for sponsoring this researchthe financial support provided by the State Key Laboratory of Geohazard Prevention and Geoenvironment Protection(Chengdu University of Technology,SKLGP2021K002)National Natural Science Foundation of China(52374101,32111530138).
文摘Discrete fracture network(DFN)commonly existing in natural rock masses plays an important role in geological complexity which can influence rock fracturing behaviour during fluid injection.This paper simulated the hydraulic fracturing process in lab-scale coal samples with DFNs and the induced seismic activities by the discrete element method(DEM).The effects of DFNs on hydraulic fracturing,induced seismicity and elastic property changes have been concluded.Denser DFNs can comprehensively decrease the peak injection pressure and injection duration.The proportion of strong seismic events increases first and then decreases with increasing DFN density.In addition,the relative modulus of the rock mass is derived innovatively from breakdown pressure,breakdown fracture length and the related initiation time.Increasing DFN densities among large(35–60 degrees)and small(0–30 degrees)fracture dip angles show opposite evolution trends in relative modulus.The transitional point(dip angle)for the opposite trends is also proportionally affected by the friction angle of the rock mass.The modelling results have much practical meaning to infer the density and geometry of pre-existing fractures and the elastic property of rock mass in the field,simply based on the hydraulic fracturing and induced seismicity monitoring data.
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funded by the National Natural Scientific Foundation of China(Nos.52304008,52404038,52474043)the China Postdoctoral Science Foundation(No.2023MD734223)+1 种基金the Key Laboratory of Well Stability and Fluid&Rock Mechanics in Oil and Gas Reservoir of Shaanxi Province(No.23JS047)the Youth Talent Lifting Program of Xi'an Science and Technology Association(No.959202413078)。
文摘Supercritical CO_(2)(SC-CO_(2))fracturing stands out a promising waterless stimulation technique in the development of unconventional resources.While numerous studies have delved into the inducedfracture mechanism of SC-CO_(2),the small scale of rock samples and synthetic materials used in many studies have limited a comprehensive understanding of fracture propagation in unconventional formations.In this study,cubic tight sandstone samples with dimensions of 300 mm were employed to conduct SC-CO_(2)fractu ring experiments under true-triaxial stre ss conditions.The spatial morphology and quantitative attributes of fracture induced by water and SC-CO_(2)fracturing were compared,while the impact of in-situ stress on fracture propagation was also investigated.The results indicate that the SCCO_(2)fracturing takes approximately ten times longer than water fracturing.Furthermore,under identical stress condition,the breakdown pressure(BP)for SC-CO_(2)fracturing is nearly 25%lower than that for water fracturing.A quantitative analysis of fracture morphology reveals that water fracturing typically produces relatively simple fracture pattern,with the primary fracture distribution predominantly controlled by bedding planes.In contrast,SC-CO_(2)fracturing results in a more complex fracture morphology.As the differential of horizontal principal stress increases,the BP for SC-CO_(2)fractured rock exhibits a downward trend,and the induced fracture morphology becomes more simplified.Moreover,the presence of abnormal in-situ stress leads to a further increase in the BP for SC-CO_(2)fracturing,simultaneously enhancing the development of a more conductive fracture network.These findings provide critical insights into the efficiency and behavior of SC-CO_(2)fracturing in comparison to traditional water-based fracturing,offering valuable implication for its potential applications in unconventional reservoirs.
