To evaluate the fracturing effect and dynamic change process after volume fracturing with vertical wells in low permeability oil reservoirs, an oil-water two-phase flow model and a well model are built. On this basis,...To evaluate the fracturing effect and dynamic change process after volume fracturing with vertical wells in low permeability oil reservoirs, an oil-water two-phase flow model and a well model are built. On this basis, an evaluation method of fracturing effect based on production data and fracturing fluid backflow data is established, and the method is used to analyze some field cases. The vicinity area of main fracture after fracturing is divided into different stimulated regions. The permeability and area of different regions are used to characterize the stimulation strength and scale of the fracture network. The conductivity of stimulated region is defined as the product of the permeability and area of the stimulated region. Through parameter sensitivity analysis, it is found that half-length of the fracture and the permeability of the core area mainly affect the flow law near the well, that is, the early stage of production;while matrix permeability mainly affects the flow law at the far end of the fracture. Taking a typical old well in Changqing Oilfield as an example, the fracturing effect and its changes after two rounds of volume fracturing in this well are evaluated. It is found that with the increase of production time after the first volume fracturing, the permeability and conductivity of stimulated area gradually decreased, and the fracturing effect gradually decreased until disappeared;after the second volume fracturing, the permeability and conductivity of stimulated area increased significantly again.展开更多
Horizontal well drilling and multi-stage hydraulic fracturing are key technologies for the development of shale gas reservoirs.Instantaneous acquisition of hydraulic fracture parameters is crucial for evaluating fract...Horizontal well drilling and multi-stage hydraulic fracturing are key technologies for the development of shale gas reservoirs.Instantaneous acquisition of hydraulic fracture parameters is crucial for evaluating fracturing effectiveness,optimizing processes,and predicting gas productivity.This paper establishes a transient flow model for shale gas wells based on the boundary element method,achieving the characterization of stimulated reservoir volume for a single stage.By integrating pressure monitoring data following the pumping shut-in period of hydraulic fracturing for well testing interpretation,a workflow for inverting fracture parameters of shale gas wells is established.This new method eliminates the need for prolonged production testing and can interpret parameters of individual hydraulic fracture segments,offering significant advantages over the conventional pressure transient analysismethod.The practical application of thismethodology was conducted on 10 shale gaswellswithin the Changning shale gas block of Sichuan,China.The results show a high correlation between the interpreted single-stage total length and surface area of hydraulic fractures and the outcomes of gas production profile tests.Additionally,significant correlations are observed between these parameters and cluster number,horizontal stress difference,and natural fracture density.This demonstrates the effectiveness of the proposed fracture parameter inversion method and the feasibility of field application.The findings of this study aim to provide solutions and references for the inversion of fracture parameters in shale gas wells.展开更多
For the purpose of improving the accuracy and effectiveness of acid fracturing design for carbonate reservoirs with developed natural fractures,an acidizingfluidflow and reaction model taking the multiple leak-off effec...For the purpose of improving the accuracy and effectiveness of acid fracturing design for carbonate reservoirs with developed natural fractures,an acidizingfluidflow and reaction model taking the multiple leak-off effect of natural fracture,wormhole and matrix into account was established according to the liquid phase reaction equilibrium principle and the local reaction equilibrium principle after the dynamic change of fracture geometry in the process of fracture created by prepadfluid was simulated in the classical pseudo-three dimensional mathematical model of fracture propagation.Then,the acid fracturing stimulation of a case well on site was taken as an example.The newly developed model was used to simulate thefiltration process of acidizingfluid in fractures and the dynamic etching morphology of acidic rocks during the acidizingfluid injection of prepad acid fracturing.The effective length of etched fractures was determined by analyzing the concentration change of acidizingfluid along the direction of hydraulic fracture length and the threshold concentration of residual acidizingfluid comprehensively,and then it was compared with the interpretation result of pressure buildup test.And the following research results were obtained.First,in the process of acid fracturing in fracturedeporous reservoirs,the acidizingfluidfiltration velocity along the direction of fracture length is not constant and thefiltration velocity curvefluctuates in a serrated shape.And the acidizingfluidfiltration velocity where etched wormholes meet natural fractures is commonly higher than that in matrix.Second,acidizingfluid is lost seriously and the effective distance of acidizingfluid gets short significantly during the acid fracturing of fracturedeporous reservoirs.Third,acid fracturing in the verification well is remarkable in blockage removing and stimulation,and its well test interpretation results are consistent with the simulation interpretation results provided by the newly developed mathematical model.It is indicated that this newly developed model is reliable.In conclusion,the mathematical model of prepad acid fracturing which considers multiple leak-off effect is more suitable for acid fracturing simulation of fracturedeporous reservoirs.展开更多
Fracture effectiveness plays a key role in gas productivity of ultra-deep tight sandstone reservoirs,Kuqa depression,Tarim Basin.Based on cores,thin sections,well logging,well testing and production data,the study eva...Fracture effectiveness plays a key role in gas productivity of ultra-deep tight sandstone reservoirs,Kuqa depression,Tarim Basin.Based on cores,thin sections,well logging,well testing and production data,the study evaluated fracture effectiveness and illustrated its impacts on gas productivity.High-angle and vertical shear fractures are the most important types.Distribution of effective fractures shows great heterogeneous.Fracture effectiveness is influenced by tectonism,diagenesis and in-situ stress.Earlier fractures or fractures in close to gypsum rock are easier to be filled.Completely filled fractures can be reopened under late tectonism.Dissolution improves local fracture effectiveness.Minerals spanning fracture surfaces protect fracture effectiveness from late compression.Fractures filled with calcite can be activated by acidification.Effective fractures parallel to maximum horizontal principal compressive stress direction show larger aperture.Overpressure can decrease the effective normal stress to maintain fracture effectiveness.With exploitation,decline in pore pressure reduces fracture effectiveness.Linear density,aperture,and strike of effective fractures influence gas productivity.Effective fractures greatly enhance matrix permeability.Therefore,more abundant and larger aperture fractures are always corresponded to higher productivity.However,effective fractures also facilitate late water invasion,especially,both mutually parallel.Intense water invasion leads to rapidly declines in productivity.展开更多
Hydraulic fracturing and permeability enhancement are effective methods to improve low-permeability coal seams.However,few studies focused on methods to increase permeability,and there are no suitable prediction metho...Hydraulic fracturing and permeability enhancement are effective methods to improve low-permeability coal seams.However,few studies focused on methods to increase permeability,and there are no suitable prediction methods for engineering applications.In this work,PFC2D software was used to simulate coal seam hydraulic fracturing.The results were used in a coupled mathematical model of the interaction between coal seam deformation and gas flow.The results show that the displacement and velocity of particles increase in the direction of minimum principal stress,and the cracks propagate in the direction of maximum principal stress.The gas pressure drop rate and permeability increase rate of the fracture model are higher than that of the non-fracture model.Both parameters decrease rapidly with an increase in the drainage time and approach 0.The longer the hydraulic fracturing time,the more complex the fracture network is,and the faster the gas pressure drops.However,the impact of fracturing on the gas drainage effect declines over time.As the fracturing time increases,the difference between the horizontal and vertical permeability increases.However,this difference decreases as the gas drainage time increases.The higher the initial void pressure,the faster the gas pressure drops,and the greater the permeability increase is.However,the influence of the initial void pressure on the permeability declines over time.The research results provide guidance for predicting the anti-reflection effect of hydraulic fracturing in underground coal mines.展开更多
Accurate diagnosis of fracture geometry and conductivity is of great challenge due to the complex morphology of volumetric fracture network. In this study, a DNN (deep neural network) model was proposed to predict fra...Accurate diagnosis of fracture geometry and conductivity is of great challenge due to the complex morphology of volumetric fracture network. In this study, a DNN (deep neural network) model was proposed to predict fracture parameters for the evaluation of the fracturing effects. Field experience and the law of fracture volume conservation were incorporated as physical constraints to improve the prediction accuracy due to small amount of data. A combined neural network was adopted to input both static geological and dynamic fracturing data. The structure of the DNN was optimized and the model was validated through k-fold cross-validation. Results indicate that this DNN model is capable of predicting the fracture parameters accurately with a low relative error of under 10% and good generalization ability. The adoptions of the combined neural network, physical constraints, and k-fold cross-validation improve the model performance. Specifically, the root-mean-square error (RMSE) of the model decreases by 71.9% and 56% respectively with the combined neural network as the input model and the consideration of physical constraints. The mean square error (MRE) of fracture parameters reduces by 75% because the k-fold cross-validation improves the rationality of data set dividing. The model based on the DNN with physical constraints proposed in this study provides foundations for the optimization of fracturing design and improves the efficiency of fracture diagnosis in tight oil and gas reservoirs.展开更多
The fracture volume is gradually changed with the depletion of fracture pressure during the production process.However,there are few flowback models available so far that can estimate the fracture volume loss using pr...The fracture volume is gradually changed with the depletion of fracture pressure during the production process.However,there are few flowback models available so far that can estimate the fracture volume loss using pressure transient and rate transient data.The initial flowback involves producing back the fracturing fuid after hydraulic fracturing,while the second flowback involves producing back the preloading fluid injected into the parent wells before fracturing of child wells.The main objective of this research is to compare the initial and second flowback data to capture the changes in fracture volume after production and preload processes.Such a comparison is useful for evaluating well performance and optimizing frac-turing operations.We construct rate-normalized pressure(RNP)versus material balance time(MBT)diagnostic plots using both initial and second flowback data(FB;and FBs,respectively)of six multi-fractured horizontal wells completed in Niobrara and Codell formations in DJ Basin.In general,the slope of RNP plot during the FB,period is higher than that during the FB;period,indicating a potential loss of fracture volume from the FB;to the FB,period.We estimate the changes in effective fracture volume(Ver)by analyzing the changes in the RNP slope and total compressibility between these two flowback periods.Ver during FB,is in general 3%-45%lower than that during FB:.We also compare the drive mechanisms for the two flowback periods by calculating the compaction-drive index(CDI),hydrocarbon-drive index(HDI),and water-drive index(WDI).The dominant drive mechanism during both flowback periods is CDI,but its contribution is reduced by 16%in the FB,period.This drop is generally compensated by a relatively higher HDI during this period.The loss of effective fracture volume might be attributed to the pressure depletion in fractures,which occurs during the production period and can extend 800 days.展开更多
Affected by reservoir heterogeneity,developed natural fractures,and bedding fractures,the fracturing pressure curves in fracturing of shale gas horizontal wells present complex shapes.A large amount of information con...Affected by reservoir heterogeneity,developed natural fractures,and bedding fractures,the fracturing pressure curves in fracturing of shale gas horizontal wells present complex shapes.A large amount of information contained in the fracturing curves is still not fully excavated.Based on the theory of shale gas fracture network fracturing,the calculation model of bottom hole net pressure is established by integrating the real-time data such as casing pressure,pump rate,and proppant concentration.Net pressure slope and net pressure index are constructed as key parameters,and the net pressure curve is divided dynamically to describe the mechanical conditions corresponding to the fracture propagation behavior during the fracturing process.Six fracture propagation modes were identified,including fracture network propagation,fracture propagation blockage,normal fracture propagation,fracture propagation long bedding,fracture height growth,and rapidfluidfiltration,and then the operation pressure curve diagnosis and identification method were formed for shale gas fracture network fracturing in horizontal wells.The shortcomings of conventional operation curve diagnosis and identification methods are abandoned and the fracture network complexity index is presented.The higher index indicates more time of fracture network propagation and fracture propagation along bedding and the better reservoir stimulation effect.The model is applied to shale gas wells in the southeastern margin of Sichuan Basin,and the average fracture network complexity index of a single well is 0.3,which is in good agreement with the microseismic monitoring results.This proves the good reliability of the method developed.The method is helpful to improve the potential and level of fracturing stimulation of shale reservoirs and is of great significance for improving the post-fracturing evaluation technology of fracture network and guiding the real-time dynamic adjustment offield fracturing operations.展开更多
Fluid and effective fracture identification in reservoirs is a crucial part of reservoir prediction.The frequency-dependent AVO inversion algorithms have proven to be effective for identifying fluid through its disper...Fluid and effective fracture identification in reservoirs is a crucial part of reservoir prediction.The frequency-dependent AVO inversion algorithms have proven to be effective for identifying fluid through its dispersion property.However,the conventional frequency-dependent AVO inversion algorithms based on Smith&Gidlow and Aki&Richards approximations do not consider the acquisition azimuth of seismic data and neglect the effect of seismic anisotropic dispersion in the actual medium.The aligned fractures in the subsurface medium induce anisotropy.The seismic anisotropy should be considered while accounting for the seismic dispersion properties through fluid-saturated fractured reservoirs.Anisotropy in such reservoirs is frequency-related due to wave-induced fluid-flow(WIFF)between interconnected fractures and pores.It can be used to identify fluid and effective fractures(fluid-saturated)by using azimuthal seismic data via anisotropic dispersion properties.In this paper,based on Rüger’s equation,we derived an analytical expression in the frequency domain for the frequencydependent AVOAz inversion in terms of fracture orientation,dispersion gradient of isotropic background rock,anisotropic dispersion gradient,and the dispersion at a normal incident angle.The frequency-dependent AVOAz equation utilizes azimuthal seismic data and considers the effect of both isotropic and anisotropic dispersion.Reassigned Gabor Transform(RGT)is used to achieve highresolution frequency division data.We then propose the frequency-dependent AVOAz inversion method to identify fluid and characterize effective fractures in fractured porous reservoirs.Through application to high-qualified seismic data of dolomite and carbonate reservoirs,the results show that the method is useful for identifying fluid and effective fractures in fluid-saturated fractured rocks.展开更多
Microscopic seepage characteristics are critical for the evaluation of tight sandstone reservoirs.In this study,a digital core approach integrating microscopic seepage simulation and CT scanning was developed to chara...Microscopic seepage characteristics are critical for the evaluation of tight sandstone reservoirs.In this study,a digital core approach integrating microscopic seepage simulation and CT scanning was developed to characterize microscopic seepage and fracture effectiveness(the ratio of micro-fractures that contributes to fluid flow)of tight sandstones.Numerical simulations were carried out for characterizations of tight sandstones.The results show that the axial permeability of the investigated cylindrical tight sandstone from Junggar Basin in China is 0.460μm~2,while the radial permeability is 0.3723μm~2,and the axial and radial effective fracture ratios are 0.4387 and 0.4806,respectively,indicating that cracks are not fully developed and the connectivity between micro-cracks is poor.Directional permeability that is difficult to measure by laboratory experiments can be obtained readily using the proposed method in this paper.The results provide important information for improving the exploration and development of tight sandstone reservoirs.展开更多
Deep shale gas reservoirs buried underground with depth being more than 3500 m are characterized by high in-situ stress,large horizontal stress difference,complex distribution of bedding and natural cracks,and strong ...Deep shale gas reservoirs buried underground with depth being more than 3500 m are characterized by high in-situ stress,large horizontal stress difference,complex distribution of bedding and natural cracks,and strong rock plasticity.Thus,during hydraulic fracturing,these reservoirs often reveal difficult fracture extension,low fracture complexity,low stimulated reservoir volume(SRV),low conductivity and fast decline,which hinder greatly the economic and effective development of deep shale gas.In this paper,a specific and feasible technique of volume fracturing of deep shale gas horizontal wells is presented.In addition to planar perforation,multi-scale fracturing,full-scale fracture filling,and control over extension of high-angle natural fractures,some supporting techniques are proposed,including multi-stage alternate injection(of acid fluid,slick water and gel)and the mixed-and small-grained proppant to be injected with variable viscosity and displacement.These techniques help to increase the effective stimulated reservoir volume(ESRV)for deep gas production.Some of the techniques have been successfully used in the fracturing of deep shale gas horizontal wells in Yongchuan,Weiyuan and southern Jiaoshiba blocks in the Sichuan Basin.As a result,Wells YY1HF and WY1HF yielded initially 14.1×10^(4)m^(3)/d and 17.5×10^(4)m^(3)/d after fracturing.The volume fracturing of deep shale gas horizontal well is meaningful in achieving the productivity of 50×108 m^(3)gas from the interval of 3500e4000 m in Phase II development of Fuling and also in commercial production of huge shale gas resources at a vertical depth of less than 6000 m.展开更多
The strengthening effect of a Zn alloy reinforced by SiC particulate was examined. Based on the results of SEM in-situ fracture observation and stress field analysis by finite element method, it is believed that the r...The strengthening effect of a Zn alloy reinforced by SiC particulate was examined. Based on the results of SEM in-situ fracture observation and stress field analysis by finite element method, it is believed that the reinforcing effect of this composite is due to the combination of strain and stress hardening in the matrix.展开更多
Objective: to analyze the results of the treatment of spinal fractures with subcutaneous micro screws + trauma screws. Methods: from September 2018 to April 2021, a total of 60 cases of spinal fractures in our hospita...Objective: to analyze the results of the treatment of spinal fractures with subcutaneous micro screws + trauma screws. Methods: from September 2018 to April 2021, a total of 60 cases of spinal fractures in our hospital were selected. The numerical table was randomly divided into two groups with 30 cases in each group. The patients in the conventional group were given conventional surgical treatment, while the patients in the micro group were given minimally invasive percutaneous vertebral screw + traumatic nail. Before and after treatment, the hump angle of the back, the height of the vertebral front, the visual shape of glasses and the frequency of complications were compared between the two groups. Results: after treatment, the spine lobe, the height of the anterior border of the spine, the visual model of the micro-group were lower than those of the conventional group, and the incidence of complications of the micro-group were lower than those of the conventional group (n < 0.05). Conclusion: the accurate clinical effect of micro-skin spinal screw and spinal trauma in spinal fracture can improve the prognosis and reduce complications.展开更多
As flow environment is poor in low permeability reservoirs, wells are always fractured in order to gain better economic benefits. Well testing analysis is very necessary for fracturing wells. However, available test a...As flow environment is poor in low permeability reservoirs, wells are always fractured in order to gain better economic benefits. Well testing analysis is very necessary for fracturing wells. However, available test analysis methods are of slow fitting speed and low fitting precision. In this paper, we first use a comprehensive evaluation method of analytical well testing, numerical well testing and well testing design. Many dynamic parameters such as fracture length, fracture conductivity, skin factor, etc are obtained. An example to illustrate accurate results of this method is given.展开更多
Based on the laboratory experiments this paper presented that the primary influence factors about the electromagnetic radiation during rock fracture are the rock mechanics characters and mineral components. The brittl...Based on the laboratory experiments this paper presented that the primary influence factors about the electromagnetic radiation during rock fracture are the rock mechanics characters and mineral components. The brittle samples and samples contained quartz, pyrite, chalopyrite produce electromagnetic radiation easily. There are three fracture radiation effects. The crystal fracture effect produces the high frequency electromagnetic signals, the piezoelectric effect produces low frequency signals and the natural semiconductor effect produces middle frequency signals possessed distinct wave shapes.展开更多
An elusive phenomenon is observed in previous investigations on dynamic fracture that the dynamic fracture toughness （DFT） of high strength metals always increases with the loading rate on the order of TPa.m1/2.s-1....An elusive phenomenon is observed in previous investigations on dynamic fracture that the dynamic fracture toughness （DFT） of high strength metals always increases with the loading rate on the order of TPa.m1/2.s-1. For the purpose of verification, variation of DFT with the loading rate for two high strength steels commonly used in the aviation industry, 30CrMnSiA and 40Cr, is studied in this work. Results of the experiments are compared, which were conducted on the modified split Hopkinson pressure bar （SHPB） apparatus, with striker velocities ranging from 9.2 to 24.1 m/s and a constant value of 16.3 m/s for 30CrMnSiA and 40Cr, respectively. It is observed that for 30CrMnSiA, the crack tip loading rate increases with the increase of the striker velocity, while the fracture initiation time and the DFT simultaneously decrease. However, in the tests of 40Cr, there is also an increasing tendency of DFT, similar to other reports. Through an in-depth investigation on the relationship between the dynamic stress intensity factor （DSIF） and the loading rate, it is concluded that the generally increasing tendency in previous studies could be false, which is induced from a limited striker velocity domain and the errors existing in the experimental and numerical processes. To disclose the real dependency of DFT on the loading rate, experimentsneed to be performed in a comparatively large striker velocity range.展开更多
The shear responses of β-SiC are investigated using molecular dynamics simulation with the Tersoff interatomic potential. Results show a clear decreasing trend in critical stress,fracture strain and shear modulus as ...The shear responses of β-SiC are investigated using molecular dynamics simulation with the Tersoff interatomic potential. Results show a clear decreasing trend in critical stress,fracture strain and shear modulus as temperature increases. Above a critical temperature, β-SiC bulk just fractures after the elastic deformation. However, below the critical temperature, an interesting pattern in β-SiC bulk emerges due to the elongation of Si-C bonds before fracture. Additionally, the shear deformation of β-SiC at room temperature is found to be dependent on the strain rate. This study may shed light on the deformation mechanism dependent on temperature and strain rate.展开更多
The ultra-low porosity and permeability,as well as complex occurrence and transport state of shale reservoir make it possess special L-type production characteristic curve and complicated shale gas flow mechanism.To s...The ultra-low porosity and permeability,as well as complex occurrence and transport state of shale reservoir make it possess special L-type production characteristic curve and complicated shale gas flow mechanism.To solve the difficulty of collecting complete production data due to short production time and operation discontinuity,a full-diameter core physical simulation experiment on the full lifecycle production process of shale gas well depletion is conducted with the purpose of obtaining many important production data including complete pressure and daily gas output in the simulated production process of shale gas well.The experimental results show the production characteristic from simulation is consistent with those from gas well.Based on the simulation data,the critical desorption pressure(12 MPa)of core,free gas production(3820.8 mL),adsorbed gas production(2151.2 mL),the proportion of the daily gas production between free and absorbed gas under different time and formation pressure,as well as the production time and final recovery rate corresponding to abandoned pressure,can be determined accurately.Numerical inversion is carried out to calculate the production performance curve of shale gas well and predict the development effect of gas well based on well testing and similarity analysis of the dimensionless time between core experiment and gas well production.Finally,the permeability and the fracturing effect(fracture network density)as the keys to the effective development of shale gas reservoirs are proposed.The permeability is the fundamental factor and the fracturing technology is the major means.展开更多
The CO_2 permeability of fractured coal is of great significance to both coalbed gas extraction and CO_2 storage in coal seams, but the effects of high confining pressure, high injection pressure and elevated temperat...The CO_2 permeability of fractured coal is of great significance to both coalbed gas extraction and CO_2 storage in coal seams, but the effects of high confining pressure, high injection pressure and elevated temperature on the CO_2 permeability of fractured coal with different fracture extents have not been investigated thoroughly. In this paper, the CO_2 permeability of fractured coals sampled from a Pingdingshan coal mine in China and artificially fractured to a certain extent is investigated through undrained triaxial tests. The CO_2 permeability is measured under the confining pressure with a range of 10–25 MPa, injection pressure with a range of 6–12 MPa and elevated temperature with a range of 25–70°C. A mechanistic model is then proposed to characterize the CO_2 permeability of the fractured coals. The effects of thermal expansion, temperature-induced reduction of adsorption capacity, and thermal micro-cracking on the CO_2 permeability are explored. The test results show that the CO_2 permeability of naturally fractured coal saliently increases with increasing injection pressure. The increase of confining pressure reduces the permeability of both naturally fractured coal and secondarily fractured coal. It is also observed that initial fracturing by external loads can enhance the permeability, but further fracturing reduces the permeability. The CO_2 permeability decreases with the elevation of temperature if the temperature is lower than 44°C, but the permeability increases with temperature once the temperature is beyond 44°C. The mechanistic model well describes these compaction mechanisms induced by confining pressure, injection pressure and the complex effects induced by elevated temperature.展开更多
基金Supported by the China National Science and Technology Major Project (2017ZX05013-001)CNPC Science and Technology Major Research Project (2018B-4907)
文摘To evaluate the fracturing effect and dynamic change process after volume fracturing with vertical wells in low permeability oil reservoirs, an oil-water two-phase flow model and a well model are built. On this basis, an evaluation method of fracturing effect based on production data and fracturing fluid backflow data is established, and the method is used to analyze some field cases. The vicinity area of main fracture after fracturing is divided into different stimulated regions. The permeability and area of different regions are used to characterize the stimulation strength and scale of the fracture network. The conductivity of stimulated region is defined as the product of the permeability and area of the stimulated region. Through parameter sensitivity analysis, it is found that half-length of the fracture and the permeability of the core area mainly affect the flow law near the well, that is, the early stage of production;while matrix permeability mainly affects the flow law at the far end of the fracture. Taking a typical old well in Changqing Oilfield as an example, the fracturing effect and its changes after two rounds of volume fracturing in this well are evaluated. It is found that with the increase of production time after the first volume fracturing, the permeability and conductivity of stimulated area gradually decreased, and the fracturing effect gradually decreased until disappeared;after the second volume fracturing, the permeability and conductivity of stimulated area increased significantly again.
基金funded by the Science and Technology Cooperation Project of the CNPC-SWPU Innovation Alliance,grant numbers“2020CX020202,2020CX030202 and 2020CX010403”.
文摘Horizontal well drilling and multi-stage hydraulic fracturing are key technologies for the development of shale gas reservoirs.Instantaneous acquisition of hydraulic fracture parameters is crucial for evaluating fracturing effectiveness,optimizing processes,and predicting gas productivity.This paper establishes a transient flow model for shale gas wells based on the boundary element method,achieving the characterization of stimulated reservoir volume for a single stage.By integrating pressure monitoring data following the pumping shut-in period of hydraulic fracturing for well testing interpretation,a workflow for inverting fracture parameters of shale gas wells is established.This new method eliminates the need for prolonged production testing and can interpret parameters of individual hydraulic fracture segments,offering significant advantages over the conventional pressure transient analysismethod.The practical application of thismethodology was conducted on 10 shale gaswellswithin the Changning shale gas block of Sichuan,China.The results show a high correlation between the interpreted single-stage total length and surface area of hydraulic fractures and the outcomes of gas production profile tests.Additionally,significant correlations are observed between these parameters and cluster number,horizontal stress difference,and natural fracture density.This demonstrates the effectiveness of the proposed fracture parameter inversion method and the feasibility of field application.The findings of this study aim to provide solutions and references for the inversion of fracture parameters in shale gas wells.
基金supported by the National Major Science and Technology Project“Demonstrative development project of large carbonate gas field in Sichuan Basin”(No.:2016ZX05052).
文摘For the purpose of improving the accuracy and effectiveness of acid fracturing design for carbonate reservoirs with developed natural fractures,an acidizingfluidflow and reaction model taking the multiple leak-off effect of natural fracture,wormhole and matrix into account was established according to the liquid phase reaction equilibrium principle and the local reaction equilibrium principle after the dynamic change of fracture geometry in the process of fracture created by prepadfluid was simulated in the classical pseudo-three dimensional mathematical model of fracture propagation.Then,the acid fracturing stimulation of a case well on site was taken as an example.The newly developed model was used to simulate thefiltration process of acidizingfluid in fractures and the dynamic etching morphology of acidic rocks during the acidizingfluid injection of prepad acid fracturing.The effective length of etched fractures was determined by analyzing the concentration change of acidizingfluid along the direction of hydraulic fracture length and the threshold concentration of residual acidizingfluid comprehensively,and then it was compared with the interpretation result of pressure buildup test.And the following research results were obtained.First,in the process of acid fracturing in fracturedeporous reservoirs,the acidizingfluidfiltration velocity along the direction of fracture length is not constant and thefiltration velocity curvefluctuates in a serrated shape.And the acidizingfluidfiltration velocity where etched wormholes meet natural fractures is commonly higher than that in matrix.Second,acidizingfluid is lost seriously and the effective distance of acidizingfluid gets short significantly during the acid fracturing of fracturedeporous reservoirs.Third,acid fracturing in the verification well is remarkable in blockage removing and stimulation,and its well test interpretation results are consistent with the simulation interpretation results provided by the newly developed mathematical model.It is indicated that this newly developed model is reliable.In conclusion,the mathematical model of prepad acid fracturing which considers multiple leak-off effect is more suitable for acid fracturing simulation of fracturedeporous reservoirs.
基金supported by the National Natural Science Foundation of China(No.U21B2062)。
文摘Fracture effectiveness plays a key role in gas productivity of ultra-deep tight sandstone reservoirs,Kuqa depression,Tarim Basin.Based on cores,thin sections,well logging,well testing and production data,the study evaluated fracture effectiveness and illustrated its impacts on gas productivity.High-angle and vertical shear fractures are the most important types.Distribution of effective fractures shows great heterogeneous.Fracture effectiveness is influenced by tectonism,diagenesis and in-situ stress.Earlier fractures or fractures in close to gypsum rock are easier to be filled.Completely filled fractures can be reopened under late tectonism.Dissolution improves local fracture effectiveness.Minerals spanning fracture surfaces protect fracture effectiveness from late compression.Fractures filled with calcite can be activated by acidification.Effective fractures parallel to maximum horizontal principal compressive stress direction show larger aperture.Overpressure can decrease the effective normal stress to maintain fracture effectiveness.With exploitation,decline in pore pressure reduces fracture effectiveness.Linear density,aperture,and strike of effective fractures influence gas productivity.Effective fractures greatly enhance matrix permeability.Therefore,more abundant and larger aperture fractures are always corresponded to higher productivity.However,effective fractures also facilitate late water invasion,especially,both mutually parallel.Intense water invasion leads to rapidly declines in productivity.
基金This work was supported by National Natural Science Foundation of China(52130409,52121003,52004291,51874314).
文摘Hydraulic fracturing and permeability enhancement are effective methods to improve low-permeability coal seams.However,few studies focused on methods to increase permeability,and there are no suitable prediction methods for engineering applications.In this work,PFC2D software was used to simulate coal seam hydraulic fracturing.The results were used in a coupled mathematical model of the interaction between coal seam deformation and gas flow.The results show that the displacement and velocity of particles increase in the direction of minimum principal stress,and the cracks propagate in the direction of maximum principal stress.The gas pressure drop rate and permeability increase rate of the fracture model are higher than that of the non-fracture model.Both parameters decrease rapidly with an increase in the drainage time and approach 0.The longer the hydraulic fracturing time,the more complex the fracture network is,and the faster the gas pressure drops.However,the impact of fracturing on the gas drainage effect declines over time.As the fracturing time increases,the difference between the horizontal and vertical permeability increases.However,this difference decreases as the gas drainage time increases.The higher the initial void pressure,the faster the gas pressure drops,and the greater the permeability increase is.However,the influence of the initial void pressure on the permeability declines over time.The research results provide guidance for predicting the anti-reflection effect of hydraulic fracturing in underground coal mines.
基金supported by the National Natural Science Foundation of China(Grant No.52174044,52004302)Science Foundation of China University of Petroleum,Beijing(No.ZX20200134,2462021YXZZ012)the Strategic Cooperation Technology Projects of CNPC and CUPB(ZLZX 2020-01-07).
文摘Accurate diagnosis of fracture geometry and conductivity is of great challenge due to the complex morphology of volumetric fracture network. In this study, a DNN (deep neural network) model was proposed to predict fracture parameters for the evaluation of the fracturing effects. Field experience and the law of fracture volume conservation were incorporated as physical constraints to improve the prediction accuracy due to small amount of data. A combined neural network was adopted to input both static geological and dynamic fracturing data. The structure of the DNN was optimized and the model was validated through k-fold cross-validation. Results indicate that this DNN model is capable of predicting the fracture parameters accurately with a low relative error of under 10% and good generalization ability. The adoptions of the combined neural network, physical constraints, and k-fold cross-validation improve the model performance. Specifically, the root-mean-square error (RMSE) of the model decreases by 71.9% and 56% respectively with the combined neural network as the input model and the consideration of physical constraints. The mean square error (MRE) of fracture parameters reduces by 75% because the k-fold cross-validation improves the rationality of data set dividing. The model based on the DNN with physical constraints proposed in this study provides foundations for the optimization of fracturing design and improves the efficiency of fracture diagnosis in tight oil and gas reservoirs.
文摘The fracture volume is gradually changed with the depletion of fracture pressure during the production process.However,there are few flowback models available so far that can estimate the fracture volume loss using pressure transient and rate transient data.The initial flowback involves producing back the fracturing fuid after hydraulic fracturing,while the second flowback involves producing back the preloading fluid injected into the parent wells before fracturing of child wells.The main objective of this research is to compare the initial and second flowback data to capture the changes in fracture volume after production and preload processes.Such a comparison is useful for evaluating well performance and optimizing frac-turing operations.We construct rate-normalized pressure(RNP)versus material balance time(MBT)diagnostic plots using both initial and second flowback data(FB;and FBs,respectively)of six multi-fractured horizontal wells completed in Niobrara and Codell formations in DJ Basin.In general,the slope of RNP plot during the FB,period is higher than that during the FB;period,indicating a potential loss of fracture volume from the FB;to the FB,period.We estimate the changes in effective fracture volume(Ver)by analyzing the changes in the RNP slope and total compressibility between these two flowback periods.Ver during FB,is in general 3%-45%lower than that during FB:.We also compare the drive mechanisms for the two flowback periods by calculating the compaction-drive index(CDI),hydrocarbon-drive index(HDI),and water-drive index(WDI).The dominant drive mechanism during both flowback periods is CDI,but its contribution is reduced by 16%in the FB,period.This drop is generally compensated by a relatively higher HDI during this period.The loss of effective fracture volume might be attributed to the pressure depletion in fractures,which occurs during the production period and can extend 800 days.
基金National Natural Science Foundation of China Basic The ory of Efficient Development of Shale Oil and Gas(No.51490653)Theory and Method of Efficient Construction of Fracture Network in Deep and Ultra-Deep Shale Gas Horizontal Wells(No.U19A2043)Theory and Method of Long-term Propping for Deep Shale Gas Hydraulic Fractures based on DEM-LBM Hydro-Mechanical Coupling(No.52104039).
文摘Affected by reservoir heterogeneity,developed natural fractures,and bedding fractures,the fracturing pressure curves in fracturing of shale gas horizontal wells present complex shapes.A large amount of information contained in the fracturing curves is still not fully excavated.Based on the theory of shale gas fracture network fracturing,the calculation model of bottom hole net pressure is established by integrating the real-time data such as casing pressure,pump rate,and proppant concentration.Net pressure slope and net pressure index are constructed as key parameters,and the net pressure curve is divided dynamically to describe the mechanical conditions corresponding to the fracture propagation behavior during the fracturing process.Six fracture propagation modes were identified,including fracture network propagation,fracture propagation blockage,normal fracture propagation,fracture propagation long bedding,fracture height growth,and rapidfluidfiltration,and then the operation pressure curve diagnosis and identification method were formed for shale gas fracture network fracturing in horizontal wells.The shortcomings of conventional operation curve diagnosis and identification methods are abandoned and the fracture network complexity index is presented.The higher index indicates more time of fracture network propagation and fracture propagation along bedding and the better reservoir stimulation effect.The model is applied to shale gas wells in the southeastern margin of Sichuan Basin,and the average fracture network complexity index of a single well is 0.3,which is in good agreement with the microseismic monitoring results.This proves the good reliability of the method developed.The method is helpful to improve the potential and level of fracturing stimulation of shale reservoirs and is of great significance for improving the post-fracturing evaluation technology of fracture network and guiding the real-time dynamic adjustment offield fracturing operations.
基金supported by the National Major Science and Technology Project of China(2016ZX05004003)the National Natural Science Foundation of China(41574103,41974120,U20B2015)Open Fund of State Key Laboratory of Coal Resources and Safe Mining(Grant No.SKLCRSM19KFA08)。
文摘Fluid and effective fracture identification in reservoirs is a crucial part of reservoir prediction.The frequency-dependent AVO inversion algorithms have proven to be effective for identifying fluid through its dispersion property.However,the conventional frequency-dependent AVO inversion algorithms based on Smith&Gidlow and Aki&Richards approximations do not consider the acquisition azimuth of seismic data and neglect the effect of seismic anisotropic dispersion in the actual medium.The aligned fractures in the subsurface medium induce anisotropy.The seismic anisotropy should be considered while accounting for the seismic dispersion properties through fluid-saturated fractured reservoirs.Anisotropy in such reservoirs is frequency-related due to wave-induced fluid-flow(WIFF)between interconnected fractures and pores.It can be used to identify fluid and effective fractures(fluid-saturated)by using azimuthal seismic data via anisotropic dispersion properties.In this paper,based on Rüger’s equation,we derived an analytical expression in the frequency domain for the frequencydependent AVOAz inversion in terms of fracture orientation,dispersion gradient of isotropic background rock,anisotropic dispersion gradient,and the dispersion at a normal incident angle.The frequency-dependent AVOAz equation utilizes azimuthal seismic data and considers the effect of both isotropic and anisotropic dispersion.Reassigned Gabor Transform(RGT)is used to achieve highresolution frequency division data.We then propose the frequency-dependent AVOAz inversion method to identify fluid and characterize effective fractures in fractured porous reservoirs.Through application to high-qualified seismic data of dolomite and carbonate reservoirs,the results show that the method is useful for identifying fluid and effective fractures in fluid-saturated fractured rocks.
基金financially supported by the National Natural Science Foundation of China(Grant No.41972138)the Technology Major Project of China(Grant No.ZD2019-183007,2016ZX05002-002)。
文摘Microscopic seepage characteristics are critical for the evaluation of tight sandstone reservoirs.In this study,a digital core approach integrating microscopic seepage simulation and CT scanning was developed to characterize microscopic seepage and fracture effectiveness(the ratio of micro-fractures that contributes to fluid flow)of tight sandstones.Numerical simulations were carried out for characterizations of tight sandstones.The results show that the axial permeability of the investigated cylindrical tight sandstone from Junggar Basin in China is 0.460μm~2,while the radial permeability is 0.3723μm~2,and the axial and radial effective fracture ratios are 0.4387 and 0.4806,respectively,indicating that cracks are not fully developed and the connectivity between micro-cracks is poor.Directional permeability that is difficult to measure by laboratory experiments can be obtained readily using the proposed method in this paper.The results provide important information for improving the exploration and development of tight sandstone reservoirs.
基金Supported by the Foundation of State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms Effective Development.
文摘Deep shale gas reservoirs buried underground with depth being more than 3500 m are characterized by high in-situ stress,large horizontal stress difference,complex distribution of bedding and natural cracks,and strong rock plasticity.Thus,during hydraulic fracturing,these reservoirs often reveal difficult fracture extension,low fracture complexity,low stimulated reservoir volume(SRV),low conductivity and fast decline,which hinder greatly the economic and effective development of deep shale gas.In this paper,a specific and feasible technique of volume fracturing of deep shale gas horizontal wells is presented.In addition to planar perforation,multi-scale fracturing,full-scale fracture filling,and control over extension of high-angle natural fractures,some supporting techniques are proposed,including multi-stage alternate injection(of acid fluid,slick water and gel)and the mixed-and small-grained proppant to be injected with variable viscosity and displacement.These techniques help to increase the effective stimulated reservoir volume(ESRV)for deep gas production.Some of the techniques have been successfully used in the fracturing of deep shale gas horizontal wells in Yongchuan,Weiyuan and southern Jiaoshiba blocks in the Sichuan Basin.As a result,Wells YY1HF and WY1HF yielded initially 14.1×10^(4)m^(3)/d and 17.5×10^(4)m^(3)/d after fracturing.The volume fracturing of deep shale gas horizontal well is meaningful in achieving the productivity of 50×108 m^(3)gas from the interval of 3500e4000 m in Phase II development of Fuling and also in commercial production of huge shale gas resources at a vertical depth of less than 6000 m.
基金This project is supported by the National Natural Science Foundation of China(No.59971017).
文摘The strengthening effect of a Zn alloy reinforced by SiC particulate was examined. Based on the results of SEM in-situ fracture observation and stress field analysis by finite element method, it is believed that the reinforcing effect of this composite is due to the combination of strain and stress hardening in the matrix.
文摘Objective: to analyze the results of the treatment of spinal fractures with subcutaneous micro screws + trauma screws. Methods: from September 2018 to April 2021, a total of 60 cases of spinal fractures in our hospital were selected. The numerical table was randomly divided into two groups with 30 cases in each group. The patients in the conventional group were given conventional surgical treatment, while the patients in the micro group were given minimally invasive percutaneous vertebral screw + traumatic nail. Before and after treatment, the hump angle of the back, the height of the vertebral front, the visual shape of glasses and the frequency of complications were compared between the two groups. Results: after treatment, the spine lobe, the height of the anterior border of the spine, the visual model of the micro-group were lower than those of the conventional group, and the incidence of complications of the micro-group were lower than those of the conventional group (n < 0.05). Conclusion: the accurate clinical effect of micro-skin spinal screw and spinal trauma in spinal fracture can improve the prognosis and reduce complications.
文摘As flow environment is poor in low permeability reservoirs, wells are always fractured in order to gain better economic benefits. Well testing analysis is very necessary for fracturing wells. However, available test analysis methods are of slow fitting speed and low fitting precision. In this paper, we first use a comprehensive evaluation method of analytical well testing, numerical well testing and well testing design. Many dynamic parameters such as fracture length, fracture conductivity, skin factor, etc are obtained. An example to illustrate accurate results of this method is given.
文摘Based on the laboratory experiments this paper presented that the primary influence factors about the electromagnetic radiation during rock fracture are the rock mechanics characters and mineral components. The brittle samples and samples contained quartz, pyrite, chalopyrite produce electromagnetic radiation easily. There are three fracture radiation effects. The crystal fracture effect produces the high frequency electromagnetic signals, the piezoelectric effect produces low frequency signals and the natural semiconductor effect produces middle frequency signals possessed distinct wave shapes.
基金supported by the 111 Project (B07050)the National Natural Science Foundation of China (10932008)
文摘An elusive phenomenon is observed in previous investigations on dynamic fracture that the dynamic fracture toughness （DFT） of high strength metals always increases with the loading rate on the order of TPa.m1/2.s-1. For the purpose of verification, variation of DFT with the loading rate for two high strength steels commonly used in the aviation industry, 30CrMnSiA and 40Cr, is studied in this work. Results of the experiments are compared, which were conducted on the modified split Hopkinson pressure bar （SHPB） apparatus, with striker velocities ranging from 9.2 to 24.1 m/s and a constant value of 16.3 m/s for 30CrMnSiA and 40Cr, respectively. It is observed that for 30CrMnSiA, the crack tip loading rate increases with the increase of the striker velocity, while the fracture initiation time and the DFT simultaneously decrease. However, in the tests of 40Cr, there is also an increasing tendency of DFT, similar to other reports. Through an in-depth investigation on the relationship between the dynamic stress intensity factor （DSIF） and the loading rate, it is concluded that the generally increasing tendency in previous studies could be false, which is induced from a limited striker velocity domain and the errors existing in the experimental and numerical processes. To disclose the real dependency of DFT on the loading rate, experimentsneed to be performed in a comparatively large striker velocity range.
基金supported by the National Natural Science Foundation of China(NSFC Grants No.11632014,11302161 and11302162)China Postdoctoral Science Foundation(Grant No.2013M542339)the Chang Jiang Scholar program
文摘The shear responses of β-SiC are investigated using molecular dynamics simulation with the Tersoff interatomic potential. Results show a clear decreasing trend in critical stress,fracture strain and shear modulus as temperature increases. Above a critical temperature, β-SiC bulk just fractures after the elastic deformation. However, below the critical temperature, an interesting pattern in β-SiC bulk emerges due to the elongation of Si-C bonds before fracture. Additionally, the shear deformation of β-SiC at room temperature is found to be dependent on the strain rate. This study may shed light on the deformation mechanism dependent on temperature and strain rate.
基金The work was supported by the National Science and Technology Major Project(2016ZX05062,2017ZX05037-001).
文摘The ultra-low porosity and permeability,as well as complex occurrence and transport state of shale reservoir make it possess special L-type production characteristic curve and complicated shale gas flow mechanism.To solve the difficulty of collecting complete production data due to short production time and operation discontinuity,a full-diameter core physical simulation experiment on the full lifecycle production process of shale gas well depletion is conducted with the purpose of obtaining many important production data including complete pressure and daily gas output in the simulated production process of shale gas well.The experimental results show the production characteristic from simulation is consistent with those from gas well.Based on the simulation data,the critical desorption pressure(12 MPa)of core,free gas production(3820.8 mL),adsorbed gas production(2151.2 mL),the proportion of the daily gas production between free and absorbed gas under different time and formation pressure,as well as the production time and final recovery rate corresponding to abandoned pressure,can be determined accurately.Numerical inversion is carried out to calculate the production performance curve of shale gas well and predict the development effect of gas well based on well testing and similarity analysis of the dimensionless time between core experiment and gas well production.Finally,the permeability and the fracturing effect(fracture network density)as the keys to the effective development of shale gas reservoirs are proposed.The permeability is the fundamental factor and the fracturing technology is the major means.
基金supported by the National Natural Science Foundation of China(Grant Nos.51374213&51674251)the State Key Research Development Program of China(Grant No.2016YFC0600705)+3 种基金the National Natural Science Fund for Distinguished Young Scholars(Grant No.51125017)Fund for Creative Research and Development Group Program of Jiangsu Province(Grant No.2014-27)Science Fund for Creative Research Groups of the National Natural Science Foundation of China(Grant No.51421003)the State Key Research Development Program of China(Grant No.2016YFC0600705)
文摘The CO_2 permeability of fractured coal is of great significance to both coalbed gas extraction and CO_2 storage in coal seams, but the effects of high confining pressure, high injection pressure and elevated temperature on the CO_2 permeability of fractured coal with different fracture extents have not been investigated thoroughly. In this paper, the CO_2 permeability of fractured coals sampled from a Pingdingshan coal mine in China and artificially fractured to a certain extent is investigated through undrained triaxial tests. The CO_2 permeability is measured under the confining pressure with a range of 10–25 MPa, injection pressure with a range of 6–12 MPa and elevated temperature with a range of 25–70°C. A mechanistic model is then proposed to characterize the CO_2 permeability of the fractured coals. The effects of thermal expansion, temperature-induced reduction of adsorption capacity, and thermal micro-cracking on the CO_2 permeability are explored. The test results show that the CO_2 permeability of naturally fractured coal saliently increases with increasing injection pressure. The increase of confining pressure reduces the permeability of both naturally fractured coal and secondarily fractured coal. It is also observed that initial fracturing by external loads can enhance the permeability, but further fracturing reduces the permeability. The CO_2 permeability decreases with the elevation of temperature if the temperature is lower than 44°C, but the permeability increases with temperature once the temperature is beyond 44°C. The mechanistic model well describes these compaction mechanisms induced by confining pressure, injection pressure and the complex effects induced by elevated temperature.
