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
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
At present,evaluation on reservoir damage induced by fracturing fluid mainly refers to The Evaluation Measurement for Properties of Water-based Fracturing Fluid:SY/T5107-2016(referred to as the industry standard below...At present,evaluation on reservoir damage induced by fracturing fluid mainly refers to The Evaluation Measurement for Properties of Water-based Fracturing Fluid:SY/T5107-2016(referred to as the industry standard below).However,the fracturing fluid displacing core process stipulated in the industry standard is not consistent with the fast invading process of fracturing fluid into the reservoir under high pressure during the actual fracturing construction.Besides,the influences of fracturing fluid residues,gel breaking mode,original water saturation and other factors are not taken into consideration in the experiments to evaluate the damage of fracturing fluids.Thus,the accuracy of evaluation results is influenced.In this paper,tight sandstone cores of the Lower Jurassic Ahe Formation(J1a)in Dibei area of Kuqa Depression of the Tarim Basin were selected as samples.The invading process of fracturing fluid into a tight sandstone reservoir was simulated by modifying experimental process and method.Then,the damage degree of fracturing fluid to gas reservoir was evaluated and the damage mechanisms of fracturing fluid were analyzed systematically.And the following research results were obtained.First,themodified evaluationmethod takes into account the influences of several factors,such as the originalwater saturation of gas reservoir,the instantaneous“breakdown”effect of high-pressure during fracturing and the fracturing fluid residues,so it can evaluate the damage degree of fracturing fluid to tight sandstone gas reservoirsmore objectively.Second,the evaluation results based on the industry standard showthat the damage degree of fracturing fluid to the permeability of tight sandstone gas reservoirs ismedium to strong,whereas the damage degree evaluated by themodifiedmethod is medium to weak.Third,the retention of fracturing fluid residues in fractures is the main cause of permeability damage.The residues can easily block fractures and fracture surface pores.Most of them retain in the pores in the surface layer of matrix cores(invasion depth less than 3 cm),so residues are filtered by matrix pores.Fourth,when fracturing fluid migrates inwards from the core surface,high molecular polymers retain in the form of thin-film lamellar,local flaky nodular and crystal inclusion in turn in the reservoir pores.Fifth,under the experimental conditions,salting-out crystals appear and are unevenly distributed in the cores.In fractures,salting-out crystals and high molecules are polymerized to form composite inclusions.In matrix pores,salting-out crystals and a small number of fragments(e.g.illite)are enclosed to form a complex.Sixth,migratory particles caused by speed sensitivity are usually combined with residues and high molecular polymers to form composite inclusions,thus blocking pores and fractures.展开更多
In response to the unclear understanding of fracture propagation and intersection interference in zipper fracturing under the factory development model of deep shale gas wells,a coupled hydro-mechanical model for zipp...In response to the unclear understanding of fracture propagation and intersection interference in zipper fracturing under the factory development model of deep shale gas wells,a coupled hydro-mechanical model for zipper fracturing considering the influence of natural fracture zones was established based on the finite element–discrete element method.The reliability of the model was verified using experimental data and field monitoring pressure increase data.Taking the deep shale gas reservoir in southern Sichuan as an example,the propagation and interference laws of fracturing fractures under the influence of natural fracture zones with different characteristics were studied.The results show that the large approaching angle fracture zone has a blocking effect on the forward propagation of fracturing fractures and the intersection of inter well fractures.During pump shutdown,hydraulic fractures exhibit continued expansion behavior under net pressure driving.Under high stress difference,as the approaching angle of the fracture zone increases,the response well pressure increase and the total length of the fractured fracture show a trend of first decreasing and then increasing,and first increasing and then decreasing,respectively.Compared to small approach angle fracture zones,natural fracture zones with large approach angles require longer time and have greater difficulty to intersect.The width of fractures and the length of natural fractures are negatively and positively correlated with the response well pressure increase,respectively,and positively and negatively correlated with the time required for intersection,the total length of hydraulic fractures,and fracturing efficiency,respectively.As the displacement distance of the well increases,the probability of fracture intersection decreases,but the regularity between displacement distance and the response well pressure increase and the total length of fractures is not obvious.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
Fractures are of great significance to tight oil and gas development.Fracture identification using conventional well logs is a feasible way to locate the underground fractures in tight sandstones.However,there are thr...Fractures are of great significance to tight oil and gas development.Fracture identification using conventional well logs is a feasible way to locate the underground fractures in tight sandstones.However,there are three problems affecting its interpretation accuracy and practical application,namely weak well log responses of fractures,a lack of specific logs for fracture prediction,and relative change omission in log responses.To overcome these problems and improve fracture identification accuracy,a fracture indicating parameter(FIP)method composed of a comprehensive index method(CIM)and a comprehensive fractal method(CFM)is introduced.The CIM tries to handle the first problem by amplifying log responses of fractures.The CFM addresses the third one using fractal dimensions.The flexible weight parameters corresponding to logs in the CIM and CFM make the interpretation possible for wells lacking specific logs.The reconstructed logs in the CIM and CFM try to solve the second problem.It is noted that the FIP method can calculate the probability of fracture development at a certain depth,but cannot show the fracture development degree of a new well compared with other wells.In this study,a formation fracture intensity(FFI)method is also introduced to further evaluate fracture development combined with production data.To test the validity of the FIP and FFI methods,fracture identification experiments are implemented in a tight reservoir in the Ordos Basin.The results are consistent with the data of rock core observation and production,indicating the proposed methods are effective for fracture identification and evaluation.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
To explore the value of helical CT volume rendering technique (VRT) in post-operative evaluation of screw fixation of axis fractures.Methods There were 21 cases of screw fixation of axis fractures between February 200...To explore the value of helical CT volume rendering technique (VRT) in post-operative evaluation of screw fixation of axis fractures.Methods There were 21 cases of screw fixation of axis fractures between February 2002 and May 2004 in the study including six cases with fractures on axis body,five on odontoid process and 10 on axis body and odontoid process.All cases received X-ray plain film,helical CT scanning,multi-planar reformatting(MPR) and VRT.Results Screw fixation through axis body and massa lateralis atlantis was performed in 10 cases and that through axis body and odontoid process in 11.VRT could clearly display full aperture of screw orbit,location of screw and angle of fixation and hence was superior to X-ray plain film and MPR.Multi-angle VRT displayed asymmetrical space of odontoid process and massa lateralis atlantis in four cases and medial deviation of 2～5 mm of half screw in screw fixation through axis body and massa lateralis atlantis in six.Conclusion VRT can eliminate false shadow of fixation screw,clearly display full aperture of screw orbit and hence supply improtant imaging evidence for post-operative evaluation of screw fixation of axis fractures.7 refs,1 fig,1 tab.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.
基金Project supported by National Natural Science Foundation of China“Basic study on drying of tight gas reservoir and enhancing gas seepage capacity”(No.51534006).
文摘At present,evaluation on reservoir damage induced by fracturing fluid mainly refers to The Evaluation Measurement for Properties of Water-based Fracturing Fluid:SY/T5107-2016(referred to as the industry standard below).However,the fracturing fluid displacing core process stipulated in the industry standard is not consistent with the fast invading process of fracturing fluid into the reservoir under high pressure during the actual fracturing construction.Besides,the influences of fracturing fluid residues,gel breaking mode,original water saturation and other factors are not taken into consideration in the experiments to evaluate the damage of fracturing fluids.Thus,the accuracy of evaluation results is influenced.In this paper,tight sandstone cores of the Lower Jurassic Ahe Formation(J1a)in Dibei area of Kuqa Depression of the Tarim Basin were selected as samples.The invading process of fracturing fluid into a tight sandstone reservoir was simulated by modifying experimental process and method.Then,the damage degree of fracturing fluid to gas reservoir was evaluated and the damage mechanisms of fracturing fluid were analyzed systematically.And the following research results were obtained.First,themodified evaluationmethod takes into account the influences of several factors,such as the originalwater saturation of gas reservoir,the instantaneous“breakdown”effect of high-pressure during fracturing and the fracturing fluid residues,so it can evaluate the damage degree of fracturing fluid to tight sandstone gas reservoirsmore objectively.Second,the evaluation results based on the industry standard showthat the damage degree of fracturing fluid to the permeability of tight sandstone gas reservoirs ismedium to strong,whereas the damage degree evaluated by themodifiedmethod is medium to weak.Third,the retention of fracturing fluid residues in fractures is the main cause of permeability damage.The residues can easily block fractures and fracture surface pores.Most of them retain in the pores in the surface layer of matrix cores(invasion depth less than 3 cm),so residues are filtered by matrix pores.Fourth,when fracturing fluid migrates inwards from the core surface,high molecular polymers retain in the form of thin-film lamellar,local flaky nodular and crystal inclusion in turn in the reservoir pores.Fifth,under the experimental conditions,salting-out crystals appear and are unevenly distributed in the cores.In fractures,salting-out crystals and high molecules are polymerized to form composite inclusions.In matrix pores,salting-out crystals and a small number of fragments(e.g.illite)are enclosed to form a complex.Sixth,migratory particles caused by speed sensitivity are usually combined with residues and high molecular polymers to form composite inclusions,thus blocking pores and fractures.
基金Supported by National Natural Science Foundation Joint Fund Project(NO.U21B2071)National Natural Science Youth Foundation of China(NO.52304041)。
文摘In response to the unclear understanding of fracture propagation and intersection interference in zipper fracturing under the factory development model of deep shale gas wells,a coupled hydro-mechanical model for zipper fracturing considering the influence of natural fracture zones was established based on the finite element–discrete element method.The reliability of the model was verified using experimental data and field monitoring pressure increase data.Taking the deep shale gas reservoir in southern Sichuan as an example,the propagation and interference laws of fracturing fractures under the influence of natural fracture zones with different characteristics were studied.The results show that the large approaching angle fracture zone has a blocking effect on the forward propagation of fracturing fractures and the intersection of inter well fractures.During pump shutdown,hydraulic fractures exhibit continued expansion behavior under net pressure driving.Under high stress difference,as the approaching angle of the fracture zone increases,the response well pressure increase and the total length of the fractured fracture show a trend of first decreasing and then increasing,and first increasing and then decreasing,respectively.Compared to small approach angle fracture zones,natural fracture zones with large approach angles require longer time and have greater difficulty to intersect.The width of fractures and the length of natural fractures are negatively and positively correlated with the response well pressure increase,respectively,and positively and negatively correlated with the time required for intersection,the total length of hydraulic fractures,and fracturing efficiency,respectively.As the displacement distance of the well increases,the probability of fracture intersection decreases,but the regularity between displacement distance and the response well pressure increase and the total length of fractures is not obvious.
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 Science and Technology Major Project(Grant No.2017ZX05009001-002 and 2017ZX05013002-004)the Fundamental Research Funds for the Central Universities(Grant No.2462020YJRC005)Science Foundation of China University of Petroleum,Beijing(Grant No.2462020XKJS02).
文摘Fractures are of great significance to tight oil and gas development.Fracture identification using conventional well logs is a feasible way to locate the underground fractures in tight sandstones.However,there are three problems affecting its interpretation accuracy and practical application,namely weak well log responses of fractures,a lack of specific logs for fracture prediction,and relative change omission in log responses.To overcome these problems and improve fracture identification accuracy,a fracture indicating parameter(FIP)method composed of a comprehensive index method(CIM)and a comprehensive fractal method(CFM)is introduced.The CIM tries to handle the first problem by amplifying log responses of fractures.The CFM addresses the third one using fractal dimensions.The flexible weight parameters corresponding to logs in the CIM and CFM make the interpretation possible for wells lacking specific logs.The reconstructed logs in the CIM and CFM try to solve the second problem.It is noted that the FIP method can calculate the probability of fracture development at a certain depth,but cannot show the fracture development degree of a new well compared with other wells.In this study,a formation fracture intensity(FFI)method is also introduced to further evaluate fracture development combined with production data.To test the validity of the FIP and FFI methods,fracture identification experiments are implemented in a tight reservoir in the Ordos Basin.The results are consistent with the data of rock core observation and production,indicating the proposed methods are effective for fracture identification and evaluation.
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.
文摘To explore the value of helical CT volume rendering technique (VRT) in post-operative evaluation of screw fixation of axis fractures.Methods There were 21 cases of screw fixation of axis fractures between February 2002 and May 2004 in the study including six cases with fractures on axis body,five on odontoid process and 10 on axis body and odontoid process.All cases received X-ray plain film,helical CT scanning,multi-planar reformatting(MPR) and VRT.Results Screw fixation through axis body and massa lateralis atlantis was performed in 10 cases and that through axis body and odontoid process in 11.VRT could clearly display full aperture of screw orbit,location of screw and angle of fixation and hence was superior to X-ray plain film and MPR.Multi-angle VRT displayed asymmetrical space of odontoid process and massa lateralis atlantis in four cases and medial deviation of 2～5 mm of half screw in screw fixation through axis body and massa lateralis atlantis in six.Conclusion VRT can eliminate false shadow of fixation screw,clearly display full aperture of screw orbit and hence supply improtant imaging evidence for post-operative evaluation of screw fixation of axis fractures.7 refs,1 fig,1 tab.
