The presence of sealed or semi-sealed,multiscale natural fracture systems appears to be crucial for the successful stimulation of deep reservoirs.To explore the reaction of such systems to reservoir stimulation,a new ...The presence of sealed or semi-sealed,multiscale natural fracture systems appears to be crucial for the successful stimulation of deep reservoirs.To explore the reaction of such systems to reservoir stimulation,a new numerical simulation approach for hydraulic stimulation has been developed,trying to establish a realistic model of the physics involved.Our new model successfully reproduces dynamic fracture activation,network generation,and overall reservoir permeability enhancement.Its outputs indicate that natural fractures facilitate stimulation far beyond the near-wellbore area,and can significantly improve the hydraulic conductivity of unconventional geo-energy reservoirs.According to our model,the fracture activation patterns are jointly determined by the occurrence of natural fractures and the in situ stress.High-density natural fractures,high-fluid pressure,and low effective stress environments promote the formation of complex fracture networks during stimulation.Multistage or multicluster fracturing treatments with an appropriate spacing also increase the stimulated reservoir area(SRA).The simulation scheme demonstrated in this work offers the possibility to elucidate the complex multiphysical couplings seen in the field through detailed site-specific modeling.展开更多
Unconventional resources like shale gas has been the focus of intense research and development for two decades. Apart from intrinsic geologic factors that control the gas shale productivity (e.g. organic matter conten...Unconventional resources like shale gas has been the focus of intense research and development for two decades. Apart from intrinsic geologic factors that control the gas shale productivity (e.g. organic matter content, bedding planes, natural fractures, porosity and stress regime among others), external factors like wellbore orientation and stimulation design play a role. In this study, we present a series of true triaxial hydraulic fracturing experiments conducted on Lushan shale to investigate the interplay of internal factors (bedding, natural fractures and in situ stress) and external factors (wellbore orientation) on the growth process of fracture networks in cubic specimens of 200 mm in length. We observe relatively low breakdown pressure and fracture propagation pressure as the wellbore orientation and/or the maximum in situ stress is subparallel to the shale bedding plane. The wellbore orientation has a more prominent effect on the breakdown pressure, but its effect is tapered with increasing angle of bedding inclination. The shale breakdown is followed by an abrupt response in sample displacement, which reflects the stimulated fracture volume. Based on fluid tracer analysis, the morphology of hydraulic fractures (HF) is divided into four categories. Among the categories, activation of bedding planes (bedding failure, BF) and natural fractures (NF) significantly increase bifurcation and fractured areas. Under the same stress regime, a horizontal wellbore is more favorable to enhance the complexity of hydraulic fracture networks. This is attributed to the relatively large surface area in contact with the bedding plane for the horizontal borehole compared to the case with a vertical wellbore. These findings provide important references for hydraulic fracturing design in shale reservoirs.展开更多
The production efficiency of shale gas is affected by the interaction between hydraulic and natural fractures.This study presents a simulation of natural fractures in shale reservoirs,based on a discrete fracture netw...The production efficiency of shale gas is affected by the interaction between hydraulic and natural fractures.This study presents a simulation of natural fractures in shale reservoirs,based on a discrete fracture network(DFN)method for hydraulic fracturing engineering.Fracture properties of the model are calculated from core fracture data,according to statistical mathematical analysis.The calculation results make full use of the quantitative information of core fracture orientation,density,opening and length,which constitute the direct and extensive data of mining engineering.The reliability and applicability of the model are analyzed with regard to model size and density,a calculation method for dominant size and density being proposed.Then,finite element analysis is applied to a hydraulic fracturing numerical simulation of a shale fractured reservoir in southeastern Chongqing.The hydraulic pressure distribution,fracture propagation,acoustic emission information and in situ stress changes during fracturing are analyzed.The results show the application of fracture statistics in fracture modeling and the influence of fracture distribution on hydraulic fracturing engineering.The present analysis may provide a reference for shale gas exploitation.展开更多
Unconventional reservoirs usually contain many weak surfaces such as faults,laminae and natural fractures,and effective activation and utilization of these weak surfaces in reservoirs can significantly improve the ext...Unconventional reservoirs usually contain many weak surfaces such as faults,laminae and natural fractures,and effective activation and utilization of these weak surfaces in reservoirs can significantly improve the extraction effect.In hydraulic fracturing,when the artificial fracture approaches the natural fracture,the natural fracture would be influenced by both the original in-situ stress field and the hydraulic fracturing-induced stress field.In this paper,the hydraulic fracturing-induced stress field is calculated based on the relative position of hydraulic fracture and natural fracture,the original in-situ stress,the net pressure inside the hydraulic fracture and the pore pressure of the formation.Furthermore,the stability model of the natural fracture is established by combining the Mohr-Coulomb rupture criterion,and extensive parametric studies are conducted to explore the impact of each parameter on the stability of the natural fracture.The validity of the proposed model is verified by comparing with the reservoir characteristics and fracturing process of the X-well 150e155 formation in the Songliao Basin.It is found that the stress field induced by the hydraulic fracture inhibits the activation of the natural fracture after the artificial fracture crossed the natural fracture.Therefore,for similar reservoirs as X-well 150e155,it is suggested to connect natural fractures with hydraulic fractures first and then activate natural fractures which can effectively utilize the natural fractures and form a complex fracture network.展开更多
Whether hydraulic fractures could connect multiple gas zones in the vertical plane is the key to fracturing treatment to jointly exploit coalbed methane and tight sandstone gas through integrative hydraulic fracturing...Whether hydraulic fractures could connect multiple gas zones in the vertical plane is the key to fracturing treatment to jointly exploit coalbed methane and tight sandstone gas through integrative hydraulic fracturing in tight sandstone–coal interbedded formations. Laboratory true triaxial hydraulic fracturing experiments were conducted on layered specimens with di erent combination types of natural sandstone and coal to simulate the propagation behavior of hydraulic fractures. The effects of the fracture initiation position, fracturing fluid viscosity and injection rate were discussed. The results showed that di erent fracture morphologies could be found. When initiating from coal seams, three patterns of fracture initiation and propagation were obtained:(1) The main hydraulic fracture initiated and propagated along the natural fractures and then diverged due to the effects of in situ stress and formed secondary fractures.(2) The hydraulic fracture initiated and propagated in the direction of the maximum horizontal stress.(3) Multiple fractures initiated and propagated at the same time. With the same fracturing fluid viscosity and injection rate, the hydraulic fractures initiating in sandstones had greater chances than those in coal seams to penetrate interfaces and enter neighboring layers. Excessively small or large fracturing fluid viscosity and injection rate would do harm to the vertical extension height of the induced fracture and improvement of the stimulated reservoir volume. Compared with operation parameters(fracturing fluid viscosity and injection rate), the natural weak planes in coals were considered to be the key factor that a ected the fracture propagation path. The experimental results would make some contributions to the development of tight sandstone–coal interbedded reservoirs.展开更多
Today, the oil and gas industry, and in particular hydraulic fracturing operations, have come under increasing pressure from regulators and the public to reduce emissions. As the industry evolves, oil and gas producer...Today, the oil and gas industry, and in particular hydraulic fracturing operations, have come under increasing pressure from regulators and the public to reduce emissions. As the industry evolves, oil and gas producers are in the position of evaluating alternative technologies which will support their objectives of reducing their overall emissions profile and carbon footprint. As a response, the deployment of technology and solutions to reduce emissions related to hydraulic fracturing applications has recently accelerated, creating various options to address these industry challenges. BJ Energy Solutions and West Virginia University have been working on the application and emissions characterization of various hydraulic fracturing technologies. A study was conducted to evaluate the efficiency and resultant emissions from various technologies, including natural gas reciprocating engines, diesel-natural gas dual-fuel engines, large (>24 MW) gas turbines, and direct drive turbines. The study involved the development of an emissions model with the purpose of estimating total emissions of carbon dioxide (CO<sub>2</sub>), nitrous oxide (N2O) and exhaust methane (CH<sub>4</sub>) slip, all Greenhouse Gases (GHGs), and converted to tons of CO<sub>2</sub> equivalent emissions per day of operation. The model inputs are the required Hydraulic Horsepower (HHP) based on pumping rate and pressure for various shale play scenarios. The model calculates emissions from the TITAN, which is a direct-drive turbine model fielded by BJ, using data collected following U.S. Environmental Protection Agency (EPA) testing protocols. The model also calculates and compares other hydraulic fracturing technologies utilizing published Original Equipment Manufacturer (OEM) data. Relevant EPA-regulated criteria emissions of oxides of nitrogen (NO<sub>x</sub>), Carbon Monoxide (CO) and Particulate Matter (PM) are also reported. Modeling results demonstrated that in most cases, the TITAN gas turbine system has lower total GHG emissions than conventional diesel and other next-generation technologies, and also has lower criteria emissions. The benefits of the TITAN gas turbine system compared to the other technologies stems from significantly lower methane slip, and the high-power transfer efficiency resulting from directly connecting a turbine to a reciprocating pump, despite the comparatively lower thermal efficiency.展开更多
Fluid leak-off phenomenon plays a critical role in hydraulic fracturing operation.This phenomenon can be very impressive in successful operation of hydraulic fracturing.This operation is very complex in fractured rese...Fluid leak-off phenomenon plays a critical role in hydraulic fracturing operation.This phenomenon can be very impressive in successful operation of hydraulic fracturing.This operation is very complex in fractured reservoirs due to the reaction between induced fracture and natural fractures.In this study with the cohesive element method,the effect of presence of natural fracture on the magnitude of hydraulic fracturing fluid leak-off is investigated.First of all,cohesive element and extended finite element method methods are described.The fluid flow inside hydraulic fracture and the affecting parameters on leak-off of this fluid on adjacent environment are analyzed.Then,effects of natural fracture on hydraulic fracturing direction such as deviation,leak-off and the mutual influences(which includes the changes of stress regime around the natural fracture)and also changes in pore pressure are processed.The results indicate that presence of natural fracture will cause reduction in aperture of hydraulic fracture.This decrease will lead to extension of fluid lag and eventually delaying of leak-off phenomenon.However,this effect is negligible against the positive impact due to shear and normal displacement on increasing leak-off.展开更多
The drainage areas(and volumes)near hydraulically fractured wells,computed and visualized in our study at high resolution,may be critically affected by the presence of natural fractures.Using a recently developed algo...The drainage areas(and volumes)near hydraulically fractured wells,computed and visualized in our study at high resolution,may be critically affected by the presence of natural fractures.Using a recently developed algorithm based on complex analysis methods(CAMs),the drained rock volume(DRV)is visualized for a range of synthetic constellations of natural fractures near hydraulic fractures.First,flow interference effects near a single hydraulic fracture are systematically investigated for a variety of natural fracture sets.The permeability contrast between the matrix and the natural fractures is increased stepwise in order to better understand the effect on the DRV.Next,a larger-scale model investigates flow interference for a full hydraulically fractured well with a variety of natural fracture sets.The time of flight contours(TOFCs)outlining the DRV are for all cases with natural fractures compared to a base case without any natural fractures.Discrete natural fractures,with different orientations,hydraulic conductivity,and fracture density,may shift the TOFC patterns in the reservoir region drained by the hydraulically fractured well,essentially shifting the location of the well’s drainage area.The CAM-based models provide a computationally efficient method to quantify and visualize the drainage in both naturally and hydraulically fractured reservoirs.展开更多
To study the casing deformation(CD)in shale gas well fracturing caused by natural fracture slip,a fracture face stress model is built based on stress analysis,and a CD prediction model is established based on complex ...To study the casing deformation(CD)in shale gas well fracturing caused by natural fracture slip,a fracture face stress model is built based on stress analysis,and a CD prediction model is established based on complex function to analyze factors affecting wellbore shear stress and CD.(1)The fracture and wellbore approach angles have significant impacts on the wellbore shear stress.In Weiyuan shale gas field,Sichuan Basin,under the common wellbore approach angle of nearly 90°,the wellbore is subjected to large shear stress and high risk of CD at the fracture approach angle range of 20° to 55° or its supplementary angle range.(2)When the fracture is partially opened,the wellbore shear stress is positively correlated with the fluid pressure,and negatively correlated with the fracture friction coefficient;when the fracture is fully opened,the wellbore shear stress is positively correlated with the natural fracture area.(3)The lower the elastic modulus and the longer the fracture length,the more serious the CD will be,and the Poisson’s ratio has a weak influence on the CD.The deformation first increases and then decreases with the increase of fracture approach angle,and reaches the maximum when the fracture approach angle is 45°.(4)At a given fracture approach angle,appropriately adjusting the wellbore approach angle can avoid high shear stress acting on wellbore,and reasonable control of the fluid pressure in the fracture can reduce the CD risk.The shear stress acting on casing is usually much greater than the shear strength of casing,so increasing casing strength or cementing quality have limited effect on reducing the risk of CD.Caliper logging data has verified that the CD prediction model is reliable,so the model can be used to establish risk analysis chart and calculate deformation value,to provide a reference for quick CD risk prediction in fracturing design.展开更多
Hydraulic fracturing is considered the main stimulation method to develop shale gas reservoirs. Due to its strong heterogeneity, the shale gas formation is typically embedded with geological discontinuities such as be...Hydraulic fracturing is considered the main stimulation method to develop shale gas reservoirs. Due to its strong heterogeneity, the shale gas formation is typically embedded with geological discontinuities such as bedding planes and natural fractures. Many researchers realized that the interaction between natural fractures and hydraulic fractures plays a crucial role in generating a complex fracture network. In this paper, true tri-axial hydraulic fracturing tests were performed on polymethyl methacrylate (PMMA), on which pre-existing fracture was pre-manufactured to simulate natural fracture. A cohesive model has been developed to verify the results of the experimental tests. The key findings demonstrate that the experimental results agreed well with the numerical simulation outcomes where three main interaction modes were observed: crossing;being arrested by opening the pre-existing fracture;being arrested without dilating the pre-existing fracture. Crossing behavior is more likely to occur with the approaching angle, horizontal stress difference, and injection rate increase. Furthermore, the higher flow rate might assist in reactivating the natural fractures where both sides of the pre-existing fractures were reactivated as the injection rate increased from 5 to 20 mL/min. Additionally, hydraulic fractures show a tendency to extend vertically rather than along the direction of maximum horizontal stress when they are first terminated at the interface. This research may contribute to the field application of hydraulic fracturing in shale gas formation.展开更多
In this study,we use the extended finite element method(XFEM)with a consideration of junction enrichment functions to investigate the mechanics of hydraulic fractures related to naturally cemented fractures.In the pro...In this study,we use the extended finite element method(XFEM)with a consideration of junction enrichment functions to investigate the mechanics of hydraulic fractures related to naturally cemented fractures.In the proposed numerical model,the lubrication equation is adopted to describe the fluid flow within fractures.The fluid-solid coupling systems of the hydraulic fracturing problem are solved using the Newton-Raphson method.The energy release rate criterion is used to determine the cross/arrest behavior between a hydraulic fracture(HF)and a cemented natural fracture(NF).The failure patterns and mechanisms of crack propagation at the intersection of natural fractures are discussed.Simulation results show that after crossing an NF,the failure mode along the cemented NF path may change from the tensile regime to the shear or mixed-mode regime.When an advancing HF kinks back toward the matrix,the failure mode may gradually switch back to the tensile-dominated regime.Key factors,including the length of the upper/lower portion of the cemented NF,horizontal stress anisotropy,and the intersection angle of the crack propagation are investigated in detail.An uncemented or partially cemented NF will form a more complex fracture network than a cemented NF.This study provides insight into the formation mechanism of fracture networks in formations that contain cemented NF.展开更多
Pre-existing natural fractures and other structurally weak planes are usually well-developed in unconventional reservoirs.When such fractures intersect with hydraulic induced fractures,they will redirect and propagate...Pre-existing natural fractures and other structurally weak planes are usually well-developed in unconventional reservoirs.When such fractures intersect with hydraulic induced fractures,they will redirect and propagate as an important mechanical principle of volume fracturing by the formation of complex fracture networks.Under the shadow effect of natural fractures and other structurally weak planes with hydraulic supported fracture stress,hydraulic fractures do not fully propagate in the direction of the maximum horizontal-principal-stress.This paper computed the stress intensity factors of hydraulic fracture types I and II by integrating the various interactions,established universally-applicable mechanical principles for the propagation behavior when a hydraulic fracture propagating in an arbitrary direction intersects with a natural fracture at an arbitrary angle,and demonstrated the mechanical principles of the intersection between hydraulic induced fractures and pre-existing natural fractures.This study proved the following conclusions:as the intersection angle between the hydraulic fracture and the maximum horizontal-principal-stress increased,the possibility of the hydraulic fracture being captured by the natural fracture with an identical approaching angle first increased and then decreased;as the net stress increased,the intersection behavior between the hydraulic fracture and the natural fracture transitioned from penetration to capture.展开更多
During the past years,the recovery of unconventional gas formation has attracted lots of attention and achieved huge success.To produce gas from the low-permeability unconventional formations,hydraulic fracturing tech...During the past years,the recovery of unconventional gas formation has attracted lots of attention and achieved huge success.To produce gas from the low-permeability unconventional formations,hydraulic fracturing technology is essential and critical.In this paper,we present the development of a three-dimensional thermalhydraulic-mechanical numerical simulator for the simulation of hydraulic fracturing operations in tight sandstone reservoirs.Our simulator is based on integrated finite difference(IFD)method.In this method,the simulation domain is subdivided into sub domains and the governing equations are integrated over a sub domain with flux terms expressed as an integral over the sub domain boundary using the divergence theorem.Our simulator conducts coupled thermal-hydraulic-mechanical simulation of the initiation and extension of hydraulic fractures.It also calculates the mass/heat transport of injected hydraulic fluids as well as proppants.Our simulator is able to handle anisotropic formations with multiple layers.Our simulator has been validated by comparing with an analytical solution as well as Ribeiro and Sharma model.Our model can simulate fracture spacing effect on fracture profile when combining IFD with Discontinuous Displacement Method(DDM).展开更多
Fracture system is an important factor controlling tight oil accumulation in the Triassic Chang 8 Member,southwestern Ordos Basin,China.A systematic characterization of the multi-scale natural fractures is a basis for...Fracture system is an important factor controlling tight oil accumulation in the Triassic Chang 8 Member,southwestern Ordos Basin,China.A systematic characterization of the multi-scale natural fractures is a basis for the efficient tight oil production.Based on outcrops,seismic reflections,well cores,well logs(image and conventional logging),casting thin sections,and scanning electron microscope observation,the multi-scale fractures occurrences and their influences on Chang 8 tight sandstone reservoirs are revealed.The results show that three periods of strike-slip faults and four scales of natural fractures developed,namely mega-scale(length>7×10^(7) mm),macro-scale(3.5×10^(5)<length<7×10^(7) mm),meso-scale(10<length<3.5×10^(5) mm),and micro-scale(length<10 mm)fractures.The mega-and macro-scale fractures developed by strike-slip faults are characterized by strike-segmentation and lateral zonation,which connect the source and reservoir.These scale fractures also influence the distribution and effectiveness of traps and reservoirs,which directly influence the hydrocarbon charging and distribution.The meso fractures include the tectonic,diagenetic,as well as hydrocarbon generation-related overpressure types.The meso-and micro-scale fractures improve the sandstone physical properties and also the tight oil well production performance.This integrated study helps to understand the distribution of multi-scale fractures in tight sandstones and provides a referable case and workflow for multi-scale fracture evaluation.展开更多
Fracking is one of the kernel technologies in the remarkable shale gas revolution. The extended finite element method is used in this paper to numerically investigate the interaction between hydraulic and natural frac...Fracking is one of the kernel technologies in the remarkable shale gas revolution. The extended finite element method is used in this paper to numerically investigate the interaction between hydraulic and natural fractures, which is an important issue of the enigmatic fracture network formation in fracking. The criteria which control the opening of natural fracture and crossing of hydraulic fracture are tentatively presented. Influence factors on the interaction process are systematically analyzed, which include the approach angle, anisotropy of in-situ stress and fluid pressure profile.展开更多
基金This work was financially supported by the National Natural Science Foundation of China(Nos.U22A20166,51904190,12172230 and U19A2098)the Department of Science and Technology of Guangdong Province(No.2019ZT08G315)。
文摘The presence of sealed or semi-sealed,multiscale natural fracture systems appears to be crucial for the successful stimulation of deep reservoirs.To explore the reaction of such systems to reservoir stimulation,a new numerical simulation approach for hydraulic stimulation has been developed,trying to establish a realistic model of the physics involved.Our new model successfully reproduces dynamic fracture activation,network generation,and overall reservoir permeability enhancement.Its outputs indicate that natural fractures facilitate stimulation far beyond the near-wellbore area,and can significantly improve the hydraulic conductivity of unconventional geo-energy reservoirs.According to our model,the fracture activation patterns are jointly determined by the occurrence of natural fractures and the in situ stress.High-density natural fractures,high-fluid pressure,and low effective stress environments promote the formation of complex fracture networks during stimulation.Multistage or multicluster fracturing treatments with an appropriate spacing also increase the stimulated reservoir area(SRA).The simulation scheme demonstrated in this work offers the possibility to elucidate the complex multiphysical couplings seen in the field through detailed site-specific modeling.
基金supported by the National Natural Science Foundation of China(Grant Nos.52064006 and 52004072)It was.also supported by the program(Grant No.202006050112)of China Scholarship Council(CSC)for the first author's visit at the Helm-holtz Centre Potsdam,GFZ German Research Centre for Geosciences.
文摘Unconventional resources like shale gas has been the focus of intense research and development for two decades. Apart from intrinsic geologic factors that control the gas shale productivity (e.g. organic matter content, bedding planes, natural fractures, porosity and stress regime among others), external factors like wellbore orientation and stimulation design play a role. In this study, we present a series of true triaxial hydraulic fracturing experiments conducted on Lushan shale to investigate the interplay of internal factors (bedding, natural fractures and in situ stress) and external factors (wellbore orientation) on the growth process of fracture networks in cubic specimens of 200 mm in length. We observe relatively low breakdown pressure and fracture propagation pressure as the wellbore orientation and/or the maximum in situ stress is subparallel to the shale bedding plane. The wellbore orientation has a more prominent effect on the breakdown pressure, but its effect is tapered with increasing angle of bedding inclination. The shale breakdown is followed by an abrupt response in sample displacement, which reflects the stimulated fracture volume. Based on fluid tracer analysis, the morphology of hydraulic fractures (HF) is divided into four categories. Among the categories, activation of bedding planes (bedding failure, BF) and natural fractures (NF) significantly increase bifurcation and fractured areas. Under the same stress regime, a horizontal wellbore is more favorable to enhance the complexity of hydraulic fracture networks. This is attributed to the relatively large surface area in contact with the bedding plane for the horizontal borehole compared to the case with a vertical wellbore. These findings provide important references for hydraulic fracturing design in shale reservoirs.
基金supported by the National Natural Science Foundation of China(Grant Nos.11872118,11627901)。
文摘The production efficiency of shale gas is affected by the interaction between hydraulic and natural fractures.This study presents a simulation of natural fractures in shale reservoirs,based on a discrete fracture network(DFN)method for hydraulic fracturing engineering.Fracture properties of the model are calculated from core fracture data,according to statistical mathematical analysis.The calculation results make full use of the quantitative information of core fracture orientation,density,opening and length,which constitute the direct and extensive data of mining engineering.The reliability and applicability of the model are analyzed with regard to model size and density,a calculation method for dominant size and density being proposed.Then,finite element analysis is applied to a hydraulic fracturing numerical simulation of a shale fractured reservoir in southeastern Chongqing.The hydraulic pressure distribution,fracture propagation,acoustic emission information and in situ stress changes during fracturing are analyzed.The results show the application of fracture statistics in fracture modeling and the influence of fracture distribution on hydraulic fracturing engineering.The present analysis may provide a reference for shale gas exploitation.
基金funded by the subprojects of the National Key R&D Program of China(2020YFA0710600)the NSFC(National Natural Science Foundation of China,grant 42374132).
文摘Unconventional reservoirs usually contain many weak surfaces such as faults,laminae and natural fractures,and effective activation and utilization of these weak surfaces in reservoirs can significantly improve the extraction effect.In hydraulic fracturing,when the artificial fracture approaches the natural fracture,the natural fracture would be influenced by both the original in-situ stress field and the hydraulic fracturing-induced stress field.In this paper,the hydraulic fracturing-induced stress field is calculated based on the relative position of hydraulic fracture and natural fracture,the original in-situ stress,the net pressure inside the hydraulic fracture and the pore pressure of the formation.Furthermore,the stability model of the natural fracture is established by combining the Mohr-Coulomb rupture criterion,and extensive parametric studies are conducted to explore the impact of each parameter on the stability of the natural fracture.The validity of the proposed model is verified by comparing with the reservoir characteristics and fracturing process of the X-well 150e155 formation in the Songliao Basin.It is found that the stress field induced by the hydraulic fracture inhibits the activation of the natural fracture after the artificial fracture crossed the natural fracture.Therefore,for similar reservoirs as X-well 150e155,it is suggested to connect natural fractures with hydraulic fractures first and then activate natural fractures which can effectively utilize the natural fractures and form a complex fracture network.
基金the support from the National Science and Technology Major Projects of China(Grant No.2016ZX05066)Major Program of National Natural Science Foundation of China(Grant No.51490650)PetroChina Innovation Foundation(No.2018D-5007-0307)
文摘Whether hydraulic fractures could connect multiple gas zones in the vertical plane is the key to fracturing treatment to jointly exploit coalbed methane and tight sandstone gas through integrative hydraulic fracturing in tight sandstone–coal interbedded formations. Laboratory true triaxial hydraulic fracturing experiments were conducted on layered specimens with di erent combination types of natural sandstone and coal to simulate the propagation behavior of hydraulic fractures. The effects of the fracture initiation position, fracturing fluid viscosity and injection rate were discussed. The results showed that di erent fracture morphologies could be found. When initiating from coal seams, three patterns of fracture initiation and propagation were obtained:(1) The main hydraulic fracture initiated and propagated along the natural fractures and then diverged due to the effects of in situ stress and formed secondary fractures.(2) The hydraulic fracture initiated and propagated in the direction of the maximum horizontal stress.(3) Multiple fractures initiated and propagated at the same time. With the same fracturing fluid viscosity and injection rate, the hydraulic fractures initiating in sandstones had greater chances than those in coal seams to penetrate interfaces and enter neighboring layers. Excessively small or large fracturing fluid viscosity and injection rate would do harm to the vertical extension height of the induced fracture and improvement of the stimulated reservoir volume. Compared with operation parameters(fracturing fluid viscosity and injection rate), the natural weak planes in coals were considered to be the key factor that a ected the fracture propagation path. The experimental results would make some contributions to the development of tight sandstone–coal interbedded reservoirs.
文摘Today, the oil and gas industry, and in particular hydraulic fracturing operations, have come under increasing pressure from regulators and the public to reduce emissions. As the industry evolves, oil and gas producers are in the position of evaluating alternative technologies which will support their objectives of reducing their overall emissions profile and carbon footprint. As a response, the deployment of technology and solutions to reduce emissions related to hydraulic fracturing applications has recently accelerated, creating various options to address these industry challenges. BJ Energy Solutions and West Virginia University have been working on the application and emissions characterization of various hydraulic fracturing technologies. A study was conducted to evaluate the efficiency and resultant emissions from various technologies, including natural gas reciprocating engines, diesel-natural gas dual-fuel engines, large (>24 MW) gas turbines, and direct drive turbines. The study involved the development of an emissions model with the purpose of estimating total emissions of carbon dioxide (CO<sub>2</sub>), nitrous oxide (N2O) and exhaust methane (CH<sub>4</sub>) slip, all Greenhouse Gases (GHGs), and converted to tons of CO<sub>2</sub> equivalent emissions per day of operation. The model inputs are the required Hydraulic Horsepower (HHP) based on pumping rate and pressure for various shale play scenarios. The model calculates emissions from the TITAN, which is a direct-drive turbine model fielded by BJ, using data collected following U.S. Environmental Protection Agency (EPA) testing protocols. The model also calculates and compares other hydraulic fracturing technologies utilizing published Original Equipment Manufacturer (OEM) data. Relevant EPA-regulated criteria emissions of oxides of nitrogen (NO<sub>x</sub>), Carbon Monoxide (CO) and Particulate Matter (PM) are also reported. Modeling results demonstrated that in most cases, the TITAN gas turbine system has lower total GHG emissions than conventional diesel and other next-generation technologies, and also has lower criteria emissions. The benefits of the TITAN gas turbine system compared to the other technologies stems from significantly lower methane slip, and the high-power transfer efficiency resulting from directly connecting a turbine to a reciprocating pump, despite the comparatively lower thermal efficiency.
文摘Fluid leak-off phenomenon plays a critical role in hydraulic fracturing operation.This phenomenon can be very impressive in successful operation of hydraulic fracturing.This operation is very complex in fractured reservoirs due to the reaction between induced fracture and natural fractures.In this study with the cohesive element method,the effect of presence of natural fracture on the magnitude of hydraulic fracturing fluid leak-off is investigated.First of all,cohesive element and extended finite element method methods are described.The fluid flow inside hydraulic fracture and the affecting parameters on leak-off of this fluid on adjacent environment are analyzed.Then,effects of natural fracture on hydraulic fracturing direction such as deviation,leak-off and the mutual influences(which includes the changes of stress regime around the natural fracture)and also changes in pore pressure are processed.The results indicate that presence of natural fracture will cause reduction in aperture of hydraulic fracture.This decrease will lead to extension of fluid lag and eventually delaying of leak-off phenomenon.However,this effect is negligible against the positive impact due to shear and normal displacement on increasing leak-off.
文摘The drainage areas(and volumes)near hydraulically fractured wells,computed and visualized in our study at high resolution,may be critically affected by the presence of natural fractures.Using a recently developed algorithm based on complex analysis methods(CAMs),the drained rock volume(DRV)is visualized for a range of synthetic constellations of natural fractures near hydraulic fractures.First,flow interference effects near a single hydraulic fracture are systematically investigated for a variety of natural fracture sets.The permeability contrast between the matrix and the natural fractures is increased stepwise in order to better understand the effect on the DRV.Next,a larger-scale model investigates flow interference for a full hydraulically fractured well with a variety of natural fracture sets.The time of flight contours(TOFCs)outlining the DRV are for all cases with natural fractures compared to a base case without any natural fractures.Discrete natural fractures,with different orientations,hydraulic conductivity,and fracture density,may shift the TOFC patterns in the reservoir region drained by the hydraulically fractured well,essentially shifting the location of the well’s drainage area.The CAM-based models provide a computationally efficient method to quantify and visualize the drainage in both naturally and hydraulically fractured reservoirs.
基金Supported by National Natural Science Foundation of China(51904258,51874250)Project of Science and Technology of Shale Gas Exploration&Development of CCDC(2019-JS-941)National Major Project of Science and Technology(2016ZX05048-004-006)。
文摘To study the casing deformation(CD)in shale gas well fracturing caused by natural fracture slip,a fracture face stress model is built based on stress analysis,and a CD prediction model is established based on complex function to analyze factors affecting wellbore shear stress and CD.(1)The fracture and wellbore approach angles have significant impacts on the wellbore shear stress.In Weiyuan shale gas field,Sichuan Basin,under the common wellbore approach angle of nearly 90°,the wellbore is subjected to large shear stress and high risk of CD at the fracture approach angle range of 20° to 55° or its supplementary angle range.(2)When the fracture is partially opened,the wellbore shear stress is positively correlated with the fluid pressure,and negatively correlated with the fracture friction coefficient;when the fracture is fully opened,the wellbore shear stress is positively correlated with the natural fracture area.(3)The lower the elastic modulus and the longer the fracture length,the more serious the CD will be,and the Poisson’s ratio has a weak influence on the CD.The deformation first increases and then decreases with the increase of fracture approach angle,and reaches the maximum when the fracture approach angle is 45°.(4)At a given fracture approach angle,appropriately adjusting the wellbore approach angle can avoid high shear stress acting on wellbore,and reasonable control of the fluid pressure in the fracture can reduce the CD risk.The shear stress acting on casing is usually much greater than the shear strength of casing,so increasing casing strength or cementing quality have limited effect on reducing the risk of CD.Caliper logging data has verified that the CD prediction model is reliable,so the model can be used to establish risk analysis chart and calculate deformation value,to provide a reference for quick CD risk prediction in fracturing design.
基金support from Major Program of the National Natural Science Foundation of China(Grant No.52192621)the National Natural Science Foundation of China for Major International(Regional)Joint Research Project(Grant No.52020105001)+1 种基金Major Science and Technology Project of Yunnan Province(Grant No.202302AF080001)Beijing Outstanding Young Scientist Program(Grant No.BJJWZYJH01201911414038).
文摘Hydraulic fracturing is considered the main stimulation method to develop shale gas reservoirs. Due to its strong heterogeneity, the shale gas formation is typically embedded with geological discontinuities such as bedding planes and natural fractures. Many researchers realized that the interaction between natural fractures and hydraulic fractures plays a crucial role in generating a complex fracture network. In this paper, true tri-axial hydraulic fracturing tests were performed on polymethyl methacrylate (PMMA), on which pre-existing fracture was pre-manufactured to simulate natural fracture. A cohesive model has been developed to verify the results of the experimental tests. The key findings demonstrate that the experimental results agreed well with the numerical simulation outcomes where three main interaction modes were observed: crossing;being arrested by opening the pre-existing fracture;being arrested without dilating the pre-existing fracture. Crossing behavior is more likely to occur with the approaching angle, horizontal stress difference, and injection rate increase. Furthermore, the higher flow rate might assist in reactivating the natural fractures where both sides of the pre-existing fractures were reactivated as the injection rate increased from 5 to 20 mL/min. Additionally, hydraulic fractures show a tendency to extend vertically rather than along the direction of maximum horizontal stress when they are first terminated at the interface. This research may contribute to the field application of hydraulic fracturing in shale gas formation.
基金financially supported by the National Science Foundation of China(Grant Nos.51804033 and 51936001)Natural Science Foundation of Jiangsu Province(Grant No.BK20170457)+3 种基金Program of Great Wall Scholar(Grant No.CIT&TCD20180313)Jointly Projects of Beijing Natural Science FoundationBeijing Municipal Education Commission(Grant No.KZ201810017023)Beijing Youth Talent Support Program(CIT&TCD201804037).
文摘In this study,we use the extended finite element method(XFEM)with a consideration of junction enrichment functions to investigate the mechanics of hydraulic fractures related to naturally cemented fractures.In the proposed numerical model,the lubrication equation is adopted to describe the fluid flow within fractures.The fluid-solid coupling systems of the hydraulic fracturing problem are solved using the Newton-Raphson method.The energy release rate criterion is used to determine the cross/arrest behavior between a hydraulic fracture(HF)and a cemented natural fracture(NF).The failure patterns and mechanisms of crack propagation at the intersection of natural fractures are discussed.Simulation results show that after crossing an NF,the failure mode along the cemented NF path may change from the tensile regime to the shear or mixed-mode regime.When an advancing HF kinks back toward the matrix,the failure mode may gradually switch back to the tensile-dominated regime.Key factors,including the length of the upper/lower portion of the cemented NF,horizontal stress anisotropy,and the intersection angle of the crack propagation are investigated in detail.An uncemented or partially cemented NF will form a more complex fracture network than a cemented NF.This study provides insight into the formation mechanism of fracture networks in formations that contain cemented NF.
基金Sponsored by National Science and Technology Major Projects(2016ZX05052,2016ZX05014).
文摘Pre-existing natural fractures and other structurally weak planes are usually well-developed in unconventional reservoirs.When such fractures intersect with hydraulic induced fractures,they will redirect and propagate as an important mechanical principle of volume fracturing by the formation of complex fracture networks.Under the shadow effect of natural fractures and other structurally weak planes with hydraulic supported fracture stress,hydraulic fractures do not fully propagate in the direction of the maximum horizontal-principal-stress.This paper computed the stress intensity factors of hydraulic fracture types I and II by integrating the various interactions,established universally-applicable mechanical principles for the propagation behavior when a hydraulic fracture propagating in an arbitrary direction intersects with a natural fracture at an arbitrary angle,and demonstrated the mechanical principles of the intersection between hydraulic induced fractures and pre-existing natural fractures.This study proved the following conclusions:as the intersection angle between the hydraulic fracture and the maximum horizontal-principal-stress increased,the possibility of the hydraulic fracture being captured by the natural fracture with an identical approaching angle first increased and then decreased;as the net stress increased,the intersection behavior between the hydraulic fracture and the natural fracture transitioned from penetration to capture.
文摘During the past years,the recovery of unconventional gas formation has attracted lots of attention and achieved huge success.To produce gas from the low-permeability unconventional formations,hydraulic fracturing technology is essential and critical.In this paper,we present the development of a three-dimensional thermalhydraulic-mechanical numerical simulator for the simulation of hydraulic fracturing operations in tight sandstone reservoirs.Our simulator is based on integrated finite difference(IFD)method.In this method,the simulation domain is subdivided into sub domains and the governing equations are integrated over a sub domain with flux terms expressed as an integral over the sub domain boundary using the divergence theorem.Our simulator conducts coupled thermal-hydraulic-mechanical simulation of the initiation and extension of hydraulic fractures.It also calculates the mass/heat transport of injected hydraulic fluids as well as proppants.Our simulator is able to handle anisotropic formations with multiple layers.Our simulator has been validated by comparing with an analytical solution as well as Ribeiro and Sharma model.Our model can simulate fracture spacing effect on fracture profile when combining IFD with Discontinuous Displacement Method(DDM).
基金We thank the financial support from the National Natural Science Foundation of China(Nos.41902147 and 41402117)the China National Science and Technology Major Project(No.2016ZX05048-001-01-CS).We also thank the Sinopec Huabei Company for providing the core samples and the other data sets,and the permission to publish this article.
文摘Fracture system is an important factor controlling tight oil accumulation in the Triassic Chang 8 Member,southwestern Ordos Basin,China.A systematic characterization of the multi-scale natural fractures is a basis for the efficient tight oil production.Based on outcrops,seismic reflections,well cores,well logs(image and conventional logging),casting thin sections,and scanning electron microscope observation,the multi-scale fractures occurrences and their influences on Chang 8 tight sandstone reservoirs are revealed.The results show that three periods of strike-slip faults and four scales of natural fractures developed,namely mega-scale(length>7×10^(7) mm),macro-scale(3.5×10^(5)<length<7×10^(7) mm),meso-scale(10<length<3.5×10^(5) mm),and micro-scale(length<10 mm)fractures.The mega-and macro-scale fractures developed by strike-slip faults are characterized by strike-segmentation and lateral zonation,which connect the source and reservoir.These scale fractures also influence the distribution and effectiveness of traps and reservoirs,which directly influence the hydrocarbon charging and distribution.The meso fractures include the tectonic,diagenetic,as well as hydrocarbon generation-related overpressure types.The meso-and micro-scale fractures improve the sandstone physical properties and also the tight oil well production performance.This integrated study helps to understand the distribution of multi-scale fractures in tight sandstones and provides a referable case and workflow for multi-scale fracture evaluation.
基金supported by the National Natural Science Foundation of China (Grant No. 11372157)the Special Research Grant for Doctor Discipline by Ministry of Education of China (Grant No. 20120002110075)the Foundation for the Author of National Excellent Doctoral Dissertation of China (FANEDD) (Grant No. 201326)
文摘Fracking is one of the kernel technologies in the remarkable shale gas revolution. The extended finite element method is used in this paper to numerically investigate the interaction between hydraulic and natural fractures, which is an important issue of the enigmatic fracture network formation in fracking. The criteria which control the opening of natural fracture and crossing of hydraulic fracture are tentatively presented. Influence factors on the interaction process are systematically analyzed, which include the approach angle, anisotropy of in-situ stress and fluid pressure profile.