The viscosity of fracturing fluid and in-situ stress difference are the two important factors that affect the hydraulic fracturing pressure and propagation morphology. In this study, raw coal was used to prepare coal ...The viscosity of fracturing fluid and in-situ stress difference are the two important factors that affect the hydraulic fracturing pressure and propagation morphology. In this study, raw coal was used to prepare coal samples for experiments, and clean fracturing fluid samples were prepared using CTAB surfactant. A series of hydraulic fracturing tests were conducted with an in-house developed triaxial hydraulic fracturing simulator and the fracturing process was monitored with an acoustic emission instrument to analyze the influences of fracturing fluid viscosity and horizontal in-situ stress difference on coal fracture propagation. The results show that the number of branched fractures decreased, the fracture pattern became simpler, the fractures width increased obviously, and the distribution of AE event points was concentrated with the increase of the fracturing fluid viscosity or the horizontal in-situ stress difference. The acoustic emission energy decreases with the increase of fracturing fluid viscosity and increases with the increase of horizontal in situ stress difference. The low viscosity clean fracturing fluid has strong elasticity and is easy to be compressed into the tip of fractures, resulting in complex fractures. The high viscosity clean fracturing fluids are the opposite. Our experimental results provide a reference and scientific basis for the design and optimization of field hydraulic fracturing parameters.展开更多
Developing low-permeability Coalbed Methane(CBM)reservoirs can significantly benefit from a comprehensive understanding of hydraulic fracture nucleation and propagation mechanisms,particularly in anthracite CBM reserv...Developing low-permeability Coalbed Methane(CBM)reservoirs can significantly benefit from a comprehensive understanding of hydraulic fracture nucleation and propagation mechanisms,particularly in anthracite CBM reservoirs.This study employs true-triaxial hydraulic fracturing experiments to investigate these mechanisms,with variables including injection flow rate,horizontal stress difference(σH-σh),and bedding orientation.Additionally,we conduct corresponding numerical cases to validate the experimental conclusions.The research also considers re-fracturing instances.For the first time,we utilize a combination of Kaiser tests and the stress transfer function in ANSYS Workbench finite element analysis to accurately restore the confining pressure of the coal sample.The findings suggest that a high initial injection flow rate during hydraulic fracturing can promote fluid leakage and aid in maintaining substantial fracture pressure.Enhanced fracturing efficiency can be achieved through higher injection rates,and it can ensure optimal fracturing efficiency,minimizing roof and floor fracturing in coal reservoirs to prevent fracturing fluid leakage.The presence of a high horizontal stress difference facilitates hydraulic fracture propagation along the direction of the maximum horizontal compressive stress,requiring a greater hydraulic pressure to produce more fracture systems in coal reservoirs.Additionally,a minor deviation in the wellbore injection direction from the bedding orientation assists in creating a complex hydraulic fractured network,although this also requires higher hydraulic pressure to initiate new fractures.In the case of multiple hydraulic fracturing,the second initiation pressure tends to be significantly higher than the first,indicating that a sequential increase in hydraulic pressure aids the formation of additional fractures.Moreover,a simplified numerical simulation has been conducted to corroborate the experimental findings.These insights are crucial in optimizing hydraulic fracturing processes to enhance the permeability of anthracite CBM reservoirs.展开更多
Volumetric fracturing is a primary stimulation technology for economical and effective exploitation of tight oil reservoirs. The main mechanism is to connect natural fractures to generate a fracture network system whi...Volumetric fracturing is a primary stimulation technology for economical and effective exploitation of tight oil reservoirs. The main mechanism is to connect natural fractures to generate a fracture network system which can enhance the stimulated reservoir volume. By using the combined finite and discrete element method, a model was built to describe hydraulic fracture propagation in tight oil reservoirs. Considering the effect of horizontal stress difference, number and spacing of perforation clus- ters, injection rate, and the density of natural fractures on fracture propagation, we used this model to simulate the fracture propagation in a tight formation of a certain oil- field. Simulation results show that when the horizontal stress difference is lower than 5 MPa, it is beneficial to form a complex fracture network system. If the horizontal stress difference is higher than 6 MPa, it is easy to form a planar fracture system; with high horizontal stress differ- ence, increasing the number of perforation clusters is beneficial to open and connect more natural fractures, and to improve the complexity of fracture network and the stimulated reservoir volume （SRV）. As the injection rate increases, the effect of volumetric fracturing may be improved; the density of natural fractures may only have a great influence on the effect of volume stimulation in a low horizontal stress difference.展开更多
For figuring out the stress interference of multiphase fracture combinations and its effect on the fracture initiation pressure of subsequent hydraulic fracturing,a calculation model for the coupled stress field with ...For figuring out the stress interference of multiphase fracture combinations and its effect on the fracture initiation pressure of subsequent hydraulic fracturing,a calculation model for the coupled stress field with multiple induced fractures preexisted was established based on the calculation model for the stress field with single induced fracture preexisted,and the change laws of circumferential stress field around the wellbore under the effect of induced stress were analyzed.Then,the fracture initiation pressure of subsequent hydraulic fracturing was calculated according to the fracturing mechanics criterion.Finally,the effects of the length,phase,horizontal principal stress difference co-efficient and quantity of preexisting fractures on its initiation pressure were analyzed.And the following research results were obtained.First,the circumferential stress difference in the area near the fractures behind the preexisting fractures increase greatly and even the horizontal principal stress is reversed.Second,as the length of new preexisting fractures increases,the initiation pressure of initial preexisting fractures rises first and then drops.And when the length of new preexisting fractures is equal to that of the initial preexisting fractures,the initiation pressure of initial preexisting fractures drops quickly first and then slowly.Third,the initiation pressure of high phase fractures is lower than that of low phase fractures.Fourth,with the increase of fracture quantity,the initiation pressure of new preexisting fractures drops gradually,but the decline trend of initiation pressure difference is not obvious.Fifth,the stress interference of multiphase fractures influences the fracture initiation pressure,and high-phase long fractures and low-phase short fractures are both favorable for the simultaneous initiation of multiphase fractures.Sixth,syn-chronous propagation of multiple fractures can generate more complicated stress interference and excite the evolution of hydraulic fracture network,so as to realize uniform fracturing stimulation.In conclusion,the research results can provide a theoretical guidance for the design of fracturing operation,e.g.perforation phase of blasting induced fracturing,scale of induced fractures,and pump pressure of subsequent hydraulic fracturing.展开更多
基金National Natural Science Foundation of China (51974176, 52174194, 51934004)Shandong Provincial Colleges and Universities Youth Innovation and Technology Support Program (2019KJH006)+1 种基金Taishan Scholars Project (TS20190935)Shandong outstanding youth fund (ZR2020JQ22).
文摘The viscosity of fracturing fluid and in-situ stress difference are the two important factors that affect the hydraulic fracturing pressure and propagation morphology. In this study, raw coal was used to prepare coal samples for experiments, and clean fracturing fluid samples were prepared using CTAB surfactant. A series of hydraulic fracturing tests were conducted with an in-house developed triaxial hydraulic fracturing simulator and the fracturing process was monitored with an acoustic emission instrument to analyze the influences of fracturing fluid viscosity and horizontal in-situ stress difference on coal fracture propagation. The results show that the number of branched fractures decreased, the fracture pattern became simpler, the fractures width increased obviously, and the distribution of AE event points was concentrated with the increase of the fracturing fluid viscosity or the horizontal in-situ stress difference. The acoustic emission energy decreases with the increase of fracturing fluid viscosity and increases with the increase of horizontal in situ stress difference. The low viscosity clean fracturing fluid has strong elasticity and is easy to be compressed into the tip of fractures, resulting in complex fractures. The high viscosity clean fracturing fluids are the opposite. Our experimental results provide a reference and scientific basis for the design and optimization of field hydraulic fracturing parameters.
基金funded by the National Natural Science Foundation of China(No.42202155)China Postdoctoral Science Foundation(No.2021MD703807)+7 种基金Heilongjiang Provincial Postdoctoral Science Foundation(No.LBH-Z20121)financial support from the China Scholarship Council(No.202008230018)the Research Fund Program of Hubei Key Laboratory of Resources and Eco-Environment Geology(No.HBREGKFJJ-202309)funding by the DGICYT Spanish Project(grant no.PID2020-118999GB-I00)funded by the MCIN/AEI/10.13039/501100011033funding by the Ramón y Cajal fellowship(grant no.RyC-2018-026335-I)funded by the MCIN/AEI/10.13039/50110001103the European Social Fund-Investing in Your Future.
文摘Developing low-permeability Coalbed Methane(CBM)reservoirs can significantly benefit from a comprehensive understanding of hydraulic fracture nucleation and propagation mechanisms,particularly in anthracite CBM reservoirs.This study employs true-triaxial hydraulic fracturing experiments to investigate these mechanisms,with variables including injection flow rate,horizontal stress difference(σH-σh),and bedding orientation.Additionally,we conduct corresponding numerical cases to validate the experimental conclusions.The research also considers re-fracturing instances.For the first time,we utilize a combination of Kaiser tests and the stress transfer function in ANSYS Workbench finite element analysis to accurately restore the confining pressure of the coal sample.The findings suggest that a high initial injection flow rate during hydraulic fracturing can promote fluid leakage and aid in maintaining substantial fracture pressure.Enhanced fracturing efficiency can be achieved through higher injection rates,and it can ensure optimal fracturing efficiency,minimizing roof and floor fracturing in coal reservoirs to prevent fracturing fluid leakage.The presence of a high horizontal stress difference facilitates hydraulic fracture propagation along the direction of the maximum horizontal compressive stress,requiring a greater hydraulic pressure to produce more fracture systems in coal reservoirs.Additionally,a minor deviation in the wellbore injection direction from the bedding orientation assists in creating a complex hydraulic fractured network,although this also requires higher hydraulic pressure to initiate new fractures.In the case of multiple hydraulic fracturing,the second initiation pressure tends to be significantly higher than the first,indicating that a sequential increase in hydraulic pressure aids the formation of additional fractures.Moreover,a simplified numerical simulation has been conducted to corroborate the experimental findings.These insights are crucial in optimizing hydraulic fracturing processes to enhance the permeability of anthracite CBM reservoirs.
文摘Volumetric fracturing is a primary stimulation technology for economical and effective exploitation of tight oil reservoirs. The main mechanism is to connect natural fractures to generate a fracture network system which can enhance the stimulated reservoir volume. By using the combined finite and discrete element method, a model was built to describe hydraulic fracture propagation in tight oil reservoirs. Considering the effect of horizontal stress difference, number and spacing of perforation clus- ters, injection rate, and the density of natural fractures on fracture propagation, we used this model to simulate the fracture propagation in a tight formation of a certain oil- field. Simulation results show that when the horizontal stress difference is lower than 5 MPa, it is beneficial to form a complex fracture network system. If the horizontal stress difference is higher than 6 MPa, it is easy to form a planar fracture system; with high horizontal stress differ- ence, increasing the number of perforation clusters is beneficial to open and connect more natural fractures, and to improve the complexity of fracture network and the stimulated reservoir volume （SRV）. As the injection rate increases, the effect of volumetric fracturing may be improved; the density of natural fractures may only have a great influence on the effect of volume stimulation in a low horizontal stress difference.
基金Project supported by the National Natural Science Foundation of China,“Microscopic Damage Mechanism of Saturated Brittle Rock during Blasting Induced Fracturing and Its Control Law on Macroscopic Damage”(No.51874339)the Natural Science Foundation of Shandong Province,“Study on the Coupled Response Mechanism of Crack Propagation and Rock Damage in the fracturing condition of multistage strong pulse”(No.ZR2016EEQ04)the Fundamental Research Funds for the Central Universities,China,“Study on the Mechanical Mechanism of Multistage Blasting Induced SRV in Deep-Seated Tight Reservoirs with High Horizontal Stress Difference”(No.17CX05004).
文摘For figuring out the stress interference of multiphase fracture combinations and its effect on the fracture initiation pressure of subsequent hydraulic fracturing,a calculation model for the coupled stress field with multiple induced fractures preexisted was established based on the calculation model for the stress field with single induced fracture preexisted,and the change laws of circumferential stress field around the wellbore under the effect of induced stress were analyzed.Then,the fracture initiation pressure of subsequent hydraulic fracturing was calculated according to the fracturing mechanics criterion.Finally,the effects of the length,phase,horizontal principal stress difference co-efficient and quantity of preexisting fractures on its initiation pressure were analyzed.And the following research results were obtained.First,the circumferential stress difference in the area near the fractures behind the preexisting fractures increase greatly and even the horizontal principal stress is reversed.Second,as the length of new preexisting fractures increases,the initiation pressure of initial preexisting fractures rises first and then drops.And when the length of new preexisting fractures is equal to that of the initial preexisting fractures,the initiation pressure of initial preexisting fractures drops quickly first and then slowly.Third,the initiation pressure of high phase fractures is lower than that of low phase fractures.Fourth,with the increase of fracture quantity,the initiation pressure of new preexisting fractures drops gradually,but the decline trend of initiation pressure difference is not obvious.Fifth,the stress interference of multiphase fractures influences the fracture initiation pressure,and high-phase long fractures and low-phase short fractures are both favorable for the simultaneous initiation of multiphase fractures.Sixth,syn-chronous propagation of multiple fractures can generate more complicated stress interference and excite the evolution of hydraulic fracture network,so as to realize uniform fracturing stimulation.In conclusion,the research results can provide a theoretical guidance for the design of fracturing operation,e.g.perforation phase of blasting induced fracturing,scale of induced fractures,and pump pressure of subsequent hydraulic fracturing.
