Numerical Simulation Of Hydraulic Fracturing Fracture Propagation In Shale Gas Reservoirs

Shale gas is an unconventional resource that exists in the shale reservoir.Shale reservoir has the characteristics of low porosity and low permeability.The shale gas must be economically modified through hydraulic fracturing and other reservoir transformation methods.Different fracturing construction parameters and natural fractures in the shale reservoir will affect the propagation of hydraulic fractures,and the final geometry of fracture directly affects the production of shale gas.Therefore,studying the influence of relevant parameters on the propagation of hydraulic fractures in the shale reservoir is the great significance for optimizing on-site hydraulic fracturing design and realizing the economical exploitation of shale gas.Aiming at the problem of hydraulic fracturing fracture propagation in shale reservoirs,a two-dimensional fluid-solid coupled hydraulic fracturing fracture propagation model was established based on the extended finite element method.The hydraulic fracturing simulation software HF＿XFEM was compiled using Fortran.A numerical simulation study of the effects of different construction parameters and natural fractures on hydraulic fracturing fracture propagation using self-made software,the main conclusions are as follows:(1)The effect of key parameters of hydraulic fracturing on hydraulic fracturing,such as prefabricated fracture angle,prefabricated fracture length,horizontal stress difference and fracturing fluid viscosity,in intact rock masses without considering natural fractures.The results show that the crack propagation path under different prefabricated crack angles deflects and expands toward the direction of the maximum principal stress.The greater the difference between the prefabricated crack angle and the horizontal stress,the more obvious the crack propagation path deflection.However,the effect of prefabricated fracture length and fracturing fluid viscosity on the propagation path of fracture is small.When the prefabricated crack angle is greater than 15 °,there is a discontinuous phenomenon in the width of the crack deflection,and the width of the prefabricated crack is smaller than the width of the surrounding fracturing cracks,and the crack surface is uneven.When the prefabricated fracture angle is larger and the prefabricated fracture length is shorter,the width discontinuity of the fracture deflection and the unevenness of the fracture surface are more obvious,which is not conducive to the transportation of proppant and fracturing fluid,and will affect the final reservoir production.The viscosity of the fracturing fluid has a significant effect on the fracture width.The fracture width gradually widens as the fracturing viscosity increases,but the fracture propagation rate decreases.The water pressure is mainly affected by the prefabricated crack angle and the horizontal stress difference.The smaller the prefabricated crack angle and the horizontal stress difference,the greater the cracking water pressure.(2)The extended R & T criterion is used to characterize the intersection of hydraulic fractures and natural fractures,and a fracture intersection model is established.The correctness of the model is verified by comparison with literature tests.It is found that the fracture morphology after the fracture intersects is affected by factors such as the initial in-situ stress difference and the fracture intersection angle.Under the condition of high-stress difference and large fracture intersection angle,hydraulic fractures easily propagate through natural fractures.When the low-stress difference or the crack intersection angle is small,the hydraulic crack is easy to open the natural crack and continue to expand along the natural crack path.(3)The effects of the intersection angle and length of natural fractures on the propagation of hydraulic fractures in rock bodies with natural fractures are studied.It was found that when a hydraulic fracture opens a natural fracture and expands along with it,when the fracture intersection angle is 90 °,the hydraulic fracture easily expands in both directions along the two ends of the natural fracture,forming a “T”-shaped fracture;when the fracture intersection angle is small,the hydraulic fracture easily follows the natural fracture One end of the crack expands unidirectionally,forming an "L" crack.Different natural fracture intersection angles and lengths have a significant effect on the fracture width.The stress concentration phenomenon is prone to occur at the intersection of the fracture and the end of the natural fracture,and the fracture width appears discontinuous.The larger the crack intersection angle and the smaller the natural crack length,the more obvious the crack discontinuity.(4)The influence of the distribution position of natural fractures,the viscosity of fracturing fluid and the horizontal stress difference on the formation of the hydraulic fracturing network under a certain number of natural fractures was studied.It is found that the fracture propagation during hydraulic fracturing will affect the stress distribution state of the reservoir,affect the opening of natural fractures and the subsequent hydraulic fracture propagation,and it is easy to generate an asymmetric hydraulic fracture network.The distribution of natural fractures is that the number of natural fractures communicated by the hydraulic fracture network generated when the azimuth angle is 90 ° and the greater the difference in in-situ stress,the more complicated the fracture network structure,the larger the area of reservoir transformation.As the key parameter of the hydraulic fracturing network,the viscosity of fracturing fluid is too large or too small to obtain a complicated hydraulic fracturing network and a large area for reservoir reconstruction.The actual fracturing operation is reasonably selected the viscosity of fracturing fluid due to the combination of geological reservoir conditionsBased on the extended finite element method,the two-dimensional hydraulic fracturing numerical software HF＿XFEM compiled by Fortran can be used to simulate the hydraulic fracturing construction parameters and simulate the hydraulic fracturing of single hydraulic fractures and the intersection of hydraulic fractures and natural fractures to generate complex fracture networks.It can provide a reference for on-site fracturing construction.