Study On Numerical Simulation Of Hydraulic Fracturing Fracture Propagation Based On PPCZ Model

In geothermal mining,hydraulic fracturing technology is often used to develop and transform reservoirs to improve the permeability of reservoir rocks and improve the mining efficiency of geothermal energy.Hydraulic fracturing technology injects water into the original cracks of the rock matrix and applies fluid pressure to cause cracks to initiate and expand,thereby creating cracks in the rock formation.In the process of hydraulic fracturing,the development of hydraulic fractures will be affected by many factors such as the physical properties of rock matrix,initial fractures,in-situ stress conditions,natural fractures,and fracturing fluid injection conditions.Therefore,in order to predict the development behavior of fractures during hydraulic fracturing,it is necessary to study the influence of various conditions on the development of hydraulic fractures.In this paper,the applicability of the finite-discrete element method(FDEM)based on the PPCZ model to simulate the hydraulic fracturing process is verified by numerical simulation combined with the theoretical solution of the KGD model.The crack propagation law of two-dimensional hydraulic fracturing is studied by numerical simulation method,which mainly includes three aspects : the propagation law of hydraulic fracture under the condition of two-dimensional single fracture water injection,the interaction between hydraulic fracture and natural fracture,and the propagation law of hydraulic fracture in anisotropic rock.The specific research contents are as follows :(1)Based on the PPCZ(Pore Pressure Cohesive Zone)model,a two-dimensional hydraulic fracturing numerical model is established.Through the hydraulic fracturing numerical simulation experiment of single fracture water injection conditions,the results of the injection hole pressure with time are compared with the KGD high analytical solution model.(2)The effects of ground stress,initial fracture angle and initial fracture length on the development of hydraulic fractures under two-dimensional single fracture water injection conditions were investigated by numerical simulation.The results show that under the condition that the ground stress conforms to the hydrostatic pressure,that is,when the principal stresses of the ground stress in the two directions on the horizontal plane are equal,the initiation pressure increases linearly with the increase of the ground stress,and for the initial cracks at different angles,the initiation pressure is basically affected by the change of the ground stress.In the presence of deviatoric ground stress,that is,the principal stress of the ground stress in the two directions on the horizontal plane is unequal.Under the condition of ground stress difference,the hydraulic fracture will deflect to the direction of the maximum principal ground stress.At this time,the crack initiation pressure of the fracture is mainly affected by the ground stress with a larger angle to the initial fracture.The crack initiation pressure increases with the increase of the ground stress,and the local stress difference is large.When the angle between the initial fracture and the horizontal direction is close to 45 °,the tip of the hydraulic fracture is prone to shear fracture.The crack initiation pressure decreases with the increase of the initial crack length,but the decreasing effect gradually slows down.(3)The interaction between hydraulic fracture and natural fracture is numerically simulated.The numerical model of hydraulic fracturing with natural fracture surface is established,and three typical interaction modes are analyzed by linear elastic fracture mechanics.By changing the in-situ stress difference and interaction angle,the influence of in-situ stress difference and interaction angle on the interaction mode of cracks is explored,and the distribution area diagram of interaction between hydraulic cracks and natural cracks under the condition of in-situ stress difference-interaction angle is given.Finally,the influencing factors of the crossing area are explored.The results show that natural fractures have a great influence on the development of hydraulic fractures and the change of injection pore pressure.The larger the in-situ stress difference,the easier it is for hydraulic fractures to cross natural fractures.The larger the interaction angle between hydraulic fractures and natural fractures,the easier it is for hydraulic fractures to cross natural fractures.Under the combination of low in-situ stress difference and low interaction angle,natural fractures are easy to open.Under the combination of high in-situ stress difference of about-45 °,natural fractures are prone to shear slip.Under the combination of high in-situ stress difference and high interaction angle,hydraulic fractures are easy to penetrate natural fractures.The use of high injection rate or high viscosity fracturing fluid can expand the crossing area in the distribution map of the interaction between hydraulic fractures and natural fractures,and hydraulic fractures are easier to cross natural fractures.(4)Through the secondary development of PPCZ model,the anisotropic rock matrix numerical model with tensile strength changing with angle is constructed,and the law of hydraulic fracturing of anisotropic rock is explored by numerical simulation.The results show that the greater the anisotropy of rock matrix,the greater the impact on fracture development.During the fracturing process,the initiation pressure is not only directly affected by the change of normal stress on the fracture surface caused by in-situ stress,but also further affected by the change of hydraulic fracture initiation and propagation direction caused by in-situ stress change.The maximum in-situ stress and the weak plane direction of rock matrix jointly affect the trend of hydraulic fractures.