Numerical Study On Hydraulic Fracturing Of Tight Reservoir Based On The Extended Finite Element

Unconventional resources are quite rich in the world,but they are difficult to exploit because of the extremely low permeability.Under the circumstances,hydraulic fracturing technology has been the main means of enhancing production for low permeability reservoirs.In addition,there exists a large number of natural fractures and bedding structure due to the complexity geology and they contribute to form network with hydraulic fracture.Therefore,it is of great importance to study the influence factors of hydraulic fracturing process to ensure the formation of effective fracture network.This thesis proposes a two-dimensional hydraulic fracturing model based on the discrete fracture model and the extended finite element.Meanwhile,the model considers the flow of fracturing fluid,rock deformation,fluid-solid coupling and the intersection of fractures.For meshing of numerical simulation,the regular orthogonal grid is applied owing to the advantage of the extended finite element.The fluid-solid coupling will cause the interaction between the seepage in the rock and the deformation of the rock skeleton.In other words,the pressure field affects rock deformation and in turn the fracture width determines the fracture permeability.In order to accurately describe the influence of in-situ stress,the displacement field is decomposed into the initial part by in-situ stress and the effective one caused by fracturing fluid.On the other hand,for solving the stress intensity factor,the domain form of interaction J integral considering the influence of body force is adopted to avoid the error caused by the stress singularity of crack tip.The calculations of the proposed model are compared with the predictions of the plane strain KGD model for verifying the reliability.The numerical solution of the fracture length and the maximum fracture width during the fracture propagation is consistent with the theoretical.Moreover,the experimental results and intersecting criteria of fractures are compared with the numerical results and they are also consistent.The influence of fracturing fluid flux,matrix permeability,fluid-solid coupling coefficient and perforation angle on hydraulic fracturing is studied.Simulation results show that the fracture length is longer and the maximum fracture width is wider as the increase of fracturing fluid flux in a certain range.However,when it exceeds a certain value,the maximum fracture width of the fracture almost remains the same.For the matrix permeability and fluid-solid coupling factor,they both affect the body force.If the matrix permeability is higher,the fracture will be shorter because of the smaller force on the rock skeleton.Oppositely,if the coupling coefficient increases,thepropagation length of fracture will also increase.The perforation angle affects the extension direction of fracture,but the final propagation of the fracture tends to the maximum principal stress direction for different perforation angles.In addition,the propagation of fractures in simultaneous fracturing of multiple hydraulic fractures is also studied.The results show that there exists stress interference among multiple hydraulic fractures,and it will weaken with the increase of perforation distance.Under the condition of a certain perforation distance,different number hydraulic fractures cause different stress interference each other.The fractures in both sides propagate towards both sides correspondingly.Nevertheless,the fractures in the middle part are not easy to crack and the extension distance is shorter.The effect of perforation direction,approaching angle and the stress difference coefficient on hydraulic fracture propagation considering natural fracture is simulated.Then this thesis also studies the effect of the initial distribution of natural fractures,fracturing fluid viscosity and stress difference coefficient on forming fracture network.The results show that for the small perforation angle and approaching angle,hydraulic fracture just extends along one side of the natural fracture.In that case,it is equivalent to merely change the propagation direction of hydraulic fracture,which cannot effectively enhance production.For small stress difference coefficient,hydraulic fracture extends along the both sides of natural fractures,which easily permeate the further reservoirs and increase the productive area.While if the stress difference coefficient is large enough,hydraulic fracture will quickly turn to the direction of maximum principal stress along natural fractures.The simulation results show that it will be difficult to form network for natural fractures of orderly distribution and the great stress difference.Therefore,detailed geological exploration should be carried out before actual engineering fracturing.The fracturing fluid viscosity should be also adjusted reasonably.The formation of bedding structure is caused by the influence of various factors in petroleum reservoir,which will inevitably affect the hydraulic fracturing.The spacing and bond strength of different bedding planes are studied.The fracturing fluid of hydraulic fracturing is easy to penetrate into the bedding plane.Due to the smaller spacing causing the smaller tensile strength of rock,it will swerve to the weak point of the rock and form the fracture network with the natural fractures of tight reservoir.For the greater bonding strength,fracturing fluid is difficult to penetrate into the bedding plane and the hydraulic fracture will extend along the initial perforation direction.In this condition the fracture network is not easily formed.