基于孔隙弹性耦合的水力压裂数值模拟研究

The Coupling Method and Numerical Simulation of Hydraulic Fracturing Based on Poroelasticity

系统地阐述基于多孔介质渗流-损伤耦合原理,进行水力压裂的FEM数值实现方法.基本架构为:(1)引入孔隙流体压力膨胀系数将孔隙流体压力与应力场进行耦合;(2)基于损伤局部化模型,提出裂缝张开度表达式;(3)提出水力压裂引起的多孔介质水-力学属性的各向异性表达式;(4)提出全流量加载的耦合分析方案.作为实例,模拟三维地层水压裂缝扩展形态,通过比较模型的数值解和经典理论解,验证该方法的正确性.

Hydraulic fracturing is an important technology in enhancing the permeability of petroleum and natural gas reservoirs.Through on-going fluid injection,the fluid pressure energy built up within the solid skeleton can break the porous formation step by step.In return,the fracture propagation will cause more accumulation of fluid pressure energy and larger fracture size.The key problems to deal with in numerical simulation of hydraulic fracturing using the FEM are the coupling effects between fluid pore pressure and in-situ stress through solid skeleton deformation and the hydro-mechanical property parameters under hydraulic fracturing.In the present paper,we set up a complete method of numerical simulation using the weak coupling FEM.Firstly,the in-situ stress is simulated for the borehole being drilled,through which the stress and permeability redistribution near the borehole are calculated.Then computations of porous flow and stress redistribution are interactively looped.In the computation of porous flow,the fluid rate is loaded on the borehole wall,the input parameters are permeability and compressible coefficients which are related to the current stress and pore pressure,the result output is the new distribution of pore pressure,which in the computation of stress redistribution,will be an expansive force acting on the solid skeleton and producing new stress distribution.The hydraulic fracturing takes place when the total tensile strain attains its strength limit.As the loop is executed,the hydraulic fracture size grows larger and larger.The main points on which the numerical method is based include：（1）the concept of expansion coefficient for pore fluid,which is employed to couple the fluid pore pressure with the stress field;（2）the calculation method for hydraulic fracture opening,which is proposed based on the concept of damage localization;（3）the calculation method for anisotropic properties such as damage,permeability,poroelasticity,etc.;（4）the coupling loading scheme using full flow rate.In the end,a numerical simulation of three-dimensional hydraulic fracturing is performed.The comparison between numerical solution and analytical solution of the hydraulic fracturing validates the correctness of the numerical method.The numerical method proposed in the paper can be used for the simulation of complex fracture growth in inclined formation under complex in-situ stresses.