裂缝性基岩气藏水力压裂起裂压力与近井形态研究

Fracture Initiation Pressure and Near-Wellbore Fracture Geometry of Hydraulic Fracturing in Bedrock Gas Reservoirs with Natural-Fracture

为了进一步明确裂缝性基岩气藏水力压裂起裂压力和裂缝起裂形态,提出了天然裂缝应力强度因子解析方法,建立了天然裂缝地层起裂压力模型。运用高斯数值积分和二分法求解非线性积分方程,编制计算程序,通过有限元分析对比验证模型的合理性。基于该模型,计算分析了天然裂缝长度、岩石断裂韧性和地应力对起裂压力的影响规律。研究表明:横切天然裂缝越长,起裂压力就越低,纵向天然裂缝起裂压力则随缝长增大而先下降后上升;天然裂缝越长,就越容易发生横切裂缝起裂,且当近井存在较短纵向天然裂缝时,纵向天然裂缝会优先起裂;岩石断裂韧性越小,横切天然裂缝起裂压力就越低,高断裂韧性有利于纵向天然裂缝起裂;纵向裂缝在高应力差时起裂压力高,难以起裂;在高垂向应力条件下,纵向裂缝起裂压力降低,近井容易形成纵向裂缝。

To investigate fracture initiation pressure and fracture geometry of hydraulic fracturing in bedrock gas reservoirs,analytical formula of stress intensity factor for transverse natural fractures and longitudinal natural fractures are established based on weight function method in fracture mechanic,and then mathematical model for fracture initiation pressure is proposed.The Gauss integral and bisection method are used to solve the nonlinear integral equation,and the calculation program is compiled.The accuracy of the model is verified by comparing with the finite element numerical simulation results.Based on this model,the influence of natural fracture length,fracture toughness and in-situ stress is calculated and analyzed.The results show that the lager the length of near-wellbore transverse natural fractures,the easier the transverse fracture will initiate.With the increase of fracture length,the fracture initiation pressure of longitudinal natural fracture first decreases and then increases.The longer the natural fracture is,the more likely it is to produce cross cutting fracture.When there is a short vertical natural fracture near the well,the vertical natural fracture will have priority to crack initiation.The smaller the fracture toughness is,the lower the fracture initiation pressure is.High fracture toughness is beneficial to the initiation of longitudinal natural fractures.High stress difference results in high initiation pressure of longitudinal factures.The pressure of longitudinal crack initiation decreases under the condition of high vertical stress,so it is easy to form longitudinal fracture near the well under the condition of high vertical stress.