页岩气水平井井壁裂缝起裂力学行为研究

Wellbore Fracture Initiation Mechanical Behavior in a Horizontal Shale Gas Well

为了弄清层状页岩各向异性对井壁裂缝起裂力学行为的影响,以四川盆地下志留系龙马溪页岩为研究对象,开展了不同角度下页岩单轴压缩和巴西劈裂实验,明确了页岩弹性和抗张强度各向异性特征,建立了考虑页岩各向异性的水平井井壁裂缝起裂力学模型,并分析了不同因素影响下井壁裂缝起裂规律。结果表明:弹性模量各向异性、地应力比值、抗张强度各向异性和孔隙压力对裂缝起裂压力影响显著,而泊松比各向异性影响较小;弹性模量各向异性、地应力比值对裂缝起裂位置和起裂倾角影响显著,而泊松比各向异性、抗张强度各向异性和孔隙压力的影响较小;井壁裂缝起裂力学行为与井眼方位密切相关,当井眼方位靠近最大或最小水平主应力方向时,井壁裂缝起裂力学行为差异随井眼方位的变化较小;当井眼方位同时远离最大和最小水平主应力方向时,井壁裂缝起裂力学行为差异随井眼方位的变化较大。研究结果可为页岩气水平井钻井井漏预防和水力压裂设计提供理论依据和参考。

In order to clarify the effect of anisotropy of layered shale on the wellbore fracture initiation mechanical behavior,the uniaxial compression testing and Brazilian split testing experiments with different angle were conducted for LMX shale in the Lower Silurian Sichuan Basin.The anisotropic characteristics of elastic modulus and tensile strength were specified,and a wellbore fracture initiation mechanical model of shale gas horizontal wells was established considering the anisotropy of shale.The wellbore fracture initiation behaviors under different influencing factors are also analysed.The results show that:modulus anisotropy,in-situ stress difference,tensile strength anisotropy and pore pressure all have an obvious effect on fracture-initiation pressure but Poisson′s ratio anisotropy has slightly effect on it.Modulus anisotropy and in-situ stress difference also have a significant influence on wellbore fracture initiation location and inclination angle,Poisson′s ratio anisotropy,tensile strength anisotropy and pore pressure,however,have slight effects on that.Wellbore fracture initiation mechanical behaviour is closely related to wellbore azimuth;when the borehole azimuth is close to the direction of maximum or minimum horizontal principal stress,the difference of wellbore fracture initiation mechanical behavior varies slightly with borehole azimuth changing,while when the borehole azimuth is far away from the direction of maximum and minimum horizontal principal stress,the difference of wellbore fracture initiation mechanical behavior varies greatly with the changing of borehole azimuth.The research results can provide theoretical basis and reference for well leakage prevention during drilling and hydraulic fracturing design of shale gas horizontal wells.