横观各向同性砂岩储层水平井井壁应力分析

The analysis of horizontal wellbore stress in transversely isotropic sandstone reservoir

在经典的井壁应力场计算模型中,将地层岩石力学性质视为各向同性,而对于砂岩这类层理性地层,垂直于层理面和平行于层理面的岩石力学性质存在明显差异,即砂岩为横观各向同性体,因此,基于各向异性材料力学理论,建立了正交各向异性地层中井壁应力场计算新模型。新模型表明,与力学性质各向同性相比,力学性质正交各向异性条件下井壁径向应力和剪应力没有发生改变,而周向应力发生了变化,周向应力是岩石力学性质、地应力、井筒压力的函数。并运用此模型分析了横观各向同性砂岩储层水平井井壁应力分布特征,分析表明,在弹性模量各向异性度较大时,或者地应力各向异性度较小时,周向拉应力会出现四个最值点,当拉应力超过岩石抗张强度时,水力裂缝在这四个最值点同时起裂,从而解释了水力压裂过程中出现的近井筒效应。岩石弹性模量各向异性度对井壁周向应力分布有较大影响,而泊松比各向异性度对井壁周向应力分布影响很小。

In the calculation model of classic borehole wall stress field, the mechanical property of formation rock is regarded as isotropic body, however, due to the obviously variation of rock mechanical properties in vertical and parallel of bedding plane, sandstone is looked as transversely isotropic body. Based on anisotropic material mechanics theory, the new calculation model of bore- hole wall stress field in orthotropy formation is established. The result shows that, compared with isotropic mechanical property, wellbore radial stress and shear stress are not changed, but the circumferential stress is changed, which is the function of rock mechanical property, earth stress and wellbore pressure. By the adoption of this model, this paper analyzes horizontal wellbore stress characteristics of transversely isotropic sandstone reservoir, and the result shows that when the anisotropy of elastic modulus is great or when the anisotropy of earth stress is small, there occurs four maximum points by circumferential stress, moreover, when circumferential stress exceeds rock tensile strength, hydraulic fractures initiate in the four maximum points simultaneously, thereby analyzing near wellbore effects in the hydraulic fracture process. The study shows that the anisotropy of rock elastic modulus has great effects on circumferential stress, while the anisotropy of Poisson＇s ratio has little effects.