层理页岩水平井井周剪切失稳区域预测方法

Prediction Method of Shear Instability Region around the Borehole for Horizontal Wells in Bedding Shale

为了揭示层理页岩强度各向异性对水平井井周剪切失稳的影响规律,基于直剪试验研究了川南龙马溪组页岩的强度各向异性特征,并与国外典型层理页岩进行了对比;建立了层理页岩井周坐标转换关系模型,结合井周应力弹性解、Mohr-Coulomb弱面强度理论、Mohr-Coulomb准则,建立了层理页岩地层井周剪切失稳区域预测方法;分析了钻进方位对井周剪切失稳区域分布规律的影响。结果表明:弱面与本体强度参数基本呈线性关系,本体内聚力约为弱面内聚力的2.10倍,本体内摩擦角约为弱面内摩擦角的1.20倍,其中川南龙马溪组页岩强度各向异性特征更加显著;考虑层理等弱面影响计算出的失稳区域的形状不是传统的"狗耳朵"形状,而是近似于长方形,这种形状与典型的物模试验结果基本吻合;若页岩地层中存在显著的层理,则井周地层更容易沿弱面剪切滑动,导致水平井坍塌失稳的风险显著增大。利用该方法可以分别计算出井周地层沿弱面剪切、本体剪切和全部剪切的失稳区域分布,为层理发育地层井眼稳定分析、成像测井反演地应力方位等研究提供基础理论支撑。

In order to understand the effects of strength anisotropy of bedding shale on shear instability re-gion around the borehole of a horizontal well ,the strength anisotropy characteristics for Longmaxi shale in the southern of Sichuan were researched based on direct shear tests ,and the test result comparison was made with typical bedding shale data of some other countries ,so as to establish the coordinate transformation model a-round the borehole in the bedding shale .In combination with stress solution around a borehole ,Mohr-Coulomb weak plane strength theory ,Mohr-Coulomb criterion ,the prediction method of shear instability region around the borehole of a horizontal well in bedding shale was established ,and the influence of borehole azimuth on shear instability region was also analyzed .It showed that strength parameters of weak plane and rock mass are basically in a linear relationship as the cohesion and internal friction angle of rock mass are about 2.10 and 1.20 times of those of weak planes ,respectively .Strength anisotropy was more evident in Longmaxi shale .The shear instability region around the borehole was not in the commonly seen “dog ear” shape due to the weak planes .Instead ,it had a square shape ,w hich agreed with typical physical experimental results .If bedding planes in shale reservoir are significantly developed ,the formation around borehole may experience shear sliding along the weak planes ,which would increase the risk of instability in horizontal wells .The prediction method in this paper could be used to calculate the shear instability regions from weak planes shearing and rock mass shearing providing the basis in borehole stability analysis of bedding formation and the in-situ stress orientation inversion from imaging logging .