摘要
受层理特征的影响,页岩地层的井壁坍塌形状与坍塌压力不再与各向同性地层表现一致。基于Jaeger弱面强度准则计算了井壁坍塌区域分布与坍塌压力大小,结果表明:井壁坍塌是页岩基质与弱面破坏叠加的结果,且当层理弱面倾斜角增大时,井壁坍塌逐渐受层理弱面破坏主导,坍塌方位不再与水平最小地应力方位一致,即井壁坍塌区域发生偏转,井壁坍塌形状由对角破坏演变为四角破坏;各向异性地层井壁坍塌压力只沿水平最小地应力(层理面倾向)对称分布,最优钻井方位不再仅沿水平最小地应力方位,而是应结合层理弱面的倾向,尽量减小井眼轨迹与层理弱面法向夹角;考虑钻井液侵入对井周地层压力的改变与地层强度的降低,钻井液的使用应在确保井壁力学稳定性的同时提高钻井液封堵性且使用尽量小的钻井液密度。本文研究结果可为预防页岩井眼钻进过程中井壁坍塌提供参考。
In view of the lamination structure of shale formations,borehole caving appearance and caving pressures are not the same as the boreholes drilled in isotropic formations. The distribution of caving zones and caving pressures were calculated according to Jaeger weak plane criterion,showing that borehole caving is the consequence of weak plane shear failure overlapped by shale matrix failure; moreover,weak plane failure becomes increasingly dominating with increasing weak plane dip angle. Deflection of caving azimuth happens and no longer aligned with the minimum horizontal stress. Caving appearance changes from being in two opposite angles to four opposite angles; caving pressure distributes symmetrically along the minimum horizontal stress orientation（ weak plane dip direction） while drilling in anisotropic formations. As a result the optimal drilling direction is not merely the minimum horizontal stress orientation,and the best way is minimizing the angle between the wellbore and weak plane normal direction. Borehole collapse cycle is studied considering redistributed pore pressure and decreasing formation strength because of drilling fluid filtration; the results indicate that drilling fluid should have a low mud weight and high sealing effect to ensure borehole mechanical stability. This study can provide reference for preventing borehole collapsing when drilling in shale layers.
出处
《中国海上油气》
CAS
CSCD
北大核心
2017年第2期114-122,共9页
China Offshore Oil and Gas
基金
国家自然科学基金项目"裂缝性油气储层水力裂缝模拟的有限元方法(编号:11502304)"
中国石油大学(北京)人才引进项目"基于增强有限元方法(A-FEM)的水力压裂数值模拟研究(编号:2462013YJRC023)"
青年创新团队项目"深层非常规储层岩石断裂特征研究(编号:C201601)"部分研究成果
