钻井液密度对裂缝性泥页岩地层坍塌压力影响的力化耦合研究

 缝性地层井壁失稳是钻井工程中经常遇到工程难题之一，裂缝发育地层常伴随严重的井壁坍塌，具有水化性质的裂缝性地层在钻井液侵入后更易发生坍塌。扫描电镜观察某油田泥页岩发育的层理实际为尺寸极小的微裂缝，使用XRD 衍射仪测定了该泥页岩黏土矿物质量分数为30%~40%，泥页岩水化性质较强。通过剪切实验测定了泥页岩吸水后的抗剪强度，求解了不同含水量裂缝面的黏聚力及内摩擦角。建立了钻井液滤液向地层内的渗流方程及裂缝性地层坍塌压力方程，计算了不同钻井液密度和浸泡时间的坍塌压力。计算结果表明，高钻井液密度导致钻井液滤液向裂缝内加速渗流，裂缝面强度降低导致地层更加容易坍塌。裂缝性地层井壁失稳不宜提高钻井液密度稳定井壁，应采用措施提高钻井液封堵性及抑制性，降低钻井液滤液侵入量。

Impact of Drilling Fluid Density on Fractured Shale Collapse Pressure by Chemical-mechanical Method

The wellbore instability of a fracture shale formation is an issue often encountered in drilling. A fracture formation of the hydration property is often associated with severe borehole collapse. The shale beddings are actually micro-fractures according to the microstructure observations by SEM. The content of the clay minerals of the shale is 30%-40% determined by XRD equipments, and shows a strong hydration property. The shearing strength of the shale is tested by shear experiments, the cohesion and the internal friction angle are determined under different water contents, and the results are regressed. Based on the Darcy linear percolation law, the seepage equation for the drilling fluid is obtained. Through the well circumferential stress analysis and combined with the above results, the collapse equation is obtained. Collapse pressures are calculated under different mud densities and soak times. The calculation results show that the high density drilling fluid accelerates the seepage with increased water amount and reduces the strength against fracture, which leads to more easy formation collapse. Improving the drilling fluid density is unfavorable for the wellbore stability of the fractured shale formation. Measures should be taken to improve the drilling fluid plugging ability and the inhibitory ability, reducing the amount of mud filtrate invasion.