硬脆性泥页岩斜井段井壁稳定力化耦合研究

A chemo-mechanical coupling model of deviated borehole stability in hard brittle shale

建立了考虑结构特点和弱面水化的硬脆性泥页岩斜井段井壁稳定力化耦合模型,利用模型分析了坍塌压力分布的影响因素,并进行了现场实例分析。基于硬脆性泥页岩理化性能特点,根据钻井液活度-岩石膨胀率、岩石活度-吸水量等关系,给出了钻井液活度窗口,进而确定岩石临界膨胀率及合理吸水量,并给出了坍塌压力的计算方法。分析发现:弱面倾角固定时,随着弱面倾向的变化,坍塌压力呈四分之一对称分布,且不存在坍塌压力单调递增或单调递减的井斜方位,危险区域和安全区域交替出现;与弱面黏聚力相比,弱面内摩擦角的水化程度对坍塌压力分布的影响更大。现场实例分析结果表明:利用提出的力化耦合模型可以准确地预测坍塌压力分布,只要将钻井液活度控制在活度窗口内,就能保证井壁稳定并降低钻井液密度。

A chemo-mechanical coupling model of borehole stability in hard brittle shale considering structure characteristics and targeted hydration was established, the influencing factors of the distribution of collapse pressure were analyzed based on the model, and a field case analysis was conducted. Based on the physicochemical properties of hard brittle shale, a drilling fluid activity window was proposed for calculating collapse pressure by establishing the relationships of drilling fluid activity vs. swelling ratio of rock and rock activity vs. moisture content to determine critical swelling ratio of rock and reasonable moisture content. The results show that, when fixing the dip angle of weak plane, the collapse pressure appears a quarter symmetric distribution with the change in tendency, there is no azimuth angle who has a monotonic increasing or decreasing collapse pressure, and dangerous sections and safe sections exist alternately; compared with cohesion of weak plane, collapse pressure is more sensitive to internal friction angle. Field case shows that, accurate prediction of collapse pressure distribution can be obtained by the chemo-mechanical coupling model, in which borehole stability can be ensured and the density of drilling fluid can be decreased as long as the drilling fluid activity is controlled in the window.