长庆西区气探井复杂地层井壁稳定性研究

Wellbore Stability of Complex Formation of Gas Exploration Well in Changqing West Area Reservoir

摘要: 长庆西区钻气探井过程中井壁失稳问题频发，严重阻碍了现场作业进程。针对乌拉力克组页岩钻井过程中发生的剥落掉块、垮塌现象，通过室内微观组构、理化性能及力学特征测试，明确井壁失稳机理；建立考虑弱面作用的力化耦合井壁稳定模型，评价复杂地层井壁稳定性，并优化井眼轨迹。结果显示：乌拉力克组页岩属于弱膨胀性脆性页岩，微裂缝和层理发育，为钻井液滤液提供有利渗流通道，产生尖端因子效应；钻井液浸泡和层理角度对页岩力学特征具有劣化效应，在层理角度为30˚时岩石力学强度最低；钻井液浸泡作用下，化学势差和水力压差作用下，近井带孔隙压力增加，岩石强度降低，井周坍塌失稳区域增大；钻井液浸泡前期，井周失稳区域较大，随着浸泡时间增加，井周失稳区域增加速率降低；岩石弱面特征使地层坍塌压力增大，采用井斜方位角为30˚~240˚、井斜角0~28˚钻井相对较为安全。

Abstract: Shale Wellbore instability occurs frequently during drilling the gas exploration well in the Changqing West area, seriously hindering the field operation. In view of the problems of shale spalling and collapse for the Wulalike Formation, its wellbore instability mechanism is determined by the testing of microfabric, physical and chemical properties and mechanical characteristics at an indoor experiment. Considering the weak plane, a dynamic coupled wellbore stability model is established to evaluate the wellbore stability of complex formations and optimize the borehole trajectory. The results show that the shale in the Wulalike Formation is weakly expanded brittle shale, with rich micro-fractures and bedding, which provide favourable seepage channels for drilling fluid filtrate and produce tip factor effect. Drilling fluid immersion and bedding angle have a deterioration effect on shale mechanical characteristics, and the rock mechanical strength is the lowest at the bedding angle of 30˚. Under the action of drilling fluid immersion, chemical potential difference and hydraulic pressure difference, pore pressure increases in the near-wellbore zone, rock strength decreases, and the collapse instability area increases. In the early stage of drilling fluid immersion, the wellbore instability area increases, and the rate of increase of the wellbore instability area decreases with the increase of immersion time. The weak plane of the rock makes the collapse pressure increase, so it is relatively safe to drill at the azimuth of 240˚~30˚ and the inclination angle of 0˚~28˚.