泥页岩井壁坍塌周期分析

应用力学／化学耦合理论对泥页岩井壁延迟坍塌机理进行了分析。以多孔介质渗流力学为基础，建立了近井壁地层孔隙压力的计算模型，计算了不同时期的临界坍塌压力，分析了钻井液物性参数对井壁稳定性的影响，得到了近井壁地层孔隙压力及强度参数在时空域内的分布，以及一定钻井液密度下泥页岩井壁坍塌破坏的时间。算例分析表明，在同一膜效率下，随着钻井液水活度增加，坍塌周期缩短；在同一钻井液水活度下，提高膜效率可以延长坍塌周期；钻井液水活度越小，膜效率的影响越显著。现场选择钻井液时，同时考虑水活度和膜效率并根据井壁坍塌压力改变钻井液密度，效果才会更好。

Analysis of borehole collapse cycling time for shale

The time-delayed collapse mechanism of shale borehole was analyzed by theory of hydraulic-chemical effects. A calculation model of pore pressure nearby borehole was developed based on the hydrodynamics, and the critical collapse pressures were calculated at different period and the effects of drilling fluid parameters on the borehole stability were analyzed. The distribution of pore pressure and strength parameters of formation nearby borehole in time-space domain were investigated by the model, and collapse time was calculated when drilling with a given drilling fluid density. The analysis results show that the collapse cycling time shortens with the water activity of drilling fluid increasing at the same membrane efficiency, and the collapse cycling time prolongs with the membrane efficiency increasing at the same water activity of drilling fluid. The effects of membrane efficiency become obviously at low water activity of drilling fluid. Both the membrane efficiency and the water activity of drilling fluid should be considered, and the drilling fluid density should be changed according to borehole collapse pressure.