渗流对欠平衡钻井井壁稳定性的影响

Effect of percolation on wellbore stability of underbalanced drilling

何世明 安文华 王书琪 陈勉 骆发前 迟军

HE Shiming AN Wenhua WANG Shuqi CHEN Mian LUO Faqian CHI Jun

西南石油大学石油工程学院，四川成都　610500 塔里木油田公司，新疆库尔勒　841000 中国石油大学，北京　102249

School of Petroleum Engineering, Southwest Petroleum University, Chengdu 610550, China Tarim Oil Field Company, Korla 841000, China China University of Petroleum, Beijing 102249, China

欠平衡钻井过程中井壁稳定是保证欠平衡钻井成功的关键。在常规钻井时，井壁不稳来自力学和化学作用，而欠平衡加大了这些作用机理。在欠平衡钻井过程中，地层流体不断流入井内，井眼形成后井筒周围的应力将伴随地层流体的渗流而重新分布，进而影响井壁的稳定性。文章将由原地应力产生的应力与地层流体向井眼径向渗流产生的应力叠加而求得了欠平衡钻井井周应力的解析解，运用Mohr-Coulomb强度准则建立了欠平衡钻井坍塌压力的计算式。计算结果表明：在欠平衡钻井过程中，考虑地层流体在岩石中的渗流作用后、地层坍塌压力更大即井壁更易失稳，应用于塔中722井的井壁稳定性分析表明，理论结果与工程实际吻合，说明了模型的正确性，可进一步推广应用。

Borehole stability is the key for successful underbalanced drilling. During conventional drilling, borehole instability is caused by the mechanical and chemical mechanism, which gets larger by the underbalance. The formation fluid flows into the borehole unceasingly during the underbalanced drilling. After the borehole is formed, the stress around the borehole will be distributed again accompanied by the percolation of the formation fluid. Then the borehole stability will be influenced. The solution of stress distribution around borehole is obtained regarding the rock as porous medium, considering the earth stress and the additional stress when the formation fluid flow into borehole through the rock. The caving pressure formula of underbalanced drilling is established based on the Mohr-Coulomb strength criteria. The calculation result shows that during the underbalanced drilling the situation of the borehole will get instable considering the percolation when the formation fluid flows into borehole. The application to the Tazhong 722 well shows that the theoretical results match the practical situation. It is indicated that the model is verified and can be popularized further.