欠平衡钻井坍塌压力计算模型

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。在欠平衡钻井过程中，地层流体不断流入井内，促使井筒周围的应力将伴随地层流体的流入而重新分布，进而影响井壁的稳定性。根据原地应力产生的应力与地层流体向井眼径向流动产生的拖拽力叠加而求得了欠平衡钻井井周应力的解析解，考虑欠平衡钻井过程中地层流体对岩石拖拽作用后地层坍塌压力更大即井壁更易失稳，运用Mohr-Coulomb强度准则建立了欠平衡钻井坍塌压力的计算模型。将所建立的计算模型应用于塔中722井的井壁稳定性分析，结果表明：理论分析结果与工程实测数据对比，计算结果较为准确。

A MODEL FOR CALCULATING CAVING PRESSURE DURING UBD

Borehole stability is the key for successful underbalanced drilling (UBD). The formation fluid will flow into the borehole unceasingly during UBD, which will cause the stress redistribution in the formations around the borehole and therefore will affect the borehole stability. The solution of stress distribution around borehole during UBD has been obtained by superimposing the stress produced by the original earth stress and the drag force produced when formation fluid flows radially into borehole through the rock. Considering the formation caving pressure during UBD is getting higher due to the rocks dragged by formation fluid, namely the wellbore is easier to collapse, the caving pressure calculating model for UBD has been set up by using Mohr Coulomb strength failure criteria. After application in the wellbore analysis for the well Tazhong 722, it is found the theoretical results match the actual situations, and the model can provide accurate calculation.