考虑有效膜压力的坍塌压力计算模型

A calculation model of collapse pressures with consideration to the effective diaphragm pressure

川渝气区风险探井欠平衡井段的井眼扩径率普遍较高,井眼扩径率介于20%~30%,最高可达140%,增加了风险探井的钻探风险,而且欠平衡钻井井壁稳定的问题也尚未得到有效解决。为此,针对欠平衡井段井壁失稳机理开展了研究,提出了计算欠平衡钻井井壁坍塌压力的方法：通过引入有效膜压力系数,结合达西定律、地层流体状态方程、连续性方程,求解出井周压力分布解析解;依据叠加原理得到了井周应力分布模型,并采用Mohr-Coulomb准则推导出维持井壁稳定的最低钻井液密度计算模型;研究了欠平衡、过平衡、欠平衡转过平衡3种典型工况下的井周压力演化规律,并总结了欠平衡钻井井壁失稳的规律。结论认为：该模型反映了泥饼对井壁稳定的影响,由其计算得到的坍塌压力比常规模型计算所得的结果要高,说明有效膜压力系数对井壁坍塌的影响显著,而岩石强度（内聚力、内摩擦角）、地应力、岩石孔隙度等参数对井壁坍塌的影响则相对较小,采用该模型计算的结果更加符合现场实际。

In Sichuan-Chongqing gas areas, the hole enlargement ratio at underbalanced well sections of wildcat wells is generally in the range of 20-30%, or even up to 140%. The high hole enlargement ratio increases the drilling risk of wildcat wells. In addition, wellbore stability during underbalanced well drilling cannot be effectively guaranteed. In this paper, therefore, the wellbore instability mechanism of underbalanced well sections was investigated. The calculation method for collapse pressure of underbalanced drilling wellbore was proposed. Specifically, after the effective diaphragm pressure coefficient was introduced, the analytical solution of the circumferential pressure distribution was solved by means of the Darcy＇s law, the formation fluid state equation and the continuity equation. According to the superposition principle, the circumferential stress distribution model was built up. The calculation model was established for the minimum drilling fluid density for stable wellbores by using the Mohr-Coulomb criterion. With this model, the evolution laws of circum- ferential pressures in three typical conditions, i.e., underbalanced-drilling （UBD）, overbalanced-drilling （OBD） and UBD turned to OBD, were analyzed. Besides, the laws of wellbore instability at underbalanced well sections were presented. It is concluded that this model re- flects the impact of mud cakes on wellbore stability and its calculated collapse pressure is higher than that by conventional models. This indicates that borehole wall collapse is significantly affected by the effective diaphragm pressure coefficient, but less affected by rock strength （cohesion and internal friction angle）, ground stress and rock porosity. The calculation results of the proposed model can describe the real situations more accurately.