基于弹性应变能强度准则评价井壁稳定性

岩石强度准则是石油工程的重要基础理论,对于井壁稳定性研究更是不可或缺。然而,以经典弹塑性理论的岩石强度准则作为井壁失稳判断依据时所得结果,往往与现场实际情况存在较大误差。开展三轴力学试验,测试岩石不同围压条件下的极限储能,从能量角度建立岩石能量强度准则,并与其他强度准则对比分析了井壁的稳定性。计算结果表明:使用不同强度准则所得坍塌压力相差较大,其中Hoek-Brown准则的坍塌压力过低且存在井壁坍塌风险,Mohr-Coulomb和Drucker-Prager准则的结果过于保守且存在压破地层风险,而根据能量强度准则所得坍塌压力则能保证井壁不塌不漏。该准则是井壁稳定评价的一种新方法,为钻井液密度设计提供了可靠的理论依据。

WELLBORE STABILITY EVALUATION BASED ON THE ENERGY STRENGTH CRITERION OF THE ELASTIC STRAIN

Rock strength criterion is the important basic theory in petroleum engineering,especially in the study of the wellbore stability.But comparing with the field fact,there is a larger error between the obtained results by the rock strength criterion of the classical elastoplastic theory regarded as the judging evidence of the wellbore stability failure.Therefore,the triaxial mechanic test was performed to get the limiting store energy of the rocks under different peripheral pressures,the energy strength criterion was established from the viewpoint of the energy,and moreover comparing with the other strength criteria,the wellbore stability was analyzed.The calculated results show that there are much bigger differences among the achieved collapse pressures by different strength criteria:in detail,the obtained collapse pressure by Hoek-Brown criterion is rather lower and furthermore it may cause the wellbore unstable;while those by Mohr-Coulomb and Drucker-Prager criteria are so conservative and the strata may be fractured;the collapse pressure obtained by the energy strength criterion can not only prevent the wellbore not-to-be-collapsed,but also prevent the wellbore not-to-be-leaked,so the criterion can be regarded as the new evaluating method of the wellbore stability and provide the reliable theoretical evidence for the density design of the drilling liquid.