含水合物地层井壁力学状态的弹塑性解析分析

Elastoplastic Analytical Investigation of Mechanical Response of Wellbore in Methane Hydrate-bearing Sediments

摘要:

含水合物地层钻井引发井壁周边温度和渗流压力变化，可能导致水合物分解，对井壁安全产生不利影响。基于Mohrp-Coulomb准则、理想弹塑性模型以及非关联流动法则，针对含水合物地层中井壁力学分析， 建立了稳态轴对称平面应变模型，考虑水合物分解对地层温度、渗流和力学特性的影响，得到了欠压和过压钻井条件下井周应力及位移的解析解答，与相同假定下有限元模型、复杂数值模型的结果吻合较好。结果表明一定范围内水合物分解范围的增大会引起塑性区显著扩展。水合物分解引起的弹性模量劣化在欠压条件下将引起塑性区范围减小，在过压条件下则引起塑性区范围增大。

Abstract:

When drilling in methane hydrate-bearing sediments， the variation of temperature and seepage pressure around the wellbore likely leads to the hydrate dissociation， reducing the safety of wellbore. The analytical solution for wellbore mechanical analysis is derived accounting for the ideal elastic-plastic behavior of the formation， the Mohr-Coulomb yield criterion and non-associated flow rule. Based on a steady-state axisymmetric plane strain model considering the effect of hydrate dissociation on temperature， seepage flow and mechanical property， the stress and displacement solutions are presented under unbalanced and overbalanced drilling conditions. The analytical solutions agree well with the results by numerical methods under the same assumptions， as well as the complex conditions. The results show that the increase of the size of dissociated region leads to the expanding of plastic zone obviously within a certain range. The size of plastic zone is decreased with the reduction of Young's modulus of dissociated region under unbalanced drilling condition， while the result is inverse under overbalanced drilling condition.