基于流固耦合的含水合物地层井壁稳定非稳态解析模型

Unsteady Analytical Investigation of Wellbore Stability in Methane Hydrate-Bearing Sediments Based on Fluid-Solid Coupling

摘要:

针对深海水合物地层钻井过程中的井壁稳定问题，考虑水合物分解、热传导、力场-渗流场全耦合作用，建立了过压和欠压钻井下渗流、温度、力场随时间和空间变化的非稳态解析模型。解析结果与相同条件下的数值结果吻合良好，且与力场-渗流半耦合解析结果进行了对比。基于解析模型对井壁稳定的关键参数如钻井液压力、水合物分解引起的地层弹性模量劣化程度等进行了分析，结果表明：①与半耦合分析结果相比，考虑体变对渗流的影响后，过（欠）压钻井时孔压减小（增大）、应力增大（减小），增量径向位移减小；②最危险位置在井壁处，过高或过低的钻井液压力均会导致井壁失稳，水合物分解引起的地层劣化将降低最安全钻井液压力；③水合物分解引起的地层刚度降低极易诱发井壁失稳。在通常条件下，过压钻井时分解域弹性模量降低50%即可导致井壁失稳。

关键词:井壁稳定;流固耦合;水合物;非稳态分析;解析解

Abstract:

In view of the wellbore stability of drilling in methane hydrate-bearing sediments， an analytical model in unsteady state is established considering hydrate dissociation， heat conduction， and fully fluid-solid coupling.And analytical solutions for seepage， temperature and mechanical fields with time and polar radius during overbalanced and underbalanced drilling are obtained. The analytical solutions are in good agreement with the numerical results under the same conditions and compared with the partial hydraulic-mechanical coupling results. The key parameters affecting wellbore stability， such as drilling fluid pressure， and the elastic modulus reduction caused by hydrate dissociation， are analyzed. The results show that compared with partial hydraulic-mechanical coupling results， if the influence of volume deformation of solid on seepage is considered， the pore pressure decreases （increases） while stresses increase （decrease） and radial displacement decreases during overbalanced （underbalanced） drilling. The most dangerous position for the formation is at the wellbore wall and a too high or too low drilling fluid pressure will lead to wellbore instability. The deterioration of formation in mechanical properties caused by hydrate dissociation will reduce the safest drilling fluid pressure. The reduction of formation stiffness caused by hydrate dissociation is very easy to induce wellbore instability and the wellbore instability can usually be caused by reducing the elastic modulus in the dissociated region by 50% during overbalanced drilling.

Key words:wellbore stability;fluid-solid coupling;hydrate;unsteady-state analysis;analytical solution