超临界CO_(2)吸附效应对页岩地层井壁稳定影响研究

Study on the influence of supercritical carbon dioxide adsorption effect on wellbore stability of shale formation

当利用超临界CO_(2)作为钻井液钻进页岩地层时,井壁稳定是重点关注问题。超临界CO_(2)与页岩相互作用复杂,而对于超临界CO_(2)吸附效应对超临界CO_(2)开发页岩油气资源过程中的井壁稳定性影响则研究较少。本文将超临界CO_(2)吸附效应对页岩变形和力学性质影响纳入到超临界CO_(2)渗流的多物理场耦合过程中,在超临界CO_(2)运输特性和热力学特性的基础上,构建考虑吸附效应的超临界CO_(2)钻井多物理场耦合井壁稳定模型。基于有限元数值计算方法,分析超临界CO_(2)钻进过程中的地层温度、孔隙压力和地层应力的时空分布规律,研究超临界CO_(2)吸附效应对井壁稳定性的影响。结果表明:超临界CO_(2)吸附效应引起的固体变形对地层温度和流体渗流没有明显影响;吸附效应对应力分布具有显著影响,吸附应变会使应力值增大,而吸附对弹性模量的影响则会使应力值降低;忽略吸附应变会小幅度低估井壁失稳风险,而忽略吸附对页岩弹性模量的影响会较大程度地低估井壁垮塌风险;当井深超过1200 m时,坍塌压力超过井筒压力,超临界CO_(2)钻井将发生井壁垮塌。该文可为超临界CO_(2)钻井的井壁稳定性研究提供理论指导。

When supercritical CO_(2)is used as drilling fluid to drill shale formation,wellbore stability is one key issue.The interactions between supercritical CO_(2)and shale are complex.When oil and gas resources are developed with supercritical CO_(2),the research is little for the influence of supercritical CO_(2)adsorption on wellbore stability of shale formation.In this study,for the multi-physics coupling process of supercritical CO_(2)seepage,the influence of supercritical CO_(2)adsorption on deformation and mechanical properties of shale are considered.Based on the transport and thermodynamic properties of supercritical CO_(2),the multi-physics coupling models of wellbore stability for supercritical CO_(2)drilling are established for considering adsorption effect.The finite element numerical method is used to analyze the distribution of formation temperature,pore pressure and stress with time and space in the process of supercritical CO_(2)drilling,meanwhile,the influence of supercritical CO_(2)adsorption on wellbore stability of shale are studied.The results indicate that the adsorption-induced solid deformation has little effect on formation temperature and pore pressure,whereas,the stress are influenced greatly by adsorption.The stress value will increase when adsorption-induced strain is considered,while the stress value will decrease when adsorption-enhanced elastic modulus is considered.If adsorption-induced strain is neglected,the risk of wellbore collapse will be slightly underestimated,whereas,if adsorption-enhanced elastic modulus is ignored,the risk of wellbore instability will be greatly underestimated.When the depth exceeds 1200 m,the collapse pressure is greater than wellbore pressure,the wellbore collapse will occur.This paper can provide theoretical guidance for wellbore stability analysis for supercritical CO_(2)drilling.