摘要
在CO_(2)地质封存过程中,井筒周围受到多场耦合作用,易产生不同程度的损伤,进而成为潜在的CO_(2)泄漏通道。为了研究CO_(2)注入对井筒完整性的影响,建立了两相渗流流固耦合模型,分析了储层岩石初始孔隙度、渗透率、弹性模量,以及应力比和注入速率对CO_(2)运移和井筒、储层岩石的损伤影响。结果表明:初始孔渗大小和注入速率对储层岩石和水泥环损伤影响显著;较低的初始孔渗和较大的注入速率更容易造成井筒损伤。采用库仑失效准则评估储层岩石和水泥环的损伤演变。分析发现:储层岩石在与水泥环界面处容易损伤,而水泥环在与套管界面处更易发生损伤而失效,损伤可能由最初的点损伤演变为面损伤;对于现场注入CO_(2),如果维持较高的注入速率,则需要改善低渗透性储层的孔渗条件,降低井筒失效风险。
Objective Wellbores can be damaged due to multiphysical coupling during geological CO_(2) storage in deep saline aquifers,which compromises wellbore integrity and creates potential leakage pathways for CO_(2).This study proposes a fully coupled hydromechanical model to simulate the two-phase flow of brine and CO_(2) near the wellbore and to investigate the mechanical responses of the reservoir rock and cement sheath.Methods The wellbore structure in the model consisted of casing,a cement sheath,and the surrounding reservoir rock.The reservoir was considered a saturated,homogeneous,isotropic elastic medium,with deformation characterized as small,linear elastic deformation.Pore pressure diffusion was governed by the Biot storage model,while the two-phase flow for brine and CO_(2) was described by Darcy’s law and the Brooks‒Corey model.Since pore pressure increase led to wellbore damage during CO_(2) injection,the indirect variation in reservoir porosity and permeability was also considered,showing a correlation with changes in average effective stress.The mechanical response and pore pressure variation were fully coupled,and the Coulomb failure criterion was utilized to evaluate the damage risk of both the reservoir rock and the cement sheath.It was found that damage to the cement sheath depended on its inherent properties,the surrounding stress state,and changes in pore pressure.Therefore,the effects of initial porosity,permeability,and elastic modulus of the reservoir rock,as well as the stress ratio and injection rate,on CO_(2) migration and damage to the wellbore and reservoir rock were analyzed.The initial porosity and permeability of the reservoir rock were described using the Weibull distribution.Results and Discussions The influence of various factors on CO_(2) migration and damage to the wellbore and reservoir rock was examined.Analyses of the benchmark model showed that CO_(2) saturation and average pressure distributions were closely related,with the highest pore pressure occurring near the injection well.Pore pressure changed significantly from 0.5 to 4.0 days,with maximum average pressure changes initially reaching 5 MPa and increasing up to 15 MPa as the pore pressure diffused.This increase reflected the influence of boundary conditions on the migration of CO_(2).Heterogeneity in reservoir porosity and permeability significantly affected distributions of both pore pressure and CO_(2).Initial CO_(2) saturation exhibited a distinctly nonuniform pattern.As CO_(2) migrated,its distribution became more uniform but still depended on variations in porosity and permeability.Previous field simulations indicated that reservoir heterogeneity and the presence of fractures can significantly alter CO_(2) migration patterns.For the wellbore model,despite boundary constraints limiting further pressure diffusion,heterogeneity impacted CO_(2) migration and pore pressure distribution,influencing potential damage to the wellbore and surrounding areas.It was found that the injection rate,reservoir porosity,and permeability controlled CO_(2) migration,with the stress ratio and elastic modulus being largely negligible,as indicated by analyzing CO_(2) saturation and pore pressure at 4 days.In cases of low porosity and permeability,a higher injection pressure was required,resulting in greater CO_(2) saturation and extended migration distances,though excessive pressure posed a risk of fracturing.Since CO_(2) migration was linked to pore pressure diffusion,pressure-induced damage was mainly influenced by injection rate and changes in reservoir characteristics.Damage to the reservoir rock was attributed to increased porosity and permeability resulting from CO_(2) injection,as well as mechanical damage at the cement sheath interface.Analysis showed that porosity changes induced by effective stress variation ranged between 0 and 0.001.Permeability changes reached up to 0.03 mD and were even higher locally,with local increments reaching up to 0.06 mD.The Coulomb failure criterion indicated varying degrees of damage under different conditions.Scenarios such as low injection rate and high permeability delayed damage onset beyond 1 day,while low permeability or high injection rates accelerated damage,potentially fracturing the rocks.Damage initially appeared as point damage,potentially evolving into surface damage with continuous injection,occurring at the reservoir-cement sheath interface.The integrity of the cement sheath depended on its properties,the surrounding stress conditions,and variations in pore pressure.For high CO_(2) injection volumes,increasing the reservoir’s porosity and permeability reduced pressure buildup.Visual analysis revealed damage at the interface between the cement sheath and the casing.Nonuniform changes in the Coulomb failure criterion at these interfaces,influenced by heterogeneity in reservoir rock properties,can result in radial cracks within the cement sheath.Conclusions This study employs a two-phase hydromechanical coupling model to assess the impact of multiple factors on wellbore and surrounding rock damage caused by CO_(2) injection.The findings indicate that CO_(2) migration is aligned with pore pressure diffusion,resulting in both compression and expansion of internal pores.Although reservoir heterogeneity influences CO_(2) migration,the injection rate and initial permeability play a more critical role in determining CO_(2) flow and pore pressure distribution.Damage typically occurs at the reservoir-cement sheath interface and the cement-casing interface.Reducing injection rates and increasing reservoir permeability can help mitigate damage to reservoir rocks and the cement sheath.The elastic modulus and stress ratio have limited influence on damage.As the damage evolves from point to surface,enhancing permeability in low-permeability reservoirs is recommended to sustain high CO_(2) injection rates and reduce the risk of cement sheath failure.
作者
张瑶
李霞颖
李琦
马纪元
钟屹岩
陈博文
ZHANG Yao;LI Xiaying;LI Qi;MA Jiyuan;ZHONG Yiyan;CHEN Bowen(State Key Laboratory of Geomechanics and Geotechnical Engineering,Institute of Rock and Soil Mechanics,Chinese Academy of Sciences,Wuhan 430071,China;University of Chinese Academy of Sciences,Beijing 100049,China)
出处
《工程科学与技术》
北大核心
2025年第5期91-100,共10页
Advanced Engineering Sciences
基金
NSFC区域联合基金项目(U23A20671)
湖北省自然科学基金创新群体项目(2021CFA030)
岩土力学与工程国家重点实验室开放基金项目(SKLGME023018)。
关键词
流固耦合
两相渗流
井筒损伤
CO_(2)注入
水泥环失效
非均质性
hydromechanical coupling
two-phase flow
wellbore damage
CO_(2)injection
cement sheath failure
heterogeneity
作者简介
张瑶(1995-),男,博士生.研究方向:CO_(2)地质封存.E-mail:Zhangyaosc@163.com;通信作者:李琦,研究员,E-mail:qli@whrsm.ac.cn。
