A micro-annulus(MA)is defined as a high permeability zone or gap initiating/occurring at the casingcement and cement-formation interfaces during the wellbore life span.An MA can significantly compromise wellbore integ...A micro-annulus(MA)is defined as a high permeability zone or gap initiating/occurring at the casingcement and cement-formation interfaces during the wellbore life span.An MA can significantly compromise wellbore integrity by establishing enhanced fluid flow pathways.This study uses a staged finite element approach to simulate wellbore integrity during various loading steps of wellbore operations under downhole conditions.Particular emphasis is placed on the processes of cement poro-elastic property evolution,volume variation,and pore pressure variation as part of the cement hardening step.The resulting state of stress during the life cycle of a typical injection well(i.e.hardening,completion,and injection)is analyzed to assess the onset and evolution of micro-annuli at various interfaces of the composite wellbore system under downhole conditions.The results show that cement shear failure is observed at the casing-cement interface during pressure testing(excessive wellbore pressure);and tensile debonding failure initiates at the cement-formation interface due to cement shrinkage during hardening and injection-related cooling(thermal cycling).Sensitivity analyses considering several parameters show that:(1)the degree of poro-elastic bulk shrinkage has significant implications for both shear and tensile failure initiation e the less the cement shrinks,the less likely the failure initiation is;(2)cement integrity increases with increasing depth;(3)cement pore pressure evolution has significant implications for tensile failure e if cement pore pressure decreases more,higher temperature differences can be sustained before an MA occurs;and(4)cement temperature fluctuations during hardening promote initiation of debonding failure.In summary,the results presented indicate that establishing downhole conditions to quantitatively analyze MA generation is necessary.The results are different compared to laboratory studies without considering/simulating downhole conditions.The knowledge from this study can raise the awareness of predicting and evaluating MA under downhole conditions and can be used to supplement and improve future laboratory experiments.展开更多
Sustained annulus pressure and casing deformation of gas wells are common in several shale gas demonstration areas in the Sichuan Basin.In view of this,the conditions for pressure change inside the wellbore in the pro...Sustained annulus pressure and casing deformation of gas wells are common in several shale gas demonstration areas in the Sichuan Basin.In view of this,the conditions for pressure change inside the wellbore in the process of drilling were analyzed,and then an elasticeplastic mechanical model of casing-cement sheath-surrounding rock system for calculating the whole change process of wellbore pressure in the stressestrain state was established.By virtue of this model,the generation and development of micro-annulus at the first and second interfaces under various internal pressure conditions were analyzed.Then,combined with the actual field data,calculation was performed.Results show that no micro-annulus is generated in the process of loading,but the loaded internal pressure determines the plastic deformation degree of cement sheath.Besides,the decrease of internal pressure during the unloading exerts tension on the interface,leading to the generation of microannulus,and consequently the integrity of cement sheath is damaged.Finally,the mechanical performance of cement sheath can be improved by optimizing the slurry formula,and operation technologies are used to guarantee the integrity of cement sheath in shale gas wells.This model was applied in 13 wells for field tests.It is indicated that the average good quality rate of horizontal section cementing is 92.67%,and 11 wells which have been put into production are not subjected to sustained annulus pressure.It is demonstrated that the sustained annulus pressure of gas wells in shale gas demonstration areas is improved effectively.This research achievement can be used as a reference for integrity management of cement sheath in the similar blocks.展开更多
基金The authors would like to thank Chevron ETC for financial support for this study.
文摘A micro-annulus(MA)is defined as a high permeability zone or gap initiating/occurring at the casingcement and cement-formation interfaces during the wellbore life span.An MA can significantly compromise wellbore integrity by establishing enhanced fluid flow pathways.This study uses a staged finite element approach to simulate wellbore integrity during various loading steps of wellbore operations under downhole conditions.Particular emphasis is placed on the processes of cement poro-elastic property evolution,volume variation,and pore pressure variation as part of the cement hardening step.The resulting state of stress during the life cycle of a typical injection well(i.e.hardening,completion,and injection)is analyzed to assess the onset and evolution of micro-annuli at various interfaces of the composite wellbore system under downhole conditions.The results show that cement shear failure is observed at the casing-cement interface during pressure testing(excessive wellbore pressure);and tensile debonding failure initiates at the cement-formation interface due to cement shrinkage during hardening and injection-related cooling(thermal cycling).Sensitivity analyses considering several parameters show that:(1)the degree of poro-elastic bulk shrinkage has significant implications for both shear and tensile failure initiation e the less the cement shrinks,the less likely the failure initiation is;(2)cement integrity increases with increasing depth;(3)cement pore pressure evolution has significant implications for tensile failure e if cement pore pressure decreases more,higher temperature differences can be sustained before an MA occurs;and(4)cement temperature fluctuations during hardening promote initiation of debonding failure.In summary,the results presented indicate that establishing downhole conditions to quantitatively analyze MA generation is necessary.The results are different compared to laboratory studies without considering/simulating downhole conditions.The knowledge from this study can raise the awareness of predicting and evaluating MA under downhole conditions and can be used to supplement and improve future laboratory experiments.
文摘Sustained annulus pressure and casing deformation of gas wells are common in several shale gas demonstration areas in the Sichuan Basin.In view of this,the conditions for pressure change inside the wellbore in the process of drilling were analyzed,and then an elasticeplastic mechanical model of casing-cement sheath-surrounding rock system for calculating the whole change process of wellbore pressure in the stressestrain state was established.By virtue of this model,the generation and development of micro-annulus at the first and second interfaces under various internal pressure conditions were analyzed.Then,combined with the actual field data,calculation was performed.Results show that no micro-annulus is generated in the process of loading,but the loaded internal pressure determines the plastic deformation degree of cement sheath.Besides,the decrease of internal pressure during the unloading exerts tension on the interface,leading to the generation of microannulus,and consequently the integrity of cement sheath is damaged.Finally,the mechanical performance of cement sheath can be improved by optimizing the slurry formula,and operation technologies are used to guarantee the integrity of cement sheath in shale gas wells.This model was applied in 13 wells for field tests.It is indicated that the average good quality rate of horizontal section cementing is 92.67%,and 11 wells which have been put into production are not subjected to sustained annulus pressure.It is demonstrated that the sustained annulus pressure of gas wells in shale gas demonstration areas is improved effectively.This research achievement can be used as a reference for integrity management of cement sheath in the similar blocks.
