| The development of shale gas reservoir is gradually moving towards deep shales,in China.The present researches of cement sheath integrity have successfully supported the development of conventional oil & gas.However,these researches didn’t completely reveal the effect of main characteristics(high difference of horizontal principal stress,high degree of anisotropy,the dip of shale’s transversely isotropic plane)of deep shales on the cement sheath integrity.And,although,the staged finite-element method has developed for the integrity analysis of wellbore in full-life cycle,the effect of cement hydration is not considered in this method.Thus,these research on the integrity of the cement sheath in full-life cycle has not been realized.Based on cement-based materials science,hydration kinetics,and thermal-hydrological-mechanical coupling theory,this study proposed a new staged finite-element modeling approach which considers the process of cement hydration and realizes the integrity analysis of cement sheath in full-life cylce.And the effect of cement hydration heat,changes in mechanical properties of cement sheath,initial stress state of cement sheath,and the dip of shale’s transversely isotropic plane on the integrity of cement sheath are detailed analyse.This study found that the hydration heat released by cement may cause the damage of the cement sheath.And the greater the cement hydration heat,the greater the damage to the cement sheath;high initial stress of the cement sheath is benefit to the integrity of the cement sheath;the integrity of the cement sheath affected by the history of formation load,the higher the horizontal principal stress difference of the formation,the higher the irregularity of the wellbore deformation caused by drilling,and the higher the degree of uneven debonding of the cement sheath-formation interface;the transverse isotropic dip angle of the shale formation affects the formation.Building a well parallel to the transverse isotropic plane of shales is beneficial to reduce the risk of debonding between the cement sheath and the formation interface,but at the same time,it will increase the stress on the outer wall of the cement sheath.The research on the integrity of cement sheath in this paper has certain guiding significance for the long-term safe and efficient development of shale gas resources. |
