Research On Poromechanical Behavior Of Well Cements

Efficient sealing of cement sheath can ensure the integrity of wellbore.In order to judge the failure of cement sheath,the mechanical properties of cements need to be accurately described.Cenments is a porous medium material,and there exists water in cements pores.In this paper,poromechanics theory is used to study the mechanical properties of oil well cements in downhole.Firstly,the poroelastic constitutive equation was used to describe the mechanical properties of cements,and the Mohr Coulomb failure criterion of effective stress was used to judge the failure behavior of cements.Then,the experimental equipment was modified and three experimental schemes were designed,including cyclic loading,unjacket test and triaxial mechanical test.Class G oil-well cement of different ages was taken as research object.Compressive strength,Young's modulus and Poisson ratio of cements were obtained by changing confining pressure,drainage condition and pore water pressure of cements.Finally,the constitutive equation and failure criterion of cements were established using experimental data.The results show that the compressive strength of cements in drained compression tests is higher than that in undrained compression tests.With the increase of pore water pressure,the compressive strength of cements decreases gradually.The compressive strength of cements does not change when pressure differnce between confining pressure and pore water pressure remains constant.The error between experiment value and theoretical value of Biot effective stress coefficient is 1.5%.In drained compression tests,the Mohr Coulomb failure criterion of cements is linear.It is feasible to describe the mechanical behaviour of cements based on the theory of poroelastic mechanics.