A Finite Element Study of Integrity of Formation/Casing Annulus Cement Sheath in Niger Delta

Abstract

Cement sheath is a well barrier that prevents the unintentional and uncontrollable flow of fluids from, into a formation or back to its surface. However, during drilling and production operations, this cement is subjected to various stresses resulting from thermal stress, non-uniform geo-stress, compressive and tensile stresses. Therefore, this study describes a finite element analysis (FEA) simulation of a cement sheath of class G type under stress in a typical drilling and production scenario. With experiments, the rheological and mechanical properties of class G cement with varying water-cement ratio of 0.4, 0.5 and 0.6 were prepared and analyzed for their performance and workability. From the results, it showed that the cement system with the lowest water-cement ratio of 0.4, demonstrated the highest mechanical strength. This was attributed to lesser water in the mix triggering efficient interaction with cement. Hence, based on this study, 0.4 cement ratio is recommended if the ability to withstand compressive and tensile forces is desired. In cases where it is to be used for drilling and production operations characterised by fatigue and cyclic forces, its composition should be designed such that it is more ductile and flexible. An FEA software, ANSYS Mechanical APDL (ANSYS Parametric Design Language) was used to analyze stress to convergence. Material properties of 0.4 cement ratio was adopted for simulation based on experimental results. Also, well loading conditions were cycled at temperature and pressure of 0-104 0C and 250-290 bar simultaneously. Simulation results showed the time changes of equivalent (Von-Mises), maximum, minimum and shear stresses. Time changes of equivalent elastic strain, total deformation and stress intensity were also recorded. From the simulation results, it can be concluded that the yield point of the material occurred at a time (t) =1.3245×10−4 s under continuous stress. It is recommended that the contact point between the casing and the cement be monitored for deformation due to high stress response during stress analysis.