Measurement of Cement In- Situ Mechanical Properties with Consideration of Poroelasticity

Accurate characterization of oilwell cement mechanical properties is key to establishing long- term wellbore integrity. The most widely used method is curing cement in an autoclave, demolding, cutting, and transferring it to a triaxial compression apparatus. The drawback of this traditional technique is that the mechanical properties are not measured under in- situ curing conditions. In this paper, we developed a high- pressure and high- temperature vessel to hydrate cement under downhole conditions and then directly measure cement Young's modulus and Poisson's ratio without cooling or depressurization. We validated the setup with water and obtained a reasonable bulk modulus of 2.37 GPa under elevated pressure. We proposed a poroelastic method to calculate cement elastic properties accounting for boundary stiffness and changing pore pressure. We compared the in- situ measurements with traditional triaxial compression tests conducted on the same specimen after retrieval from the vessel. The results show that in- situ measured Young's modulus is more than double, and the Poisson's ratio is 20 to 100% higher than that measured by the traditional triaxial method. One mechanism could be that the depressurization and repressurization process in those traditional tests may generate microdefects or induced stresses that weaken cement mechanical properties. Finally, we applied our mechanical properties measurements to cement wellbore integrity analysis by using a thermoporoelastic model. We found that the initial state of stress plays a significant role in maintaining wellbore integrity. With only mechanical properties differences considered, the estimation with traditional measured properties may mistakenly show cement is safe under some pressure and temperature perturbations.