Rapid Measurement of Biot’s Effective Stress Coefficient for Oil Well Cements with Application to Well Integrity

Cement is a poroelastic material rather than elastic material. Early researchers have measured the strengths and elastic properties of oil well cement, but Biot’s effective stress coefficient α remains an important unknown despite its significance to well integrity. Here, we present measurements of α for different types of oil well cement, which is calculated by the gradient of confining pressure to pore pressure under changing total stress conditions with bulk volumetric strain held constant. To ‘pause’ the hydration process at a specific number of days after mixing, the specimens were dried in a vacuum oven. This enabled nitrogen gas to be used for the Biot’s effective stress coefficient experiments, which accelerated the equilibration times from greater than 3 h to less than 10 min for each pressure step. Results show that for all cement systems except the Pozmix cement, the typical range of α is 0.38~0.80, compressibility is 1.0~5.7 \(\times {10}^{-10}\)/Pa, and permeability of 0.002~0.11 mD. We also measured the effect of effective stress, curing period, and curing condition on cement's poroelastic parameters of \(\alpha\), compressibility, permeability, and microstructures. Finally, we applied \(\alpha\) in a fully coupled thermoporoelastic model and found that Biot's effective stress coefficient plays a more important role in impermeable formations. This is caused by the induced/undrained pore pressure inside cement being significantly higher when the rock and cement permeability becomes lower. With a higher induced pore pressure, α will play a more significant role in maintaining well integrity. Under such circumstances, the flexible cement with lower elastic modulus and Poisson's ratio provides better performance in zonal isolation.