In the last years, the industry has focused on ensuring that cement is efficiently placed in the wellbore and that it does not become mechanically damaged during the life of the well. However, little work has been done on how cement mechanical integrity (CMI) relates to cement hydraulic integrity (CHI), i.e., evaluating the flow rate that could take place through the cement barrier, even if CHI is one of the ultimate objectives of cement integrity.

The analysis of hydraulic integrity requires that a CMI model is used to compute the state of stress and pore pressure in the cement and to estimate which type of mechanical failure may occur during the life of the well. It also requires that a CHI model is integrated with the CMI model to estimate the rate and amount of fluid that may flow through a cement barrier, should it mechanically fail. This provides the engineer insight into the long-term integrity of the cement plug.

This paper describes the work done on CMI/CHI models for cement plugs and presents a sensitivity analysis that demonstrates the value of an integrated CMI/CHI model. The study indicates that: 1) Well geometry, cement properties, reservoirs' pressures, cement heat of hydration, and fluids' properties are required inputs for proper analysis; 2) The changes of stresses and pore pressure over time need to be computed along the length of the cement plug, with sensitivity analysis to take the existing uncertainties into account; 3) A cement plug may preserve its sealing capability, even if the CMI model shows the existence of a micro-annulus, for example, when the fluid viscosity is very high; 4) A cement plug may lose its sealing capacity, even if the CMI model shows no induced defect, for example when a micro- annulus is propagated as a hydraulic fracture.

These last two observations are of significant importance because they show that what a CMI model cannot predict, a CHI model can.

You can access this article if you purchase or spend a download.