Evaluating Cement-Plug Mechanical and Hydraulic Integrity

Abstract

Summary In past 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, few efforts have been made to determine how cement mechanical integrity (CMI) relates to cement hydraulic integrity (CHI) (i.e., evaluating the flow rate that could occur through the cement barrier), even though CHI is one of the main objectives of placing a cement plug in a wellbore. The analysis of hydraulic integrity requires that a CMI model be used to compute the state of stress and pore pressure in the cement and to estimate which type of mechanical failure might occur during the life of the well. It also requires that a CHI model be integrated with the CMI model to estimate the rate of fluid that might flow through a cement barrier, should it mechanically fail. This provides the engineer with insight into the long-term integrity of a cement plug. This paper describes the work conducted on CMI/CHI models for cement plugs, and it presents a sensitivity analysis that demonstrates the value of an integrated CMI/CHI model. The study indicates that (1) well geometry, cement properties, reservoir pressures, cement heat of hydration, and fluid 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 consider the existing uncertainties; (3) a cement plug might preserve its sealing capability, even if the CMI model shows the existence of a microannulus (e.g., when the fluid viscosity is very high); and (4) a cement plug might lose its sealing capacity, even if the CMI model shows no induced defect (e.g., when a microannulus is propagated as a hydraulic fracture). These last two observations are important because they show that what a CMI model cannot predict, a CHI model can.