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.

Recent Advances in Magnesia Blended Cement Studies for Geotechnical Well Construction—A Review

“…The cement sheath integrity is not only about mechanical characteristics, but also including hydraulic features. Bois et al proposed a crossing check method on a cement plug by use of cement mechanical integrity (CMI) and cement hydraulic integrity (CHI) models (Bois et al, 2019). Although they didn't give the complete set of equations, the hypotheses, physical case configurations, and specific parameters have been illustrated in their paper.…”

Digital Cement Integrity: A Methodology for 3D Visualization of Cracks and Microannuli in Well Cement

“…Moreover, the CT resolution is also a limitation for the subsequent CFD simulations, as potential flow path geometries smaller than the resolution are not accounted for. The main limitation is however that it is assumed that the obtained leak path geometries are fixed and do not change during flow, and this assumption is incorrect as it is known that increased pressure differences may increase the available flow path size [27].…”

“…In other words, are microannuli homogeneous and uniform around the entire circumference of the cement, or not? Model simulations have shown that the size of microannuli can be dependent on wellbore characteristics such as formation properties, casing size and cement stiffness [27,67], however, the resulting shapes can be challenging to determine from such simulations. Stormont et al [24] and Garcia Fernandez et al [25] found experimentally that the microannulus aperture can vary considerably around the circumference when they studied gas flow through different microannuli, and concluded that microannuli have complex geometries.…”

“…It should be noted however, that a major limitation of this approach is that it is assumed that the leak path geometries are fixed and do not change during flow. This assumption is incorrect, and for example, increased pressure differences may open up microannuli, and thus, increase the available flow path size [27,74]. Figure 7 shows three examples where experimentally created cracks and microannuli have been used as leak path geometries in CFD flow simulations [50][51][52].…”

“…Although well-suited as function tests of cement sealing ability and zonal isolation, such a characterization approach provides limited information about how the cement fails and few details about the resulting leak paths. For example, Bois et al [27] have pointed out the difference between cement mechanical integrity, i.e., the presence of cracks and microannuli, and cement hydraulic integrity, i.e., the resulting flow rates through the cement barrier. There are several unanswered questions, such as: are both radial cracks and microannuli formed, how many radial cracks are there, what are the aperture and geometry of the radial cracks, are the microannuli homogeneous and uniformly present around the entire circumference, what is the aperture of the microannuli, and how do fluids flow through these leak path geometries?…”