Characterization of the Microstructure of the Cement/Casing Interface Using ESEM and Micro-CT Scan Techniques

Abstract

Summary Integrity of the cement/casing interface is an essential element of establishing an effective barrier system in cased and cemented wellbores. Failure to establish and maintain an effective barrier can result in negative environmental and economic impacts, such as leakage of formation fluids into the environment or loss of production and costly remediation. The main focus of this paper is defined as the characterization of the cement/casing interface microstructure, which is critical for better understanding of the requirements for establishing effective zonal isolation and the long-term integrity of cemented wellbore sections of active wells, as well as the integrity of the abandoned wells. The primary objectives of this study were: first, to confirm the most suitable methods for preparing cement/casing samples and characterize the microstructure of the cement/casing interface (i.e., quantify the size of the microchannel due to possible debonding (gap) at the cement/casing interface and its progression along the wellbore axis); and second, to understand how different test methods can affect the microstructure, and once we have a firm baseline, analyze the effect of different cement compositions on the performance. More specifically, we have investigated effects of the cement composition and preparation, environmental conditions [i.e., relative humidity (RH) of the storage and testing conditions], cement shrinkage, and the expansion additive on the integrity of the cement/casing interface at the micrometer scale by using environmental scanning electron microscope (ESEM, 0.05 µm resolution) and microcomputed tomography (μ-CT, 11.92 µm resolution) scanning techniques. Cemented casing samples were prepared by using Class G and special abandonment cement blends with and without expansion additives and stainless-steel pipes. Results showed that any significant change in the RH of the environment during the cement preparation, curing, and testing process significantly affects the size of the gap at the cement/casing interface in test samples. Analyses of the ESEM images have shown that the 2D nonuniform gap size between the cement and the casing is inversely proportional to the change in the RH of the environment. Results suggested that cement slurries set and cured under downhole conditions with relatively constant RH may not undergo significant shrinkage and yield only minimal debonding effect. The 3D gap model reconstructed from the µ-CT images confirmed that the gap between cement and casing mostly occurred at the cement polished surface and the gap didn’t show any significant connectivity below the cement polished surface, indicating that common sample preparation methods can significantly affect the near-surface interface. Cement blends prepared with expansion additives have shown smaller gap size. The use of expansion additives enhances the cement/casing interface integrity by effectively reducing the gap size at the cement/casing interface. We conclude that as long as the RH of the environment does not change significantly, the cement is expected to undergo a limited shrinkage, and the gap between the cement and the casing may not induce any significant leakage pathway because the gap is only locally distributed at the cement polished surface without showing any significant connectivity along the wellbore axis.