Direct Imaging and Subsequent Modeling of the Cement Microstructure and Integrity of Cement/Casing and Cement/Formation Interfaces

• Author / Creator

  Yang, Xinxiang

• As a key component of the wellbore barrier system, cement provides zonal isolation and structural support during the entire life of a well. However, even when the cement is properly placed in the well, the zonal isolation may be lost over time. 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 productions and costly remediation operations. The microstructure of the wellbore cement body and the various possible interfaces (e.g. cement-casing and cement-rock) have a large influence on controlling the long-term integrity of the well.

  This study presents a systematic workflows for preparing, characterizing and analyzing the downscaled samples simulating cement body and various interface conditions (e.g., cement-casing, cement-rock), which represent various potential leakage pathways in a well at the microscale. Formation rocks, steel pipes and various cement blends were utilized to prepare the downscaled samples. Microscope, micro computed tomography (micro-CT), traditional and environmental scanning electron microscopy (SEM/ESEM) were employed to characterize the prepared samples at a resolution from 0.01 µm to 16.87 µm. Digital image processing technique and CT-based CFD modelling were conducted to examine and analyze the 2D and 3D microstructures and their impact on permeability.

  At the micron scale (2.64 µm), non-uniform porosity distribution in cement sample was observed, which caused the permeability disparities at different locations. A characteristic correlation between effective porosity and permeability was used to estimate the permeability of the cement sample. A comparative study on the porosity and permeability of early-age well cement and formation rocks reveals that the cement porosity and permeability data are mainly comparable to those of a tight sandstone. Comparisons also show that the early-age well cement has a narrower permeability range than formation rocks. The trend of cement linear fitting curve suggests that if the hydration process of neat early-age well cement sample continues, poroperm characteristics will approach those of shale.

  Analysis on the stress-induced cement fractures generated by uniaxial compression demonstrates that the fractures in cement matrix created by the monotonic compressive stress (up to the limit of uniaxial compressive strength, UCS), are not likely to form continuous leakage pathways. This is because the 2D fractures in cement matrix as shown by SEM images are in limited dimensions while the 3D fractures in cement matrix observed from CT-based 3D models have poor connectivity, generally indicating that leakage pathways which have significant permeability would not form as a result of compressing the cement samples up to their UCS limits. Inclusion of a fiber additive is expected to enhance cement integrity by limiting the fracture propagation.

  Any significant change in the relative humidity (RH) of the environment during the cement preparation, curing and testing processes 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 non-uniform gap size between the cement and the casing is inversely proportional to the change in the relative humidity (RH) of the environment. 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 as the gap is only locally distributed at the cement polished surface without showing any significant connectivity along the wellbore axis.

  Analyses on the cement-rock CT images revealed that the cement-rock interface zone had higher porosity than its neighboring zones. CT-based flow simulation study implies that the cement-rock interface is more likely to provide the leakage pathway when intact caprock exists. The size of the gaps observed at the cement-rock interface through ESEM images was significantly reduced by optimizing the cement chemistry. The effect of the expansion agent on gap size change, however, varied depending on the rock properties such as rock density and porosity. The reduction rate of the gap size due to addition of expansion agent was found to be improved with the low-density rocks.

• Subjects / Keywords

  • microstructure
  • micro-CT
  • cement
  • SEM
  • bonding

• Graduation date

  Fall 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-g4gy-e505

• License

  Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.