Gas flow through cement-casing microannuli under varying stress conditions

The cement-casing interface, or microannulus, has been identified as a common pathway within wellbores for leakage of fluids yet little is known about its size or character. We conducted laboratory measurements of gas flow through microannuli under varying confining, casing and pore pressures to provide insight into how microannuli respond to a range of stress conditions. Wellbore specimens consisting of a cement sheath cast on steel casing were produced with microannuli between the steel casing and the cement. Gas flow through these specimens was measured under varying confining, casing and pore pressures. After accounting for non-linear flow, we interpreted the measured gas flow as providing the effective wellbore permeability from which the hydraulic aperture of the microannulus was calculated using the cubic law. Results are presented as hydraulic aperture as a function of confining, casing and pore pressure for wellbore specimens with different microannuli. We found that microannuli are fracture-like in how they deform in response to stress changes. Even when loaded to large external pressures, microannuli remain open and capable of conveying significant flow. The microannuli response was less sensitive to casing pressure changes compared to confining pressure. We also demonstrated that the hydraulic aperture of the microannulus is sensitive to temperature changes within the casing. We made measurements of the actual (mechanical) aperture around the entire circumference after injecting epoxy into a microannulus and sectioning the specimen. These results indicate that the microannulus is not a uniform annular gap but is highly variable around the circumference. In addition to testing specimens with discrete microannuli, we also found that specimens with corroded casing were many orders of magnitude more permeable than intact specimens presumably due to flow through the corrosion product.