Effect of stress-dependent microannulus aperture on well leakage

Debonding at the cement-casing interface is recognized as a principal failure mechanism leading to CO2 leakage in wells. This detachment gives rise to a microannulus, which notably possesses greater permeability than undamaged cement, undermining its sealing efficacy. Conventionally, the permeability of the microannulus is regarded as a uniform value throughout the well. However, fundamentally, a microannulus is one type of fracture, and its gap or aperture size is affected by the effective stress. In this work, we developed a unique experimental apparatus. This equipment facilitates the curing of cement inside a steel casing, the formation of a microannulus between the casing and the cement, and the investigation of the fluid flow dynamics along the microannulus under laboratory-replicated in situ conditions. The microannulus was formed by injecting fluid from one end of the setup, and receiving similar amount of fluid on the other end signified the development of the leakage channel. Additionally, strain gauges affixed to the casing’s external surface yielded key information on the microannulus’s opening and closure. We observed a noticeable decline in microannulus hydraulic aperture (or permeability) in relation to effective stress and an exponential equation fits their relationship. Our findings also indicate a distinct behavior when comparing liquid CO2 with water. Specifically, it is easier for liquid CO2 to create the microannulus. However, the hydraulic aperture range for this microannulus (0.7–6 μm) is considerably smaller than that created by water flow (2–17 μm). Finally, we integrated the stress-dependent microannulus aperture size into the combined analysis of well mechanical integrity and well leakage. The outcomes consistently demonstrated that when factoring in the stress-dependent aperture sizes, the leakage rates are 3–5 times compared to a fixed aperture model. The traditional assumption of a constant aperture significantly underestimates fluid leakage risks.