The interface between the steel casing and cemented annulus of a typical wellbore may de-bond and become permeable; this flow path is commonly referred to as a microannulus. Because there are often multiple fluids associated with wellbores, understanding two-phase flow behavior in the microannulus is important when evaluating the risks and hazards associated with leaky wellbores. A microannulus was created in a mock wellbore specimen by thermal debonding, which is one of the possible mechanisms for microannulus creation in the field. The specimen was saturated with silicone oil, and the intrinsic permeability through the microannulus was measured. Nitrogen was then injected at progressively increasing pressures, first to find the breakthrough pressure, and secondly, to obtain the relation between capillary pressure and gas relative permeability. The nitrogen was injected through the bottom of the specimen, to simulate the field condition where the gas migrates upwards along the casing. The measured data was successfully fit to common functional forms, such as the models of Brooks-Corey and Van Genuchten, which relate capillary pressure, saturation, and relative permeability of the two phases. The results can be used in computational models of flow along a wellbore microannulus.
Wellbores provide access to the subsurface for a variety of purposes, including oil and gas production, geothermal energy, hydrocarbon and waste storage, groundwater access, and CO2 sequestration. Leaky wellbores can compromise production or storage integrity, pollute groundwater, create hazardous operation conditions, or result in unwanted fluids at the wellhead. Among all the possible leakage paths that have been identified in the literature, the interface between the steel and cemented annulus, commonly known as a microannulus, is one of the most critical (Gasda et al., 2004). Microannuli can be formed by stresses generated by the well operation or because of cement shrinkage (Bois et al., 2011; Kjøller et al., 2016), and it has been shown that microannuli are not expected to be uniform and it is fracture-like (Garcia Fernandez et al., 2019; Skorpa and Vrålstad, 2018; Stormont et al., 2018).