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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.

Given the challenge of definitively discriminating between chemical and nuclear explosions using seismic methods alone, surface detection of signature noble gas radioisotopes is considered a positive identification of underground nuclear explosions (UNEs). However, the migration of signature radionuclide gases between the nuclear cavity and surface is not well understood because complex processes are involved, including the generation of complex fracture networks, reactivation of natural fractures and faults, and thermo-hydro-mechanical-chemical (THMC) coupling of radionuclide gas transport in the subsurface. In this study, we provide an experimental investigation of hydro-mechanical (HM) coupling among gas flow, stress states, rock deformation, and rock damage using a unique multi-physics triaxial direct shear rock testing system. The testing system also features redundant gas pressure and flow rate measurements, well suited for parameter uncertainty quantification. Using porous tuff and tight granite samples that are relevant to historic UNE tests, we measured the Biot effective stress coefficient, rock matrix gas permeability, and fracture gas permeability at a range of pore pressure and stress conditions. The Biot effective stress coefficient varies from 0.69 to 1 for the tuff, whose porosity averages 35.3% ± 0.7%, while this coefficient varies from 0.51 to 0.78 for the tight granite (porosity