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Relative permeability is an important attribute influencing subsurface multiphase flow. Characterization of relative permeability is necessary to support activities such as carbon sequestration, geothermal energy production, and oil and gas exploration. Previous research efforts have largely neglected the relative permeability of wellbore cement used to seal well bores where risks of leak are significant. Therefore this study was performed to evaluate fracturing on permeability and relative permeability of wellbore cement. Studies of relative permeability of water and air were conducted using ordinary Portland cement paste cylinders having fracture networks that exhibited a range of permeability values. The measured relative permeability was compared with three models, 1) Corey-curve, often used for modeling relative permeability in porous media, 2) X-curve, commonly used to represent relative permeability of fractures, and 3) Burdine model based on fitting the Brooks-Corey function to fracture saturation-pressure data inferred from x-ray computed tomography (XCT) derived aperture distribution results. Experimentally-determined aqueous relative permeability was best described by the Burdine model. Though water phase tended to follow the Corey-curve for the simple fracture system while air relative permeability was best described by the X-curve. [ABSTRACT FROM AUTHOR]

Cement samples were reacted with CO2-saturated synthetic groundwater, with or without added H2S (1 wt.%), at 50°C and 10 MPa for up to 13 months (CO2 only) or for up to 3.5 months (CO2 + H2S) under static conditions. After the reaction, X-ray computed tomography (XCT) images revealed that calcium carbonate (CaCO3) precipitation occurred extensively within the fractures in the cement matrix, while micro-fractures with aperture size <∼50 μm at the cement-basalt interface were completely sealed by CaCO3 precipitation. Exposure of a fractured cement sample to CO2-saturated groundwater (50°C and 10 MPa) over a period of 13 months demonstrated progressive healing of cement fractures by CaCO3 precipitation. After reaction with CO2 + H2S-saturated groundwater, CaCO3 precipitation also occurred within the cement fracture as well as along the cement-basalt caprock interfaces. X-ray diffraction analysis showed that major cement carbonation products of the CO2 + H2S-saturated groundwater were calcite, aragonite, and vaterite, all consistent with cement carbonation by CO2-saturated groundwater. While pyrite is thermodynamically favored to form, due to the low H2S concentration it was not identified by XRD in this study. The cement alteration rate into neat Portland cement samples by CO2-saturated groundwater was similar at ∼0.02 mm/d based on XCT images, regardless of the cement-curing pressure and temperature (P-T) conditions, or the presence of H2S in the groundwater. The experimental results imply that wellbore cement with micro-fractures within the cement matrix or along the cement-caprock interface (<∼200 μm aperture under current experimental conditions) is likely to be healed by CaCO3 precipitation during exposure to CO2- or CO2 + H2S-saturated groundwater. Published 2016. This article is a U.S. Government work and is in the public domain in the USA [ABSTRACT FROM AUTHOR]

• Supercritical CO 2 +brine through cement fracture results in mineral precipitation, decreased aperture and permeability. • Fracture aperture at the effluent end was reduced from 137 μm to 50 μm, which compares well to reactive transport model. • Surface porosity increased by a factor of four due to dehydration crack formation at influent end of fracture. • Mineral formation had largest influence on permeability, compared to dehydration crack formation. For carbon capture and storage operations, wells are used as a necessary conduit for injecting CO 2 at depth, but they can also act as leakage conduits if the cement seals are compromised. The specific objective of this research was to experimentally investigate the nature of self-healing of a fracture within cement under multiphase flow of CO 2 and brine, and to compare the findings with the predictions of a recently developed model. This was accomplished by flowing a multiphase mixture of supercritical CO 2 and brine through a cement fracture. The influent end of the fracture was degraded as evidenced by the formation of cracks across the surface. At the effluent end of the fracture, the initial aperture of 137 μm was reduced to 50 μm. This reduction by 87 μm compared well with an aperture reduction of 80 μm predicted by a recently developed model tested in this study. Self-healing of the fracture contributes to permeability reduction through the potential leakage pathway. [ABSTRACT FROM AUTHOR]