PNNL

Abstract

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 extraction. Previous research efforts have largely neglected the relative permeability of wellbore cement used to seal well bores where risks of leak may be significant. Therefore this study was performed to evaluate fracturing on permeability and relative permeability of wellbore cement. Studies of relative permeability of water (0.1M NaNO3) and air were conducted using ordinary Portland cement monoliths having fracture networks that exhibited a range of permeabilities. The measured relative permeability were compared with three models, the Corey-curve, often used for modeling relative permeability in porous media, and the X-curve, commonly used to represent relative permeability of fractures, the Burdine model based on fitting the Brooks-Corey function to fracture saturation-pressure data inferred from XCT-derived aperture distribution results. Relative permeability in a more complex fracture network showed better fits to the X-curve, while a more simple fracture network followed the Corey-curve and Brooks-Corey curve more closely, with the Burdine and Brooks-Corey models providing nominal improvement to fit. Additional tracer tests through the simple fracture showed that fitted hydrodynamic dispersion coefficients decreased from 3.7 cm2 s-1 to 2.2 and 0.9 cm2 s-1 for water saturations of 100%, 80% and 66%, respectively due to decrease tortuosity. The authors postulate that 0.2-0.5 mm aperture fractures represent an aperture where there is a crossover aperture between where X-curve and Corey-curve are a preferred model fit.