Using Velocities, Density, and Bulk Modulus to Predict the Permeability Evolution of Microfractured Rocks

Abstract

Thermal fracturing in reservoir rocks can cause significant increase in permeability. The change in permeability due to fracturing was predicted using the change in elastic wave velocity, density, and elastic moduli. Westerly granite samples were thermally treated to 250, 450, 650 and 850 °C. Increasing the temperature produces an increase in fracture length, aperture and density that is isotropically distributed. The permeability was measured at 10, 30 and 50 MPa effective pressure using deionized water as the pore fluid. The permeability was correlated to P- and S-wave velocities, bulk density, and static bulk modulus that were measured at similar effective pressures on the same suite of samples. Kachanov’s (Elastic solids with many cracks and related problems, in: John, Theodore (eds) Advances in applied mechanics, Elsevier, pp 259–445, 1994) model was used to calculate the evolution of fracture density. Closure of fracture, due to increasing effective pressure, caused the permeability and fracture density to decrease, and the other parameters to increase. Thermal treatment caused a systematic increase in permeability and fracture density, and a decrease in the other parameters. Empirical relationships with high correlation coefficient exist between permeability and the other parameters for both dry and saturated conditions, which vary with effective pressure. Overall, we conclude that the elastic wave velocity, density and bulk modulus can be used to predict the change in permeability due to isotropic fracturing. However, a reliable method would be required to upscale these laboratory measurements for modelling and describing the permeability within a granitic reservoir.