Rock Fracture Sorptivity as Related to Aperture Width and Surface Roughness

Abstract

Fractures in low-porosity rocks can provide conduits for fluid flow. Numerous researchers have investigated fluid flow through fractures under saturated conditions. However, relatively little information exists on spontaneous imbibition in fractures, whereby a wetting fluid displaces a non-wetting fluid by capillarity. We investigated spontaneous imbibition of water displacing air in a suite of fractured low-porosity sedimentary and igneous rock cores (5.08-cm length by 2.54-cm diameter). Mode I fractures were induced in the cores by compression between opposing parallel flat plates. The following physical properties were measured: bulk density, ρ; solid-phase density, ρ; porosity, ϕ; contact angle, θ; fracture aperture width, and fracture surface roughness, . The wetting front in each fracture was imaged using dynamic neutron radiography. Early-time uptake exhibited a square root of time dependency, and was quantified by linear regression, with the slope equal to the fracture sorptivity, . Estimates of ranged from 10.1 to 40.5 mm s, with a median value of 25.0 mm s. There was a statistically significant effect of rock type on , with igneous rocks generally having lower mean values than sedimentary rocks. Differences in ρ, ρ, ϕ, and θ between the rock types did not contribute significantly to the variation in . However, and were significantly correlated with . These correlations indicated that increases with increasing , as predicted by early-time capillary theory, and decreases with increasing , analogous to the decrease in fracture permeability with increasing surface roughness observed under saturated flow conditions.