Rock Fracture Sorptivity as Related to Aperture Width and Surface Roughness

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, rho(b); solid-phase density, rho(s); porosity, phi; contact angle, theta(e); fracture aperture width, x(geo); and fracture surface roughness, W-r. 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, S-f. Estimates of S-f ranged from 10.1 to 40.5 mm s(-0.5), with a median value of 25.0 mm s(-0.5). There was a statistically significant effect of rock type on S-f, with igneous rocks generally having lower mean values than sedimentary rocks. Differences in rho(b), rho(s), phi, and theta(e) between the rock types did not contribute significantly to the variation in S-f However, x(geo) and W-r were significantly correlated with S-f. These correlations indicated that S-f increases with increasing x(geo), as predicted by early-time capillary theory, and decreases with increasing W-r analogous to the decrease in fracture permeability with increasing surface roughness observed under saturated flow conditions.