Rock Fracture Sorptivity as Related to Aperture Width and Surface Roughness

Abstract

Core Ideas Spontaneous imbibition was measured in rock fractures using neutron radiography. Early‐time uptake of water displacing air showed a square root of time dependency. Fracture sorptivity was quantified from the slope of the early‐time uptake data. Fracture sorptivity increased with increasing fracture aperture width. Fracture sorptivity decreased with increasing fracture 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, ρb; solid‐phase density, ρs; porosity, ϕ; contact angle, θe; fracture aperture width, xgeo; and fracture surface roughness, Wr. 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, Sf. Estimates of Sf 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 Sf, with igneous rocks generally having lower mean values than sedimentary rocks. Differences in ρb, ρs, ϕ, and θe between the rock types did not contribute significantly to the variation in Sf. However, xgeo and Wr were significantly correlated with Sf. These correlations indicated that Sf increases with increasing xgeo, as predicted by early‐time capillary theory, and decreases with increasing Wr, analogous to the decrease in fracture permeability with increasing surface roughness observed under saturated flow conditions.