Rock Fracture Sorptivity as Related to Aperture Width and Surface Roughness
Journal Article - Open Access
University of Tennessee Knoxville United States
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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, rhob solid-phase density, rhos porosity, phi contact angle, thetae 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 rhob , rhos , phi, and thetae 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 S f increases with increasing x geo , 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.
- Test Facilities, Equipment and Methods
- Statistics and Probability