Forward prediction of early-time spontaneous imbibition of water in unsaturated rock fractures

Abstract

Spontaneous imbibition is a capillary-driven phenomenon in which a wetting fluid displaces a nonwetting fluid from voids without any increase in external pressure. This study compared forward predictions of early-time displacement of air by water within Mode I fractures in 14 low-porosity rock cores using a new fractal model with those based on an established parallel plate model. Spontaneous imbibition was measured using dynamic neutron radiography, along with independent determinations of equilibrium contact angle, fracture aperture width, and fracture surface fractal dimension, Ds. The predicted uptake curves generally agreed with the experimental data. However, both models overpredicted the height of the wetting front at any given time. This overprediction may be due to lateral losses of wetting fluid to the matrix by spontaneous imbibition through fracture surfaces. The predictions of the fractal model were consistently closer to the observed values than those of the parallel plate model, and for the best cases, their upper and lower confidence intervals bounded the data points. In 12 out of 14 cases, the RMSD for the fractal model was less than that for the parallel plate model. A paired t test indicated that, on average, the RMSD for the fractal model was significantly lower than that for the parallel plate model. This statistically improved prediction can be attributed to the retardation of predicted uptake achieved through the introduction of a fracture surface roughness parameter (i.e., Ds) in the fractal model.