SPE International Symposium & Exhibition on Formation Damage Control

Spontaneous imbibition experiments were carried out on three core samples from the Mardie Greensand and one core sample of Barrow Group sandstone from the Australian North West Shelf. Water uptake into the samples, which were initially airsaturated, was measured by dynamic neutron radiography. The observed data were interpreted by (1) the linear diffusionequation approximation for fluid flow and (2) the Li and Horne model. Both these models were consistent with the observed data, and have similar consequences. The linear diffusion-equation model suggests that a constant diffusion parameter, Dw, can adequately describe the observed spontaneous imbibition of water into these samples, under ambient temperature and pressure, throughout most of the imbibition process. The Li and Horne model suggests that (1) the effective water permeability of the sample and (2) water permeability times the capillary pressure gradient within the sample are approximately constant through most of the imbibition process, during these experiments. The combined conclusion of these interpretational methods implies that the water permeability and the capillary pressure-water saturation gradient in these samples are constant throughout most of the imbibition process, under ambient temperature and pressure conditions. Introduction Studies of spontaneous imbibition of fluids into capillaries have been dated to the mid-1700s. Spontaneous imbibition experiments can be used to determine (a) effective permeability and capillary pressure in spontaneous water imbibition in water/gas systems, (b) countercurrent imbibition in fractured reservoirs, and (c) initial stage spontaneous imbibition into relatively large radius capillaries. Further, the X-ray CT technique has been a commonly applied tool to evaluate water saturation in porous samples 3, 5, . Recently, neutron radiography has also been applied to evaluate water saturation in petrophysical studies. Rant et al. carried out some studies of spontaneous imbibition into concrete, using neutron radiography, which supported the Washburn square-root time approximation for horizontal flow. Two of the most common approaches to the interpretation of spontaneous imbibition results are attempts to match the (a) Washburn equation for relatively small imbibition times, and (b) Aronofsky model 4,12 for large times, when the effects of the boundaries of the porous medium influence the flow. The Washburn equation suggests that the depth of penetration (h) of the wetting fluid into a porous medium is proportional to the square-root of time (t):