Two-phase flow experiments in a geocentrifuge and the significance of dynamic capillary pressure effect

In recent years, geocentrifuge facilities have been used for the study of two-phase flow in porous media. For example, capillary pressure-saturation curves have been obtained by carrying out drainage and imbibition experiments on soil samples in a centrifuge. However, because two-phase flow in a centrifuge is accelerated, the question arises whether the resulting curves are representative. This issue is important in light of overwhelming evidence from the literature that the capillary pressure curves obtained under dynamic (flow) conditions may be a function of the rate of change of saturation. Thus, one may define a dynamic capillary pressure and a static capillary pressure, whose difference may be proportional to the time rate of change of saturation, with the coefficient of proportionality denoted by tau. In this work, we report on a series of drainage and imbibition experiments involving water and perchloroethylene (PCE) in two different kinds of sand (fine and coarse), carried out in a geocentrifuge. Using selective pressure transducers, both water and PCE pressures are measured dynamically at three locations within the soil column. Saturation is also measured at those locations by means of time domain reflectrometry. Results obtained here show that because of the dynamic effect, capillary pressure curves obtained in a geocentrifuge are distinctly different from static curves. The results have been used to calculate the coefficient tau for the two sands.