Gas relative permeability and pore structure of sandstones

Accurate quantification of gas relative permeability is important in several engineering problems including gas storage and production, drying and wetting processes, oil production etc. In this study, experimental curves of gas relative-permeability for three different kinds of sandstone were defined using a modified version of the pulse-decay method. In order to understand the role played by the porous structure in defining the nonwetting phase permeability, the porous networks of the rocks tested were identified by two complementary methods: mercury porosimetry and sorption techniques. The results obtained show two distinct types of behaviour which are closely dependent on the pore structure. The pores located near the peak of the mercury intrusion curve seem to control the variation in gas relative-permeability. In order to clarify the influence of wetting fluid viscosity on the nonwetting phase relative permeability, each sample was subjected to two series of tests using two different saturating fluids. Our results show that, for a viscosity ratio mu(nw)/mu(w)much less than 1, the gas relative-permeability remains unchanged even if the wetting fluid viscosity is twenty times higher than that of water. A simple empirical relation (the Brooks and Corey equation) was used to correlate the gas relative-permeability with the degree of saturation. The comparison shows that this correlation accurately predicts the nonwetting-phase relative permeability. (C) 1999 Elsevier Science Ltd. All rights reserved.