Relative permeability has a significant impact on gas or oil and water production, but is one of the most complicated properties in unconventional reservoirs. Current understanding on relative permeability for unconventional reservoir rocks is very limited, mainly because of a lack of direct measurement of relative permeability for these rocks that have matrix permeability of sub-micron-Darcy level. Due to the difficulties related to the direct measurement, most studies on relative permeability in unconventional reservoirs are based on indirect or modeling methods. In this paper, a modified gas expansion method for shale matrix permeability measurement (Peng et al., 2019a) was adopted to measure gas relative permeability directly under the scenario of water imbibition for samples from different unconventional reservoir formations. Evolution of gas permeability, along with gas porosity and fracture-matrix interaction, during the process of water redistribution (mimic of what occurs in shut-in period in real production) were also closely measured. Results show that gas relative permeability in matrix decreases during water redistribution because of water imbibition from fracture to matrix and water block effect. Water block effect is more significant at low water saturations than higher water saturations, leading to a rapid-to-gradual drop of gas relative permeability with increasing water saturation.
A conceptual model on water redistribution in a fracture-matrix system and the change of gas and water relative permeability is proposed based on the experimental results and observations. Influencing factors including pore size, shape, connectivity, and wettability are taken into account in this conceptual model. The combined effect of these four influencing factors determines the level of residual gas saturation, which is the most important parameter in defining the shape of relative permeability curves. Water relative permeability is predicted based on the conceptual model and the measured gas relative permeability using modified Brooks-Corey equations. Deduction of oil-water relative permeability is also discussed, and experimental methods on determination of the key parameter, i.e., residual oil saturation, are proposed. Implication of relative permeability on gas or oil and water production and potential strategy for optimal production are also discussed in the paper. Hysteresis effect is not included in this study and will be addressed in future work.
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