General Gas Permeability Model for Porous Media: Bridging the Gaps Between Conventional and Unconventional Natural Gas Reservoirs

摘要

Many field observations have indicated that permeabilities of both conventional and unconventional gas reservoirs are not constant when gas pressure drops. For conventional reservoirs, permeability will decrease while for unconventional gas rocks, the apparent permeability may increase as gas pressure decreases to a lower magnitude. Evolution trends of permeability for different natural gas reservoirs are distinct. These differences are observed by laboratory experiments of sandstones, coals, or shales. In this study, we present a general permeability model to bridge the gaps between conventional and unconventional gas reservoirs. This model coupled three critical factors namely effective stress, adsorption, and flow regimes to reflect dynamic performances of permeability. On the basis of specific reservoirs properties, the model degenerates into four reduced types. The first reduced model is applicable for reservoirs with lower adsorption capacity. The second reduced model is adopted by unconventional reservoirs like coal seams when the intrinsic permeability is big and adsorption capacity is high. For the third reduced model, effective stress is the dominating factor for permeability evolution, which means that it is applicable for conventional reservoirs like sandstones. Unconventional gas reservoirs with low adsorption capacity like gas shales can apply the fourth reduced model because the flow regimes dominate the evolution. These reduced models are verified against the experimental data. Results show that effective stress is the main reason for the change of permeability for conventional gas reservoirs. Both effective stress and flow regimes together determine the apparent permeability of unconventional gas reservoirs. The impact of adsorption on permeability is relatively small. Permeability evolution trends can be classified into different zones for conventional and unconventional gas reservoirs. When the gas is depleted from reservoirs, the gas permeability has two bounds. For the upper bound, permeability is only affected by flow regimes and the apparent permeability will increase when gas pressure drops. For the lower bound, permeability is only affected by effective stress and the apparent permeability will decrease when the gas is depleted from the reservoirs.