Effect of water saturation on gas slippage in tight rocks

Abstract

Gas slippage phenomenon in single phase gas flow conditions has been extensively investigated. However, only a few researchers have focused on gas slippage in gas/water two-phase flow conditions. Unfortunately, initial water saturation always exists in tight gas reservoirs, and its impact on gas slippage and flow capacity should not be neglected. In this work, gas slippage for single phase and two-phase flows in tight rocks were experimentally investigated, and our results directly demonstrated that gas slip factors increase with an increase in water saturation. But interestingly, the measured values were significantly higher than those predicted by the Klinkenberg idealized model. This finding indicated that the heterogeneous pore-network structure could largely affect the gas/water distribution characteristics, e.g., leading to ‘water blocking’ or ‘gas trapping’ inside actual samples, which further influenced the two-phase gas slippage behaviors. Thus, a heterogeneity coefficient χ was proposed to correct the deviation between actual cases and the Klinkenberg’s ideal model (χ = 0.5 is for the ideal case, and χ ranges from 0.5 to 2.0 for actual tight sandstones), and thus the two-phase gas slippage for actual tight rocks could be well characterized. Besides, the impact of two-phase gas slippage on the relative gas permeability also needs to be paid more attention. Without a correction of gas slippage, the relative permeability for gas flow in a high-pressure reservoir condition would be overestimated, and this error could be up to 20% for our studied samples. Our present work illustrates a better understanding on how water saturation affects gas slippage in a two-phase flow condition, and paves a path for a more accurate evaluation of the gas flow capacity in actual reservoir systems.