Effect of pore structure on slippage effect in unsaturated tight formation using pore network model

The gas slippage phenomenon under dry conditions has been investigated extensively both numerically and experimentally. However, very limited research has focused on gas slippage behavior under wet conditions. Unlike conventional formation, the influence of water on the gas transport process can’t be neglected in tight formations due to the comparable amount of thin water film attached along the rock surface. It is found experimentally that the gas slippage factor is positively related to water saturation if water saturation is small, while it decreases with water saturation if it is larger than a critical value. Most of the existing models failed to capture the measured downtrend of gas slippage factor with increasing water saturation, which resulted from water blocking or gas trapping phenomenon. In this work, a pore-scale network model is
proposed to look at the water distribution characteristic and investigate the effect of water on the gas slippage factor. The proposed pore-scale model incorporates the capillary dominated multiphase fluid distribution, real gas effect and gas transport mechanisms at pore-scale. Based on our pore network model, the effect of pore structure characteristics including the frequency of mean pore radius, size of mean pore radius, aspect ratio and coordination number on the gas slippage behavior are investigated and discussed in detail. Similar to previous experimental observations, the simulated gas slippage factor shows a nonmonotonic increase trend with water saturation, it starts to decrease under high water saturation and the critical water saturation depends on the pore structure factors. It increases with mean pore radius and coordination number but decreases with aspect ratio. We used the pore network model to investigate the effect of water phase on gas slippage behavior at the pore scale for
the first time. It emphasized the predominance of water blocking and gas trapping phenomenon in the estimation of gas slippage factor at high water saturation.