VISUALIZATION STUDY ON THE EVOLUTION OF GAS-WATER INTERFACE AT PORE-SCALE IN TIGHT SANDSTONE ROCKS

Water phase trapping is one of the main engineering issues that restrict the efficiency of development in unconventional gas reservoirs. The damage essence of water phase trapping is the decline of gas flow capacity induced by water invasion at pore-scale. Due to the limitations that core-scale displacement experiments cannot visually reveal the evolution of gas-water interface and distribution of gas and water at pore-scale, a visualization study of gas-water two-phase flow at pore-scale using microfluidics was conducted in this study. Two types of micromodels including the single flow channel model and pore network model were designed based on analysis of the pore structures from the CT images of some real tight sandstone rocks, and related microchip models were fabricated and etched accordingly. Then the dynamic evolution of gas-water interface and flow phenomena at pore-scale were visually investigated using these two micromodels. The core-scale flow experiments were also carried out to analyze the gas and water flow mechanisms, which was used to link the pore-scale flow mechanisms. The results show that snap-off and bypass flow were the two most important pore-scale events that occurred during the evolution of gas-water interface to break the continuity of gas flow. Once the gas phase is discontinuous, the Jamin effect becomes remarkable to hinder the gas flow and water drainage, resulting in gas entrapment and residual water produced. The advantage flow path is considerable for gas and water flow at pore-scale, and can be also an unfavorable factor to harm the gas discharging water process. Combined with the core-scale analysis and pore-scale visualization, the damage mechanisms of water phase trapping in tight sandstone reservoirs is elucidated from the perspective of pore-scale evolution of gas-water interface. These findings will improve the knowledge of gas and water flow mechanisms and reveal the intrinsic mechanism of the water phase trapping in tight formations.