PORE-SCALE MODELING OF GAS-WATER FLOW IN PERMEABILITY JAIL OF TIGHT SANDSTONES

The concept of a permeability jail in tight formations refers to a range of values of fluid saturations where both gas and water cannot flow effectively. Consequently, if a permeability jail exists, there is a real possibility of compromising the productivity of gas wells. However, the understanding of fluid flow affected by a permeability jail is very limited. The objective of this paper is to study the flow mechanism in the presence of permeability jail by simulating gas-water flow using a pore-scale model for tight sandstones. First, we model the microstructure of pores and pore throats of tight sandstones using a capillary tube with a tortuous geometry. Then, considering water wettability of reservoir rocks and the Jamin effect, the gas-water interface is traced by computational fluid dynamics (CFD) modeling and a phase field method. Finally, the flow mechanism affected by a permeability jail is analyzed by discussing the fluid flow capability, the movement of the gas-water interface, and the flow resistance. A critical pressure gradient to escape the permeability jail and influence of wettability are also investigated using the model. Results reveal the process of two-phase flow influenced by the permeability jail: when gas bubbles are entrained in a water phase in a tortuous pore geometry, a high resistance of flow due to the Jamin effect exists. If the pressure gradient fails to overcome the flow resistance, then gas bubbles cannot flow through pore throats. They retain at the corners of pore space, hindering the flow of water. The fluid flow capability, which is described using a concept of displacement time, is significantly reduced owing to the permeability jail. The displacement time rises by 48.61% when water saturation increases from 52.58% (out of the permeability jail) to 55.12% (in the permeability jail). A critical pressure gradient that helps two phases to escape the permeability jail is observed: once the pressure gradient exceeds the critical pressure gradient, the impact of permeability jail diminishes.