Summary This paper presents the results of an experimental and theoretical study of mechanisms responsible for the evolution and structure of the gas bubble during air-water injection-withdrawal cycles in 2D glass micromodels. Wetting films located in the corners of throat section conduct flow. Wetting film flow generates gas loop structure which increases injection efficiency and also the amount of trapped gas during withdrawal. During the next injection, blob distribution and wetting film flow induce a quicker breakthrough. Using a network simulator which takes into account wetting film flow, the influence of flow-rate on cycling is highlighted.
Introduction The operation of an underground gas storage reservoir is distinguished from that of a gas field by the alternating displacement of fluids and much higher flow rates. Displacement efficiency during each gas injection or withdrawal phase is history dependent and linked to the fluid distribution at the end of the previous injection-withdrawal cycle.
Two-phase flow in porous media is a balance between capillary forces and viscous forces acting under very complex boundary conditions. It is calculated using the generalized Darcy's law. The equation system can be solved if the capillary pressure-saturation and relative permeability-saturation relations are known. These relations are obtained by experimental measurement but their validity is uncertain as they are determined under conditions very different from those encountered in reality. Very often they must be adjusted to fit operating data, mainly during high-order injection-withdrawal cycles, and it is very difficult to determine the degree of influence of the porous medium as opposed to that of flow. Though, under certain conditions, the generalized Darcy's law may be applied to two-phase flow in a porous medium, dominant mechanisms change when flow is very slow or very fast and this law becomes irrelevant. To gain a better understanding of these phenomena, a 2D porous medium such as a resin or glass micro-model can be used to observe basic mechanisms up to pore scale while controlling perfectly the porous medium boundary conditions.