Pore‐network modelling of combined molecular diffusion and gravity drainage mechanisms in a porous matrix block: The competitive role of driving forces

A large part of the world's hydrocarbon resources are located in fractured reservoirs, and mass transfer phenomena play a crucial role in enhanced hydrocarbon recovery from these reservoirs. Pore‐network models have been widely used to study kinetic and pore‐scale micro‐mechanisms. Molecular diffusion involves mass transfer and liquid–vapour phase change and can be simulated by a modified invasion percolation model. Despite the existence of separate pore‐scale studies on molecular diffusion and gravity drainage, no articles have been published that evaluate the combined effect of both mechanisms. This study investigates the competitive roles of the two phenomena and the effective factors controlling each mechanism with the aid of pore‐network models. According to the results obtained, gravity drainage and molecular diffusion would have a synergic effect when they are simultaneously active. Although for a single‐component liquid system, there would be a capillary holdup residual saturation in the pure gravity drainage process (between 11% and 14% for the evaluated cases) and a slow and lengthy evaporation in pure molecular diffusion (between 47% and 57% longer for the cases under study), our investigation revealed that when the two mechanisms coexist, a faster process with no residual liquid is expected. Our findings clarify that when the system is strongly gravity dominated, the liquid body remains integrated, gas–liquid contact recedes in a piston‐like manner, and three‐stage liquid desaturation is observed. Furthermore, highly clustered liquid saturation is observed in strongly capillary‐dominated systems, and the liquid desaturation curve in a capillary‐dominated model has two distinguishable stages. The competitive contribution of gravity drainage and molecular diffusion as the main driving forces of liquid extraction from a single‐block model is quantified for the entire period of desaturation. Depending on the dominance of the production mechanisms, the process is either gravity‐assisted molecular diffusion or diffusion‐assisted gravity drainage.