Pore-Network Modeling of Combined Molecular Diffusion and Gravity Drainage Mechanisms in a single Matrix Block

A significant part of Iranian hydrocarbon resources are located in fractured reservoirs. The existence of two different fracture and matrix systems creates two models for fluid storage and flow. Evaluation of the rock and fluid interaction and identification of micro-mechanisms at the pore scale is effective in better understanding the production mechanisms in these reservoirs. Pore network modeling makes it possible to simulate a wide range of different conditions, different flow regimes, and identifying micro-mechanisms at the pore scale. Since the injection of non-equilibrium gas into fractured reservoirs, a combination of gravity and molecular diffusion contribute to the production process, and so far, no pore-scale study involving the combined effects of both mechanisms has been performed, this study has examined this issue. In this research, by developing an exist pore network model based on the analogy between the isothermal drying process of a porous medium and the molecular diffusion process, by adding the effect of gravity in a single-block model and by sensitizing the various parameters of the porous medium and fluids in the process such as: fluid type, different pressures, pores and throats size, the gravity drainage and molecular diffusion mechanisms were evaluated. According to the results, at a pressure of 101.3 kPa, the desaturation time of the liquid phase of the Heptane-Nitrogen and Heptane-Carbon dioxide systems is about 18 and 170% longer than the Heptane-Methane system, respectively. This trend is also true at high pressures. By changing the liquid phase from Heptane to Octane and Decane, the desaturation time of the liquid phase occurs 3.6 and 19 times later, respectively. The results also showed that the effect of increasing the throat length does not prolong the depletion time of the liquid phase as much as increasing the throat radius.