Quantitative Analysis of Natural Gas Diffusion Characteristics in Tight Oil Reservoirs Based on Experimental and Numerical Simulation Methods.

As an important mechanism in gas injection development, the diffusion characteristics of natural gas in tight reservoirs are important in the dynamic prediction of the development effect and optimization of injection-production parameters. In this paper, a high-pressure and high-temperature oil-gas diffusion experimental device was built, which was used to study the effects of the porous medium, pressure, permeability, and fracture on oil-gas diffusion under tight reservoir conditions. Two mathematical models were used to calculate the diffusion coefficients of natural gas in bulk oil and cores. Besides, the numerical simulation model was established to study the diffusion characteristics of natural gas in gas flooding and huff-n-puff, and five diffusion coefficients were selected based on experimental results for simulation study. The remaining oil saturation of grids, the recovery of single layers, and the distribution of CH mole fraction in oil were analyzed based on the simulation results. The experimental results show that the diffusion process can be divided into three stages: the initial stage of instability, the diffusion stage, and the stable stage. The absence of medium, high pressure, high permeability, and the existence of fracture are beneficial to natural gas diffusion, which can also reduce the equilibrium time and increase the gas pressure drop. Furthermore, the existence of fracture is beneficial to the early diffusion of gas. The simulation results show that the diffusion coefficient has a greater influence on the oil recovery of huff-n-puff. For gas flooding and huff-n-puff, the diffusion features both perform such that a high diffusion coefficient results in a close diffusion distance, small sweep range, and low oil recovery. However, a high diffusion coefficient can achieve high oil washing efficiency near the injecting well. The study is helpful to provide theoretical guidance for natural gas injection in tight oil reservoirs.