Dynamic diffusion processes of miscible CO2 geological utilization with complex microscopic pore-throat structures

Abstract

It is the key to transform the energy structure and restrain the greenhouse effect for CO₂ utilization and storage. Tight reservoir has attracted special attentions due to the huge reserves and mature technology. The gas diffusion is a main and important element to improve the CO₂ utilization efficiency and storage potential. In this paper, first, the pore constructure, pore pressure and confining pressure are simulated and a series of experiments of CO₂ diffusion are established to describe the diffusion process with CO₂ utilization in tight formations. Second, a dynamic model of CO₂ diffusion in tight formation is developed based on the pressure-decay experiments and Fick’s law. Subsequently, the diffusion coefficient of CO₂ is ensured by fitting experimental results, and the accuracy of established model is also evaluated. Finally, the effects of water slug, permeability, temperature and diffusion coefficient on diffusion rate, concentration distribution, diffusion front and diffusion distance at different stage are investigated, and the diffusion distance under different temperature, diffusion coefficients and diffusion time are charactered quantitatively in the processes of CO₂ utilization in tight formations. The results show that the CO₂ diffusion rate at stage I and II is 0.075 cm/d–0.15 cm/d and 0.18 cm/d–1.62 cm/d respectively, and the water slug will weaken the diffusion rate of CO₂. Meanwhile, the CO₂ concentration in the upper part of the core is high due to the effect of gravity overlap, which will reduce the oil’s flow capacity and sweep efficiency. The diffusion rate, concentration and diffusion distance will increase when the temperature rises form 58 ℃ to 88 ℃, and the effects of temperature on diffusion characteristics at stage I of CO₂ diffusion are weaker than that of stage II. The diffusion time of reaching the boundary is 90000 s, 60000 s and 30000 s at stage I, and is 190000 s 160000 s and 130000 s at stage II, respectively when the diffusion coefficients rise from 1.45 × 10⁻⁹m²/s to 14.5 × 10⁻⁹m²/s. The CO₂ concentration is primarily in the range of 1.0 mol/m³ to 4.2 mol/m³, 3.2 mol/m³ to 9.8 mol/m³ and 11.8 mol/m³ to 18.9 mol/m³ with different diffusion coefficients, respectively. This paper provides physical simulation and theoretical analysis for the investigation and application of the diffusion processes of CO₂ utilization and storage in tight formations.