(582a) Competitive Adsorption and Displacement Behavior of CO2/CH4 Binary Mixture in Shale Matrix with Heterogeneous Surfaces

The CO₂ sequestration and enhanced gas recovery (CS-EGR) technology provides a very effective way to alleviate the greenhouse effect and energy crisis. The characteristics and mechanisms of CO₂/CH₄ competitive adsorption and displacement behavior in shale matrix as well as the effects of reservoir parameters play a vital role in enhancing shale gas production and CO₂ storage. However, it has not been fully understood especially in pores with heterogeneous surfaces composed of organic matter and inorganic compounds. A graphene-MMT heterogeneous surface was proposed as a pore model and molecular dynamics (MD) simulations have been applied.

Absolute adsorption selectivity based on the fixed mole fractions of CO₂ and CH₄ in the total shale pores has been proposed to evaluate the competitive adsorption features with relative adsorption selectivity. A strong asymmetric competitive adsorption behavior of CO₂ and CH₄ has been observed. In the case of certain temperature and pore size, the partial pressures of CO₂ and CH₄ are found to play the decisive role in the competitive adsorption by comparing the pores with homogeneous and heterogeneous surfaces. The competitive adsorption behaviors of CO₂/CH₄ near the corresponding surface in two types of pores are consistent when the partial pressure of the components of the mixture in pores with homogeneous surfaces are correspondingly equal to that in pores with heterogeneous surfaces. Therefore, it's better to describe the competitive adsorption behavior by gas partial pressure. Langmuir equation was applied to fitting adsorption isotherm in the first adsorption layer and the second adsorption layer. The good fitting results indicate that although the competitive adsorption mechanism of CO₂/CH₄ mixture can't be considered as monolayer adsorption due to the existence of multiple adsorption layers, each adsorption layer can be explained by Langmuir adsorption theory. To enhance the CH₄ recovery and CO₂ storage, the appropriate timing and amount of CO₂ injection were proposed. First reduce the reservoir pressure to as low as possible by, for example, fracturing, because the first pressure drawdown is obviously beneficial to CH₄ production. Second, inject a certain amount of CO₂ and soak. Third, reduce the pressure to the final pressure of the first pressure drawdown process. It should be noted that the Ratio should be controlled not to exceed 1.11 ~ 1.21 when injecting CO₂ in the second step, otherwise a huge separation cost will be required. After that, much CO₂ should be injected to restore reservoir pressure.

In addition, a CO₂−CH₄ displacement model that can provide a continuous pressure gradient in the displacement direction and a stable back-pressure at the pore outlet was developed based on the graphene-MMT heterogeneous surface pore. The model was divided into three areas: CO₂ injection area, pore area, and backpressure area. It was found that the displacement process always starts from the CH₄ reverse flow stage and then experiences the injection pressure action stage and positive displacement stage in sequence. Moreover, the extent of CH₄ reverse flow directly affects the system development process and the final displacement efficiency. A small system presorption pressure and a large injection pressure are beneficial to the displacement. It is believed that the reservoir pressure should be dropped to the lowest possible level during depressurization exploitation, and the CO₂ injection pressure needs to be selected by considering displacement efficiency, reservoir safety, and economic cost. The CO₂ occupies the adsorption sites near graphene faster than that near montmorillonite (MMT) in the direction of displacement, while the CH₄ desorption is faster near MMT. Therefore, it cannot be concluded that the displacement process near graphene is ahead of MMT. It is considered that the gas desorption/adsorption behavior near the graphene dominates the displacement process in terms of gas amount, while the fluctuation near MMT with time and injection pressure directly affects the gas adsorption variation in the pore space from the trend.

As carbon emissions cause more and more serious damage to the environment, CO₂ geological storage is one of the most concerned solutions. The decisive role of gas partial pressure and the adsorption/displacement mechanism of binary mixture discovered in this study may help to understand the adsorption behavior from a micro perspective, and the operation recommendations can effectively enhance the shale gas recovery and CO₂ storage. This is of great significance to environmental production and alleviation of energy crisis.