Study of CO₂ Enhancing Shale Gas Recovery Based on Competitive Adsorption Theory

As an indispensable part of unconventional natural gas resources, the shale reservoir is huge and widely distributed. It is of great significance to study how to enhance the shale gas recovery for improving the energy structure. In order to solve the problem of low gas production rate and long recovery period in the process of shale gas production, in this paper, the influences of pressure, temperature, moisture, and gas type on isothermal adsorption and desorption of shale gas are analyzed based on shale adsorption and desorption experiments, and the adsorption and desorption abilities of CO₂ and CH₄ in shale are compared to verify the feasibility of CO₂ enhancing shale gas recovery. Depletion production experiments and CO₂ injection experiments with different injection pressures (6 and 7 MPa), different injection rates (5, 10 and 20 mL/min), and different injection amounts are carried out. The mechanism of CO₂ enhancing shale gas recovery is proposed, and the parameters of CO₂ injection are optimized. The results show that the adsorption capacity of CH₄ increases with the increase in pressure and the decrease in temperature and moisture in a certain range. Under the same experimental conditions, the sorting of adsorption capacity is CO₂ > CH₄ > N₂, while desorption capacity is CH₄ > CO₂ > N₂. The desorption curves of the three gases lag behind the adsorption curves, in which the lag phenomenon of CO₂ is most obvious. The ultimate recovery of depletion production ranges from 66 to 73%. CO₂ injection can effectively increase the gas production rate of CH₄, and it can also keep the cumulative gas production of CH₄ growing steadily and rapidly. Within a certain range, CH₄ recovery increases with the increase in CO₂ injection pressure, the injection rate, and injection amount, but its increase range is related to the porosity and permeability of shale.