With the current technology, only 3-10% of gas from tight shale is recovered economically through natural depletion, demonstrating a significant potential for enhanced shale gas recovery (ESGR). Experimental studies have demonstrated that shale kerogen/organic matter has a higher adsorption affinity for CO2 than methane, CH4. CO2 is preferentially adsorbed over CH4 with a ratio of up to 5:1. This paper examines CO2-ESGR in compressible shale during huff-and-puff injection to better understand the parameters controlling its feasibility and effectiveness. A mathematical model will be presented where the CO2-CH4 substitution mechanism is implemented in an injection-production setting representative of field implementation.
A single hydraulic fracture surrounded symmetrically by a shale matrix will be considered. During primary depletion, gas is produced from the shale to the fracture by pressure driven flow and gas desorption. The porosity and permeability of matrix reduces due to compressibility. Stopping production and then injecting CO2 through the fracture and into the shale leads to increase in pressure, causing both the preferential adsorption of CO2 over CH4 and the restoration of porosity and permeability. The shale matrix then releases more CH4 in favor of the more chemically sorptive CO2. Restarting production from the well then allows more CH4 gas to be produced. Experimental adsorption data were used to quantify the substitution parameters while typical field data were used for the operating and geometrical conditions. Additional investigated production mechanisms include gas diffusion and changes in apparent permeability and porosity due to compressibility (addressed in Section 2.3.2).
Through a series of simulations, it was found that for the example studied, diffusion contributes the most to the performance of CO2-ESGR. Compressibility significantly reduces the matrix porosity and permeability during primary depletion, giving a positive effect to gas recovery. During CO2 injection, matrix porosity and permeability mostly recovers depending on pressure increase in the reservoir. The gas production seemed little sensitive to fracture permeability. The pore radius, the diffusion coefficient and the rock compressibility, parameters related to flow in the matrix, all affected the performance of CO2-ESGR.