Experimental Investigation of Oil Recovery From Siliceous Shale by Miscible CO₂ Injection

This work presents the results of tests using a low permeability (1.3 mD), medium porosity (34%) siliceous shale reservoir core sample initially saturated with oil followed by CO₂ injection at miscible conditions. The tests had 3 primary flow conditions that replicate aspects of expected field conditions: (i) countercurrent exchange of carbon dioxide from fracture to matrix with consequent production of oil (ii) cocurrent exchange of carbon dioxide from fracture to matrix with oil production at the downstream core face, and (iii) a flushing stage where carbon dioxide is injected at a pressure drop of 25 to 50 psi to drive the core to residual saturation. The countercurrent and cocurrent conditions are conducted at very low pressure difference across the fracture at the face of the core and along the length of the core, respectively. The miscible injection of CO₂ recovered virtually all of the original oil in place.

Results were simulated with a compositional reservoir simulator. The simulation model was constructed to be as similar as possible to the experimental conditions, including a heterogeneous permeability field. An exact match of the experimental results was difficult to obtain. Calculations agreed with the experiments on the total oil production but not the timing of recovery. Experimentally, more oil was recovered during countercurrent flow in comparison to simulation. Nevertheless, simulations did reveal CO2 penetration of the matrix via a countercurrent diffusion/dispersion mechanism. Incorporation of fracture-to-matrix CO2 transfer and matrix-to-fracture oil transfer were key elements of the simulation.