Experimental Investigation of Hydrocarbon and Non-Hydrocarbon Gas Injection in a Tight Danish North Sea Oil Reservoir

Lower Cretaceous oil reservoirs in the Danish part of the North Sea, are generally deep, with high porosity and very low permeability rock, which makes the applicability of any EOR method and any reservoir development plan challenging. One potentially feasible enhanced oil recovery (EOR) method is the gas injection, where the produced gas, flue gas, and CO₂ have sufficient injectivity to be the candidates. Therefore, this study aims to assess the applicability of gas injection as an EOR method for the challenging Lower Cretaceous oil reservoirs in the Danish North Sea. To exclude the effect of rock quality on recovery results, a single core plug from the Lower Cretaceous Tuxen formation without any open longitudinal fractures was selected, cleaned, and re-used for all experiments. The clean and dry core was saturated with formation water, and after that, the irreducible water saturation was achieved by displacing the water with dead crude oil. The dead oil was then displaced by live oil after four weeks of aging. The gas injection experiments afterward were conducted at two different pressures, 250 and 350 bar (current and initial reservoir pressures, respectively), and the reservoir temperature of 85 °C. During the experiments, effluent fluid density, differential pressure response across the core, and effluent gas flow rate are recorded in addition to the recovery data. This makes it possible to evaluate the two/three-phase flow characteristics accurately. The initial scenarios investigated are hydrocarbon gas injection, in this case, synthetic natural gas, at two different pressures. The results show that the recovery factor for the hydrocarbon gas injection is significantly higher at 350 bar (at near miscible condition) compared to 250 bar. Conversely, Since CO₂ is at the supercritical condition at both 250 and 350 bar at 85 °C, and is fully miscible with oil at both pressures, no significant difference is expected at different injection pressures for CO₂. Unlike, flue gas is immiscible at both pressures, showed a significantly lower recovery factor compared to natural gas and CO₂. However, comparing the efficiency of hydrocarbon gases and CO₂ is of great interest for the choice of gas for injection. The produced hydrocarbon gas might be most readily available, however, CO₂ injection includes the potential for storage of the injection gas.