On Oil Recovery Mechanisms and Potential of DME-Brine Injection in the North Sea Chalk Oil Reservoirs

North Sea tight chalk oil reservoirs are well-known for their submicron pore throat sizes and heterogeneous porosity pattern that includes fractures and microfractures. The host rock of these reservoirs is extremely sensitive and can easily react with the injected fluid, which in turn adversely affects the permeability and thus injectivity. The combined effect of these parameters makes oil production in chalk reservoirs extremely difficult. A novel solvent-based enhanced oil recovery (EOR) method that can address these issues is investigated for the first time in the chalk reservoirs. We thouroughly investigate the oil recovery potential and dominant oil recovery mechanisms by dimethyl ether (DME)-brine injection under conditions pertinent to the North Sea tight chalk oil reservoirs. A series of systematically designed high-pressure and high-temperature flooding experiments were carried out using reservoir core and crude oil. The experimental results revealed the strong oil recovery potential of tertiary DME-brine injection with two different DME contents. Furthermore, both secondary and tertiary DME-brine injection scenarios significantly improved the oil recovery with the better performance in the secondary scenario. The results show that the dominant oil recovery mechanism is rapid and strong oil swelling is caused by the preferential partitioning of DME into the oil phase. During DME-brine injection, no indications of rock mineral dissolution and adverse effects on rock permeability were observed. This is one of the advantages of this method over CO2, CO2-water alternating gas (WAG), and alkaline injections in which the EOR agent causes calcite dissolution, wormhole formation, and scaling issues in fragile chalk reservoirs.