Carbonated water injection in the North Sea chalk reservoirs: Energy analysis and environmental assessment

Carbon dioxide is one of the most effective fluids for improving and enhancing oil recovery. It dissolves in oil and reduces its density (i.e., swells the oil) and viscosity, giving oil a higher mobility. However, when the reservoir condition is not suitable for the CO₂ to become miscible in oil, the high mobility and low density of the CO₂ causes channeling and gravity override, and as a result a poor sweep and early breakthrough. These problems can be addressed by dissolving the CO₂ in water, a fluid with lower mobility, and injecting it into the reservoir, known as carbonated water injection. It is observed experimentally that the injection of a water-soluble solvents such as CO₂ or DME into a chalk core (in tertiary mode) mobilizes a large fraction of the remaining oil and vastly improves the recovery factor. Moreover, the injected CO₂ if trapped in the reservoir, can mitigate the harmful impact of the CO₂ that is otherwise released to the atmosphere. This work tries to quantify the effectiveness of the carbonated water injection into a North Sea chalk reservoir in terms of the extra oil recovery, the overall process energy balance, and the net amount of stored carbon dioxide. The prerequisite to a successful implementation of the carbonated water flooding is the availability of the CO₂. Different options are considered in this work, viz., pipeline transport of the captured CO₂ from the nearby fossil-fuel power plants, liquefied CO₂ transported by a ship, and the wind-farm electricity-driven separation of CO₂ from the atmosphere. All the energy requirements for the separation, transport, and injection of CO₂ are included in the energy analysis. The carbonated water injection into the chalk reservoir is modeled using an in-house finite volume solver. The amount of the stored CO₂ in the reservoir is quantified from the simulation results. It is assumed that the produced CO₂ in the production wells is separated and re-injected into the reservoir. The final results is presented as the net amount of recovered hydrocarbon energy from the reservoir and the net amount of captured CO₂ per unit recovered energy. The effectiveness of this process is compared to other CO₂ capture and storage processes in terms of the energy requirement per unit mass of captured carbon dioxide. The energy analysis in this work, which is founded on the fundamental laws of thermodynamics, can be easily converted to economic analysis.