A new enhanced gas recovery scheme using carbonated water and supercritical CO₂

The transition for clean, efficient, reliable, and affordable sources of energy is one of the top challenges of this century. Despite significant advances in renewable energies such as solar and wind, fossil fuels will remain a primary source of energy for years to come. Natural gas, fueling power generation and other vital industrial sectors such as desalination, is expected to play a more prominent role in the energy mix for many countries around the world. The demand for natural gas, especially in the Middle East, is on the rise. With the shortfall in recurrent discoveries of conventional gas resources, improving the recovery factors from legacy gas reservoirs is crucial. On the other hand, burning gas, like other fossil fuels, is associated with significant greenhouse gas emissions that should be mitigated, which is critical to comply with the pressing call for low-emission economies. CO2 injection in gas reservoirs is appealing as it may provide a dual benefit, including enhanced gas recovery (EGR), and partial entrapment of CO2 in the subsurface reservoir. Different CO2-EGR recovery schemes have been studied in the literature and showed that key thermodynamic properties of CO2, such as its high viscosity and density at reservoir conditions, make CO2 an efficient displacing fluid for the hydrocarbon gas. On the one hand, injecting CO2 can maintain the reservoir pressure by providing voidage replacement amid gas production. On the other hand, it can displace and mobilize the gas in place towards the production wells; nonetheless, CO2-EGR may not be economically viable. The main drawback of CO2 injection is the relatively rapid breakthrough at the production wells leading to gas recovery inefficiency due to CO2 separation and recycling. This pre-mature breakthrough of CO2 is driven by CO2/gas dilution from mechanical mixing, dispersion, and molecular diffusion. Producing gas with high impurity because of CO2 mixing adds significant CAPEX and OPEX related to the surface facilities and gas treatment, especially for fields whose initial content of CO2 within the gas composition is negligible. In this work, we propose a CO2-based EGR scheme that involves two stages. The first stage corresponds to the situation when the reservoir pressure is below that CO2 bubble point pressure. At this condition, CO2 will be in the gas state, and therefore, its sweep and displacement efficiency will be poor owing to its low viscosity and density. At this stage, CO2/gas mixing will be pronounced, and CO2 mobility will be high, leading to an early breakthrough. To avoid this unfavorable situation, we propose to co-inject brine and CO2 (carbonated water) in which CO2 will be in the aqueous solution and non-buoyant. As the carbonated water is heavier than the in-situ brine, CO2 remains trapped in the aqueous phase and flows down-dip towards the bottom of the reservoir. In stage 2, CO2 is injected in its dense supercritical phase, where the presence of water reduces its mobility and delays its breakthrough time. This two-stage injection process is more efficient than the traditional continuous CO2 injection. We demonstrate the effectiveness of this recovery scheme for a synthetic model mimicking an actual gas field. We analyze the optimum transition conditions from stage 1 to stage 2 and show a significant incremental recovery and CO2 entrapment compared to the traditional CO2-EGR that relies on continuous CO2 injection. This proposed scheme serves both objectives, which are an efficient EGR and optimum CO2 sequestration resulting from delayed CO2 breakthrough. We believe that this scheme provides significant improvements to the traditional CO2-EGR and has the features to be deployed at the field scale.