Brine Reinjection Into Shallower Aquifers During CO2 Sequestration: A Pressure Management Practice

Carbon sequestration is the process of removing carbon from the atmosphere and storing it underground. For it to effective, it has to accommodate large volumes of CO₂ but at the same time pressure has to stay below the seal fracture pressure. High pressures resulting from the compartmentalization of systems with closed boundaries will limit the injection rate and hinder the feasibility of the whole project. Injecting into shallower formations increases the potential for leakage of CO₂ or brine into shallow fresh groundwater that could result in long and expensive remediation operations. Brine extraction from the injection zone and injection into overlying shallower aquifers has been presented as a pressure management tool multiple times in literature. With this technique deep aquifer storage capacity increases and, at the same time, injection of brine into shallow aquifer is safer compared to direct CO₂ injection. In this communication, using geothermal gradient typical of the Gulf Coast, we investigate the implication of this method from pressure management and storage capacity point of view at a typical Gulf Coast storage site. We consider three different scenarios for CO₂ injection into saline aquifer: injection of CO₂ only in deep aquifer without the support of shallow aquifer; injection of CO₂ into both shallow and deep aquifers; and finally injection of CO₂ into deep aquifer and extraction and reinjection of the brine into shallow aquifer. Simple volumetric calculations based on compressibilities -considering natural pressure and geothermal gradients- can provide a simple tool to study range of storage capacity under different injection scenarios. Numerical simulations confirm the results of the analytical derivations. Compositional simulations carried out using the PVT and petrophysical data similar to a well-known experimental sequestration site in the Gulf Coast.