Integrated, geothermal-CO₂ storage reservoirs: Adaptable, multi-stage, sustainable, energy-recovery strategies that reduce carbon intensity and environmental risk

We analyze an adaptable, multi-stage, energy-recovery approach to reduce carbon intensity through sustainable geothermal energy production and secure geologic CO₂ storage (GCS) with low environmental risk in saline, sedimentary formations. We combine the benefits of the approach proposed by Buscheck (2010), which uses brine as the heat-transfer working fluid, with those of the approach first suggested by Brown (2000) and analyzed by Pruess (2006), using CO₂ as the working fluid, and then extended to saline-formation GCS by Saar et al. (2010) and Randolph and Saar (2011a). We also use pressure management to reduce the risks of induced seismicity and CO₂ and brine leakage (Buscheck et al., 2012a; Court et al., 2012, 2011a, 2011b). Our approach can involve up to three stages, with stage one of the three-stage version involving recirculation of formation brine as the working fluid. In this paper we analyze a two-stage version, with stage one involving production (and net removal) of formation brine for heat recovery and to provide pressure management/relief for CO₂ injection. Net removal of produced brine is achieved by applying it to a beneficial consumptive use: feedstock for fresh water production through desalination, saline cooling water, or make-up water injected into a nearby reservoir operation, such as in Enhanced Geothermal Systems (EGS), where there can be a shortage of working fluid. For stage one, it is important to find feasible utilization/disposition options to reduce the volume of blowdown, which is the residual brine requiring injection into the geothermal-GCS reservoir (Buscheck et al. 2012a, 2012b). During stage two, which begins as CO₂ reaches the producers; co-produced brine and CO ₂ are the working fluids. We present reservoir analyses of two-stage, integrated geothermal-GCS, using a simple conceptual model of a homogeneous, permeable reservoir, bounded by relatively impermeable sealing units. We assess CO₂ storage capacity and geothermal energy production as a function of well spacing between CO₂ injectors and brine/CO₂ producers for various well patterns.