Using geologic CO2 storage for enhanced geothermal energy and water recovery and energy storage

Reductions in CO2 emissions at a scale consistent with limiting the increase in the global average temperature to below 2oC above pre-industrial levels requires a range of measures, including increased use of renewable and low-carbon energy and reduced CO2 intensity of fossil energy use, with each of these measures having major deployment barriers. The variability of the predominant renewable resources (wind and solar) requires major advances in utility-scale diurnal-to-seasonal energy storage. Base-load energy, such as nuclear, that cannot be cycled during periods of over-generation will have difficulty co-existing on electric grids with a large presence of variable renewables. Major deployment barriers for CO2 capture, utilization, and storage (CCUS) in saline reservoirs include: (1) net cost (after accounting for utilization benefits); (2) water intensity of CO2 capture, and (3) overpressure, which is fluid pressure that exceeds the original reservoir pressure due to CO2 injection, because it drives key storage risks: induced seismicity, caprock fracture, and CO2 leakage. We present a synergistic approach to CCUS in sedimentary basins designed to address each of these deployment barriers. Our approach uses the huge fluid and thermal storage capacity of the subsurface, together with overpressure driven by CO2 storage, to harvest, store, and dispatch energy from subsurface (geothermal) and surface (solar, nuclear, fossil) thermal resources, as well as excess energy from electric grids. Captured CO2 is injected into saline reservoirs to store pressure, generate artesian flow of brine, and provide a supplemental working fluid for efficient heat extraction and power conversion. Concentric rings of injection and production wells create a hydraulic divide to confine the stored pressure, CO2, and thermal energy below the caprock seal that overlies the CO2 storage reservoir. This energy storage can take excess power from the grid and excess/waste thermal energy from thermal power plants, and dispatch that energy when it is demanded and thus enable higher penetration of variable renewables, while utilizing thermal energy that would otherwise be wasted. CO2 stored in the subsurface functions as a cushion gas to provide enormous pressure-storage capacity and displace large quantities of brine, which will flow under artificially-created artesian pressure up production wells. Geothermal power generated from produced CO2 and brine and energy-storage applications may generate enough revenues to compensate for (or to even exceed) CO2 capture and storage costs. To address the CCUS deployment barrier of overpressure, we apply a pressure-management strategy that diverts a portion of the produced brine once a target overpressure is reached at the injection wells. The target overpressure is that determined to be low enough to reduce the risk of induced seismicity, caprock fracture, and CO2 leakage. Diverted brine is available for beneficial consumptive use, such as for power-plant cooling, or it can be used to generate fresh water using desalination technologies, such as reverse osmosis. The benefit of water generation can be particularly valuable in water-stressed regions. Our analyses indicate that only a small portion (\u3c 5% unless CO2is stored at a very high rate) of the produced brine needs to be diverted for the injection wells to remain below the target overpressure. Because the required recovery factor for desalination is relatively small ( Our approach has several advantages over conventional (e.g., hydrothermal) and enhanced geothermal energy systems (EGS). CO2 is a very efficient geothermal working fluid. Combined with the benefits of harnessing the overpressure driven by CO2 storage and the greater lateral extent, permeability, and porosity of sedimentary basins, compared to hydrothermal upflows or artificially-created EGS reservoirs, it allows for much greater spacing between injection and production wells. This efficient use of wells enables utilizing resources with lower temperatures than those of typical geothermal systems, resulting in wider deployment potential. The added benefit of bulk energy storage (BES) creates an arbitrage opportunity that enhances economic viability. Our analyses show that BES achieved by time-shifting the parasitic load of pressurizing our system does not reduce the efficiency of driving fluid recirculation; hence our approach is more efficient than other BES technologies, such as lithium-ion batteries or pumped hydro. Because the primary cost of BES is that associated with oversizing the pumps for fluid reinjection, the capital cost is much less than that of other BES approaches. Moreover, the huge capacity of the subsurface can enable seasonal energy storage, while most other approaches are limited to diurnal storage. This study was funded by the U.S. Department of Energy (DOE) Geothermal Technologies Office (GTO) under grant number DE-FOA-0000336, managed by Elisabet Metcalfe and Sean Porse, and a U.S. National Science Foundation (NSF) Sustainable Energy Pathways (SEP) grant (1230691). This work was performed under the auspices of the USDOE by LLNL under contract DE-AC52-07NA2734

The value of CO2-geothermal bulk energy storage to CO2

Two primary challenges for modern societies are to reduce the amount of carbon dioxide (CO2) that is emitted to the atmosphere and to increase the penetration of renewable energy technologies into electricity systems. CO2-bulk energy storage (CO2-BES) is a CO2 capture and storage (CCS) approach that can address both of these challenges by using CO2 emitted from large point sources (e.g., fossil fuel power plants, cement manufacturers) that is sequestered in sedimentary basin geothermal resources to take power from, and deliver power to, electricity grids. Electricity can be generated by wind and solar energy technologies regardless of whether there is demand for that electricity because wind and sunlight are variable resources. When over-generation occurs, the excess electricity can be used to compress and inject CO2 into sedimentary basin geothermal resources. Electricity can then be dispatched when needed by producing the pressurized and geothermally-heated CO2 from the storage reservoir and converting the heat to electricity in a CO2-geothermal power plant. In this way, CO2-BES can time-shift excess electricity that is generated by wind and solar energy facilities to when there is demand for that electricity. This ability can increase the utilization of installed wind and solar energy capacity. Thus, CO2-BES can (1) directly reduce CO2 emissions to the atmosphere by isolating them in porous and permeable subsurface reservoirs and (2) indirectly reduce CO2 emissions by displacing electricity from power plants that emit CO2 (e.g., fossil fuel plants) with electricity from wind and solar energy facilities. We present an approach to estimate the value of these direct and indirect benefits. Our approach uses an optimization model that we developed to determine the cost-minimizing dispatch of electricity-generating facilities to meet diurnal demand in regional electricity systems. In our analysis, electricity can be generated by base load and variable load power plants, wind- and solar-energy technologies, and CO2-BES facilities. We varied prices on CO2 emissions (e.g., a CO2 emissions tax) in order to determine the optimal CO2-BES storage capacity for each CO2 price. This method allows us to assign a monetary value to the optimized energy storage capacity. We use time increments of one hour, during which we assume electricity generation and demand are constant. Initial results using hypothetical but realistic scenarios for electricity demand and electricity generation by solar energy technologies suggest that the optimal energy storage capacity of CO2-BES is sensitive to a range of CO2 prices. That is, a small increase in the price on CO2 emissions can cause substantial change in the optimal distribution of electricity generation and the energy storage capacity of CO2-BES. Thus, independent system operators (ISOs) could dispatch CO2-BES without needing additional ancillary service compensation schemes if CO2 emissions were modestly taxed. This work was funded by the U.S. National Science Foundation Sustainable Energy Pathways program (grant 1230691)

Influence of Hydrologic Heterogencity on Thermal-Hydrologic Behavior in Emplacement Drifts

Fracture networks have been characterized as highly permeable continuum within the porous rock matrix in thermal-hydrologic models used to support performance assessments of the proposed nuclear-waste repository at Yucca Mountain. Uncertainty and spatial variability of the fracture permeability are important considerations for understanding thermal-hydrologic behavior within the host rock surrounding an emplacement drift. In this paper, we conducted numerical experiments with a number of realizations of intrinsic fracture permeability and examine thermal conditions around an emplacement drift. Peak temperature and boiling duration on the drift wall are used as indices to quantify, the influence of fracture permeability. The variability of peak temperature and boiling duration resulting from small-scale fracture-permeability heterogeneity is compared with the variability resulting from variability of host-rock thermal conductivity and infiltration flux. An examination of rock dryout and condensate drainage shows that small-scale heterogeneity in fracture permeability results in a relatively small range in dryout volume and does not prevent the shedding of condensate through the pillar-separating emplacement drifts

Geologic CO2 storage using pre-injection brine production in tandem reservoirs: A strategy for improved storage performance and enhanced water recovery

Deployment barriers for CO2 capture, utilization, and storage (CCUS) in saline reservoirs can be grouped under three categories: (1) net cost (after accounting for utilization benefits); (2) water intensity of CO2 capture, and (3) uncertainty about storage capacity and permanence. The third category is often considered to be the most challenging. Overpressure, which is fluid pressure that exceeds the original reservoir pressure due to CO2 injection, is the limiting metric for storage capacity and permanence because it drives key physical risks: induced seismicity, caprock fracture, and CO2 leakage. Variables that control overpressure include: (1) the quantity of CO2 and the rate at which it is injected, (2) the size of the reservoir storage compartment, and (3) reservoir permeability. Geologic surveys, geologic logs, and core data from exploration wells provide information that can be used to estimate the size and permeability of the reservoir compartment, but large uncertainties will only be narrowed after there is operational experience with moving large quantities of fluid to move into and/or out of the reservoir. Unlike CCUS applied to CO2 Enhanced Oil Recovery (CO2-EOR) in mature oil fields, CCUS in a saline reservoir will typically (a) have less geologic information and little or no production and injection history to estimate how much CO2 can be safely and permanently stored and (b) not have the advantage of depleted reservoir pressure prior to CO2 injection. Numerous studies have evaluated strategies for managing CO2 storage reservoirs by producing brine to reduce the pressure buildup due to CO2 injection. Most of these studies assume that separate injection and production wells will be used and that brine production will begin during or after the CO2 injection phase. We present a strategy where brine production begins prior to the CO2 injection phase, using the wells that will subsequently be used for CO2 injection. In this strategy, all wells are initially used for exploration and monitoring and then to produce brine prior to injecting CO2. Our strategy also includes the option of using reservoirs in tandem, including: CO2-storage reservoirs: due to their high seal integrity, these are preferred for CO2 storage. Brine produced from these reservoirs may or may not be directly used for water generation. Brine-storage reservoirs: these are used to store brine and/or residual brine and, with treatable brine composition, to produce brine for water generation. For zero net injection, high seal integrity is not required. This strategy has several advantages. First, pressure drawdown observed during brine production mirrors the pressure buildup during CO2 injection, providing necessary data to directly estimate reservoir storage capacity before any CO2 is injected. Second, pressure drawdown is greatest where CO2 will be injected, which is more efficient both on a per well basis and per mass of removed brine basis. Pre-injection brine production in saline reservoirs shares two key advantages of CO2-EOR: (a) greater knowledge about reservoir properties and storage capacity and (b) depleted reservoir pressure, which increases storage capacity. A third advantage is that the flexibility of our tandem-reservoir approach can be used to improve the economics of Enhanced Water Recovery (EWR). The primary metric for selecting a brine-storage reservoir is for its brine composition to be more amenable for treatment for beneficial uses, such as saline cooling water or water generated through desalination. Where applicable, EWR will reduce the water intensity of CCUS, which is particularly valuable in water-stressed regions. For a range of tandem-reservoir scenarios, we assess the influence of CO2-storage and brine-storage reservoir properties (e.g., reservoir compartment size, seal permeability, and salinity) on reservoir pressure management and EWR. We also illustrate how pre-injection brine production can be used as a tool for site selection and characterization, including assessments of CO2 storage capacity and permanence. This work was sponsored by the USDOE Fossil Energy, National Energy Technology Laboratory, managed by Traci Rodosta and Andrea McNemar. This work was performed under the auspices of the USDOE by LLNL under contract DE-AC52-07NA27344

Integrated Geothermal-CO2 Reservoir Systems: Reducing Carbon Intensity through Sustainable Energy Production and Secure CO2 Storage

AbstractLarge-scale geologic CO2 storage (GCS) can be limited by overpressure, while geothermal energy production is often limited by pressure depletion. We investigate how synergistic integration of these complementary systems may enhance the viability of GCS by relieving overpressure, which reduces pore-space competition, the Area of Review, and the risks of CO2 leakage and induced seismicity, and by producing geothermal energy and water, which can defray parasitic energy and water costs of CO2 capture

Double Diffusive Natural Convection in a Nuclear Waste Repository

Abstract -In this study, we conduct a two-dimensional numerical analysis of double diffusive natural convection in an er?lplacement drift for a nuclear waste repository. In-drift heat and moisture transport is driven by combined thermal-and conlpositional-induced buoyancy forces. Numerical results demonstrate buoyancy-driven convective flow patterns and configurations during both repository heat-up and cool-down phases. It is also shown that bounda ry conditions, particularly on the drip-shield surfnce, have strong impacts on the in-drift convective flow and transport