No abstract

Summary Disposal of CO2 from stationary sources (fossil-fired power plants) into brackish (saline) aquifers has been suggested as a possible means of reducing emissions of greenhouse gases into the atmosphere. Injection of CO2 into such aquifers would be carried out at supercritical conditions and would give rise to the evolution of a two-phase fluid system, in which most of the injected CO2 will reside in a dense supercritical gas phase, while also dissolving partially into the aqueous phase and reacting with native minerals. This paper presents scoping studies of the amounts of CO2 that can be trapped into the various phases (gas, aqueous, and solid) for a range of conditions that may be encountered in typical disposal aquifers. Our analyses employ a realistic fluid property (PVT) description of brine/CO2 mixtures for supercritical conditions, which takes into account real gas density and viscosity effects for CO2, and includes pressure, temperature, and salinity dependence of CO2 dissolution into the aqueous phase. The fluid property description has been incorporated into a multipurpose reservoir simulator, and has been used to evaluate dynamic effects of CO2 injection into aquifers. A survey of minerals commonly encountered in crustal rocks was made to identify possibilities for chemical sequestration of CO2 through the formation of carbonates of low solubility. We also performed batch reaction modeling of the geochemical evolution of representative aquifer mineralogies. Results indicate that under favorable conditions the amount of CO2 that may be sequestered by precipitation of secondary carbonates is comparable to the amount of CO2 dissolved in pore waters. The accumulation of carbonates in the rock matrix and induced rock mineral alteration caused by the presence of dissolved CO2 lead to a considerable decrease in porosity. Introduction Combustion of fossil fuels such as oil, natural gas, and coal currently generates in excess of 27 billion tonnes of carbon dioxide per year worldwide,1 virtually all of which is discharged into the earth's atmosphere. Because of the expanded use of fossil fuels, the atmospheric concentration of CO2 has risen from preindustrial levels of 280 ppm (parts per million) to present-day values of approximately 365 ppm.2 The Intergovernmental Panel on Climate Change (IPCC) has projected that for a "business as usual" energy scenario, the atmospheric concentrations of CO2 may double by the middle of the 21st century and continue to rise at increasing rates beyond.3 Atmospheric CO2 is a "greenhouse gas," so called because it traps outgoing infrared and thermal radiation, thereby increasing near-surface temperatures. There is some evidence from climate modeling that increased atmospheric concentrations of CO2 may be the chief contributor to "global warming," currently estimated to amount to 0.3-0.6°C during the last 150 years.4 The U.S. Department of Energy (DOE) and other organizations have initiated broad technology programs to assess and develop methods for reducing atmospheric emissions of CO2.1 One of the more promising concepts involves disposal of CO2 into geologic formations, including oil and gas reservoirs, coal beds, and saline aquifers. CO2 injection into oil and gas reservoirs, and methane bearing coal beds, can provide collateral benefits in terms of enhancing recovery of oil and natural gas. Saline aquifers are attractive as CO2 disposal reservoirs because they are generally unused and are available in many parts of the U.S. Geologic disposal of CO2 into aquifers would be made at supercritical pressures in order to avoid adverse effects from CO2 separating into liquid and gas phases in the injection system. The critical point of CO2 is at Pcrit=73.82 bar, Tcrit=31.04°C,5 so that minimum aquifer depths of approximately 800 m would be required to sustain a supercritical pressure regime. Injection of CO2 into such aquifers would give rise to the evolution of a two-phase fluid system, in which most of the injected CO2 will reside in a dense supercritical gas phase, while also partially dissolving into the aqueous phase and reacting with native minerals. From an engineering perspective, the main issues for CO2 disposal in aquifers relate to the following factors:1. The rate at which CO2 can be disposed.2. The available storage capacity (ultimate CO2 inventory).3. The presence of a caprock of low permeability, and the potential for CO2 leakage through imperfect confinement, which may be natural or induced.4. Identification and characterization of suitable aquifer formations and caprock structures.5. Uncertainty and possibility of failure caused by incomplete knowledge of subsurface conditions and processes.6. Corrosion resistance of materials to be used in injection wells and associated facilities. CO2 disposal in aquifers has been discussed in the technical literature since the early 1990s.6-14 There is an obvious analogy to natural gas storage in aquifers, a mature technology that is widely applied in the northeastern U.S.15 While there is considerable experience with underground injection of CO2 for enhanced oil recovery,16 the Sleipner Vest project in the Norwegian sector of the North Sea is the only practical example of CO2 disposal into an aquifer.17,18 Flow systems that involve water, CO2, and dissolved solids have been extensively studied in geothermal reservoir engineering.19-22 The geothermal work mostly addresses higher temperatures and lower CO2 partial pressures than would be encountered in aquifer disposal of CO2; some of it involves not only multiphase flow but also chemical interactions between reservoir fluids and rocks.23 The amounts of CO2 that would need to be disposed of at fossil-fueled power plants are very large. A coal-fired plant with a capacity of 1,000 MWe (electric) generates approximately 30,000 tonnes of CO2 per day.14 Large-scale injection of CO2 into aquifers will induce a range of strongly coupled physical and chemical processes, including multiphase fluid flow, changes in effective stress, solute transport, and chemical reactions between fluids and formation minerals. The displacement of aquifer water by CO2 is subject to hydrodynamic instabilities, including viscous fingering and gravity override,13,24 which may give rise to bypassing and preferential flow along localized pathways; this in turn impacts, favorably as well as unfavorably, CO2 containment and storage capacity. Fluid pressures will rise as CO2 displaces aquifer water in which it partly dissolves. CO2 dissolution will lower the pH of the aqueous phase, and will give rise to chemical interactions with aquifer minerals. Dissolution of primary and precipitation of secondary minerals will alter formation porosities and permeabilities. Continuous injection of CO2 will cause formation pressures to rise over large areas of the order of 1000 km2 or more, which will modify the local mechanical stress field, causing deformation of the aquifer.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractDisposal of CO2 from stationary sources (fossil-fired power plants) into brackish (saline) aquifers has been suggested as a possible means for reducing emissions of greenhouse gases into the atmosphere. Injection of CO2 into such aquifers would be carried out at supercritical conditions, and would give rise to the evolution of a two-phase fluid system, in which most of the injected CO2 will reside in a dense supercritical gas phase, while also dissolving partially into the aqueous phase, and reacting with native minerals. This paper presents scoping studies of the amounts of CO2 that can be trapped into the various phases (gas, aqueous, and solid) for a range of conditions that may be encountered in typical disposal aquifers. Our analyses employ a realistic fluid property (PVT) description of brine-CO2 mixtures for supercritical conditions, which takes into account real gas density and viscosity effects for CO2, and includes pressure, temperature, and salinity dependence of CO2 dissolution into the aqueous phase. The fluid property description has been incorporated into a multi-purpose reservoir simulator, and has been used to evaluate dynamic effects of CO2 injection into aquifers. A survey of minerals commonly encountered in crustal rocks was made to identify possibilities for chemical sequestration of CO2 through the formation of carbonates of low solubility. We also performed batch reaction modeling of the geochemical evolution of representative aquifer mineralogies. Results indicate that under favorable conditions the amount of CO2 that may be sequestered by precipitation of secondary carbonates is comparable to the amount of CO2 dissolved in pore waters. The accumulation of carbonates in the rock matrix and induced rock mineral alteration due to the presence of dissolved CO2 lead to a considerable decrease in porosity.

The Northwest Geysers Enhanced Geothermal System (EGS) demonstration project aims to create an EGS by directly and systematically injecting cool water at relatively low pressure into a known High Temperature (280-400°C) Zone (HTZ) located under the conventional (240°C) geothermal steam reservoir at The Geysers geothermal field in California. In this paper, the results of coupled thermal, hydraulic, and mechanical (THM) analyses made using a model developed as part of the prestimulation phase of the EGS demonstration project is presented. The model simulations were conducted in order to investigate injection strategies and the resulting effects of cold-water injection upon the EGS system; in particular to predict the extent of the stimulation zone for a given injection schedule. The actual injection began on October 6, 2011, and in this paper a comparison of pre-stimulation model predictions with micro-earthquake (MEQ) monitoring data over the first few months of a one-year injection program is presented. The results show that, by using a calibrated THM model based on historic injection and MEQ data at a nearby well, the predicted extent of the stimulation zone (defined as a zone of high MEQ density around the injection well) compares well with observed seismicity. The modeling indicates that the MEQ events are related to shear reactivation of preexisting fractures, which is triggered by the combined effects of injection-induced cooling around the injection well and small changes in steam pressure as far as half a kilometer away from the injection well. Pressure-monitoring data at adjacent wells and satellite-based groundsurface deformation data were also used to validate and further calibrate reservoirscale hydraulic and mechanical model properties. The pressure signature monitored from the start of the injection was particularly useful for a precise back-calculation of

In this report, we present a numerical representation for the partial molar volume of CO 2 in water and the calculation of the corresponding aqueous solution density. The motivation behind this work is related to the importance of having accurate representations for aqueous phase properties in the numerical simulation of carbon dioxide disposal into aquifers as well as in geothermal applications. According to reported experimental data the density of aqueous solutions of CO 2 can be as much as 2-3 % higher than pure water density. This density variation might produce an influence on the groundwater flow regime. For instance, in geologic sequestration of CO 2 , convective transport mixing might occur when, several years after injection of carbon dioxide has stopped, the CO 2rich gas phase is concentrated at the top of the formation, just below an overlaying caprock. In this particular case the heavier CO 2 saturated water will flow downward and will be replaced by water with a lesser CO 2 content.

We performed a series of 3D thermo-hydro-mechanical (THM) simulations to study the influences of hydromechanical and thermal processes on the development of an enhanced geothermal system, strongly influenced by a network of short fault zones. The model we developed was calibrated by comparing the simulated THM responses to field observations, including ground-surface deformations, well pressure, and microseismic activity. Of particular importance was the comparison between the observed temporal and spatial distribution of microseismic activity, and the calculated shear reactivation of preexisting fractures inferred from simulated elasto-plastic mechanical responses in the short fault zone. Using this approach, we could identify when fault zones were reactivated (as manifested in the field by a surge of local microseismic activity within the fault zone), and we could back-calculate the in situ stress field as being close to the stress conditions required for shear reactivation. Our results show that the main mechanisms of inducing seismicity are related to injection-induced pressure increase and cooling. During injection, the reservoir expansion caused by the pressure increase led to mechanical stress transfer through the reservoir, which prevented or delayed the reactivation of preexisting fractures. After injection stopped, there was an inversion of the mechanical stress transfers that favored shear reactivation, which may explain why microseismic activity occurred after the cessation of the injection.

a b s t r a c tAn Enhanced Geothermal System (EGS) Demonstration Project is currently underway in the Northwest Geysers. The project goal is to demonstrate the feasibility of stimulating a deep high-temperature reservoir (HTR) (up to 400 • C, 750 • F). Two previously abandoned wells, Prati State 31 (PS-31) and Prati 32 (P-32), were reopened and deepened to be used as an injection and production doublet to stimulate the HTR. The deepened portions of both wells have conductive temperature gradients of 10 • F/100 ft (182 • C/km), produce connate native fluids and magmatic gas, and the rocks were isotopically unexchanged by meteoric water. The ambient temperature meteoric water injected into these hot dry rocks has evidently created a permeability volume of several cubic kilometers as determined by seismic monitoring. Preliminary isotopic analyses of the injected and produced water indicate that 50-75% of the steam from the created EGS reservoir is injection-derived.

No abstract