Reservoir performance is one of the key issues that have to be addJessed before going ahead with the development of a geothermal field. In order to select the type and size of the power plant and design other surface installations, it is necessary to know the characteristics of the production wells and of the produced fluids, and to predict the changes over a 1030 year period. This is not a straightforward task, as in most cases the calculations have to be made on the basis of data collected before significant fluid volumes have been extracted from the reservoir. The paper describes the methodology used in predicting the long-term perfonnance of hydrothermal systems, as well as DOE/G'ID-sponsored research aimed at reducing the uncertainties associated with these predictions.

The goals of the GEO-SEQ Project are to reduce the cost and risk of geologic sequestration and decrease the time to implementation. In order to reduce costs, it has been shown that enhanced oil recovery (EOR) methods can be optimized for sequestration, and enhanced gas recovery (EGR) with sequestration, is feasible. Art evaluation of the effects of SOx and NOx on geochemical reactions between CO2, water, and reservoir rocks, has been done to assess the use of impure waste streams as a means to reduce overall sequestration costs. In order to reduce sequestration risks a methodology for site-specific selection of subsurface monitoring technologies has been demonstrated, baseline data needed for interpretation of isotopic tracers used to monitor reservoir processes have been developed, and a new definition of formation capacity factor for use in assessing sequestration efficiency has been developed. Code comparison studies are underway for oil, gas, brine and coalbed reservoir simulators for predicting the fate Of CO2 in the subsurface. The GEO-SEQ Project has conducted field tests of monitoring technology at CO2 EOR projects in California and New Mexico, and is collaborating on a pilot brine formation sequestration test in Texas.

A study has been conducted to gather and interpret information regarding potential approaches for assessing, managing and mitigating risks associated with the deep geologic storage Of CO2. Information was gathered from two principle sources: (1) industrial analogues such as natural gas storage, deep injection of hazardous wastes and nuclear waste storage and (2) natural analogues, especially those with CO2 leaks at the surface. For the industrial analogues the following were evaluated: history, status and scope of the activity; risk assessment framework and methods; including key issues, performance specifications and performance assessment methods; risk management approaches, including regulatory oversight and permitting; site characterization methods, monitoring and performance confirmation; risk mitigation and remediation methods employed or planned in the event that performance specifications are not met or other unintended consequences arise; and case studies documenting responses to historical situations. Information about natural analogues for surface leakage Of CO2 were reviewed and evaluated in light of what is known about human and ecological impacts of exposure to elevated concentrations Of CO2.A set of lessons-learned from these analogues was compiled and forms the basis for recommendations in the areas of risk assessment framework and methodology, risk management approaches, and risk mitigation and remediation methods. This paper reviews the lessons-learned and also provides recommendations for additional research.

The Cerro Prieto field of northern Mexico is the largest water-dominated geothermal field under commercial production at the present time. In 2000 more than 115 million tons of fluids were extracted, but only about 20% was injected back into the reservoirs. This deficit has contributed to a drop in pressures resulting in localized boiling, influx of cooler groundwater and other reservoir processes. The influence of geologic features, especially faults, on these processes and on the response of different parts of the field to production is described. The study illustrates the importance of a multidisciplinary approach to gain an understanding of the behavior of geothermal systems under production.

The Cerro Prieto field in northern Mexico has been under commercial exploitation for 25 years. At present its three power plants are generating around 600 MWe (the total installed capacity is 620 MWe). Almost 1800 million tons of fluids have been produced between 1973 and 1997 with only about 140 million tons injected back into the reservoir (injection started in 1989). In spite of the large net fluid (and heat) extraction ftom the system, wells continue to supply steam to the power plants. This is largely due to the natural recharge of the reservoir. The inflow of cooler waters does not occur everywhere. Groundwater recharge along the north boundary of the present wellfield seems to be minor, but the lateral influx through the western and southern edges of the field, as welI as vertical through two normal faults, has been documented. The amount of natural fluid recharge at Cerro Prieto is estimated based on changes in chloride in the produced fluids. Because of the pressure support provided by natural recharge, not only has the life of the field been extended, but also that of individual wells. The behavior of wells in areas a&cted by this fluid inflow contrasts with that of wells located where recharge is only minor, or non existent. Other wells are influenced by the injection of waste geothermal waters.

In the Salton Trough, of southern California and northern Baja California, the most important geothermal fields are those at the Salton Sea (SSGF) and Cerro Prieto (CPGF). They have nearly the same geologic framework; both are located in active pullapart basins within the Trough, an actively growing rift valley which is the northern landward extension of the Gulf of California. The lithologic columns in these fields are dominated by deltaic and alluvial deposits, with that of the SSGF being richer in lacustrine sediments and evaporites. The heat sources in both fields are oceanic ridge-type intrusions of sheeted dike complexes. The volcanoes found in the CP and SS areas are genetically related to the hypabyssal rocks intercepted by some of the geothermal wells. The maximum temperatures measured in the wells are similar (i.e., around 350370° C). The main difference between the two fields is in the salinity of the geothermal fluids. The maximum total dissolved solids in the SSGF geothermal brines is about 30% and only about 3% in CPGF brines. The hydrogeological regimes and the lithology prevailing in these fields could reflect this contrast.

The liquid-dominated Cerro Prieto geothermal field of northern Baja California, Mexico has been under commercial exploitation since 1973. During the early years of operation, all waste brines were sent to an evaporation pond built west of the production area. In 1989, cooled pond brines began to be successfully injected into the reservoir along the western boundary of the geothermal system. The injection rate varied over the years, and is at present about 20% of the total fluid extracted. As expected under the continental desert conditions prevailing in the area, the temperature and salinity of the pond brines change with the seasons, being higher during the summer and lower during the winter. The chemistry of pond brines is also affected by precipitation of silica, oxidation of H{sub 2}S and reaction with airborne clays. Several production wells in the western part of the field (CP-I area) showed beneficial effects from injection. The chemical (chloride, isotopic) and physical (enthalpy, flow rate) changes observed in producers close to the injectors are reviewed. Some wells showed steam flow increases, in others steam flow decline rates flattened. Because of their higher density, injected brines migrated downward in the reservoir and showed up in deep wells.

Extraterrestrial environments expose engineering components to severe operating conditions for extended durations. Challenges of extreme temperature changes and high vacuum are being met by careful selection of available materials or by development of novel high-performance materials and processes. Bulk Metallic Glasses (BMGs) are amorphous alloys that exhibit very high strengths, hardness, strain energy storage, corrosion resistance, and the capacity for thermoplastic formability, and are of interest in space engineering design. Using criteria driven by the requirements of specific space-based mechanisms, BMG alloys were selected from the literature for processing and performance evaluation. Alloys particularly suited for gearing applications and flexure-based compliant mechanisms were identified and standard test specimens produced. Four Zr-based BMG alloys were selected for analysis: Zr53Al16Co23.25Ag7.75, Cu47Zr46Al5Y2, Zr49Ti1.96Cu37.24Al9.8Y2, and Zr60Ti2Nb2Al7.5Ni10Cu18.5, with compositions in at.%. Presented in this work are the results of compression testing, fatigue testing, and differential scanning calorimetry; previous results of wear and hardness testing have been presented elsewhere. A maximum space service temperature of 400 °C is considered. The alloys exhibited significant differences in processability and thermomechanical performance. The alloy Zr49Ti1.96Cu37.24Al9.8Y2 exhibited the best overall performance for gearing applications; and Zr60Ti2Nb2Al7.5Ni10Cu18.5 was the best alloy for compliant flexures.

Variations in temperature and salinity in hypersaline liquid-dominated geothermal systems like the Salton Sea Geothermal System (SSGS) tend to be correlated such that liquid density is relatively constant in the system. The tendency toward small density variations may be due to connectivity with a surrounding regional aquifer at multiple depths in the stratigraphic column. We present numerical simulation results for natural convection in geothermal systems like the SSGS in hydraulic connection with a constant-density aquifer. Natural convection where there are two sources of buoyancy such as heat and salt, with different diffusivities, is called double-diffusive convection. Simulations of double-diffusive convection are carried out using our general-purpose reservoir simulator TOUGH2 with a newly developed twodimensional heat and brine transport module (T2DM) that includes Fickian solute dispersion. The model includes an accurate formulation for liquid density as a function of temperature and salinity. Our simulation results show many features that are consistent with observations of the SSGS, making conceptual models that involve hydraulic connectivity with a surrounding aqulfer appear plausible. The generality of our model makes the results broadly applicable to systems similar to the SSGS.

The deep Cerro Prieto (Baja California, Mexico) beta reservoir is offset vertically by the southwest-northeast trending, normal H fault. Under exploitation pressures in the upthrown block have decreased strongly resulting in boiling and high-enthalpy production fluids. Significant differences in fluid chemical and isotopic compositions are observed in the two parts of the reservoir and particularly in an anomalous zone associated with the H fault. These differences result from intense boiling and adiabatic steam condensation, as well as from leakage of overlying cooler water along the fault.