Michael C. Adams

Thermal stabilities of aromatic acids as geothermal tracers

Abstract Thirty-nine aromatic acids were tested for their suitability as geothermal tracers. The parameters of the experiments included temperatures up to 300°C for periods of up to one month in fluids of various salinities, the presence or absence of rocks, and atmospheric levels of molecular oxygen. Of the compounds tested, at least 24 are suitable as tracers in a moderate-temperature geothermal environment while 5 may be used at temperatures as high as 300°C. The compounds displayed no adsorption on the rocks used in the tests. Some of the compounds were used successfully in a major tracer test at the Dixie Valley, Nevada geothermal system.

The Coso geothermal field: A nascent metamorphic core complex

Investigation of the Coso Range using seismicity, gravity, and geochemistry of rocks and fluids, supports the interpretation that the structure hosting the geothermal resource is a nascent metamorphic core complex. The structural setting is a releasing bend in a dextral strike-slip system that extends from the Indian Wells Valley northward into the Owens Valley. This tectonic setting results in NW-directed transtension, which is accommodated by normal and strike-slip faulting of the brittle upper 4–6 km of the crust, and shearing and ductile stretching below this depth, accompanied by shallow igneous intrusions. Focal mechanisms of some small earthquakes that have occurred from 1996 to the present beneath the Coso Range exhibit depth-dependent rotation of seismic P and T axes, indicating that the local orientations of the principal stresses likely favor resolved shear stress on low-angle faults. These small earthquakes occur near the base of seismicity, which we interpret as coincident with the brittle-ductile transition. Geochemical results show a significant asthenospheric influence in the isotopic composition of rocks and fluids, indicating that the crust is thinned within the Coso structure. Thinned upper crust is underlain by a more dense mafic body at depths of 10 km or less. This is consistent with observed gravity anomalies and models. The mafic body may represent cumulates left over from the fractional crystallization of rhyolite, which occurs as endogenous domes at Coso, or it could be a sheeted-dike complex in the upper mid-crustal area. Transtension began at 2–3 Ma, and continues today. Using a long-term crustal deformation rate of 2 mm/yr, we infer that the basal detachment fault commonly observed in fully exhumed metamorphic core complexes will reach the surface in two to four million years.

Kinetics of fluorescein decay and its application as a geothermal tracer

Abstract Fluorescein is a dye used to trace the path of injected fluids through geothermal reservoirs. We have measured its thermal stability at temperatures up to 300°C in hydrothermal autoclaves at various fluid compositions, pHs, and oxygen concentrations. The results of these experiments indicate that fluorescein will decay less than 10% during a one month tracer test in geothermal reservoirs with temperatures below 210°C. For tracer tests involving longer times and/or higher temperatures, the activation parameters presented in this study can be used to correct for thermal decay. These parameters were applied to a tracer test conducted at the Dixie Valley, Nevada geothermal system to correct for the thermal decay of fluorescein and to deduce the effective temperature of the injection-production flow path.

The formation and distribution of CO2-enriched fluid inclusions in epithermal environments

Abstract Fluid inclusions from two geothermal systems associated with volcanic activity were studied to document the distribution of CO 2 in modern epithermal environments. The fluid inclusion data, combined with mineral distributions and chemical analyses of the production fluids from both systems, provide a record of steam and gas flux to depths in excess of 1 to 2 km and of transient variations in the gas contents of the reservoirs. The liquid-rich fluid inclusions can be grouped into two types on the basis of their CO 2 contents. Inclusions with CO 2 contents of less than about 4 wt% typically have calculated gas contents that are higher than the present-day reservoir fluids. However, the calculated pressures and temperatures of these inclusions are consistent with their depth of formation, indicating that they may have formed in response to boiling and mixing processes in the reservoir. Liquid-rich fluid inclusions with CO 2 contents between approximately 4 and 6 wt% are characterized by CO 2 clathrate dissociation temperatures greater than 0.0°C. These inclusions occur on the margins of the thermal systems where they define umbrella-shaped caps around the main zones of upwelling. The CO 2 contents of the inclusions require that they formed at pressures several tens of bars above hydrostatic. Elevated pressures and gas contents may have developed through compression and condensation of CO 2 - enriched steam by tectonic stress.

Hydrofluorocarbons as geothermal vapor-phase tracers

Abstract Two hydrofluorocarbons, R-134a and R-23, have been developed for use as a vapor-phase tracer in geothermal systems. These low molecular weight compounds are volatile, electrically neutral, nontoxic, relatively inexpensive, and have detection limits as low as 10 −5 ppm. Data from laboratory and field tests indicate that they are stable enough to be used as tracers in vapor-dominated systems such as The Geysers. However, these compounds have a higher volatility than water, which affects the rate of transfer of the tracer from the liquid injectate to reservoir steam during boiling. Simple analytic models of boiling were used to estimate the effects of the high tracer volatilities on the outcome of tracer tests in vapor-dominated systems. The results imply that the effects of volatility are exaggerated under conditions of high superheat, which promote the continuous removal of steam from the vicinity of the boiling interface. In contrast, low to moderate superheat reduces the effects of volatility to the extent that the volatile-tracer test results qualitatively resemble those in which tritiated water is used as a tracer. Thus, volatile tracers can be used with confidence to qualitatively describe the distribution of injected water in vapor-dominated systems where superheat is low to moderate.

Chemistry of fluids from ascension #1, a deep geothermal well on ascension island, South Atlantic Ocean

The volcanic Ascension Island is located about 100 km west of the median valley of the Mid-Atlantic Ridge. A 3126 m-deep well was drilled on the island to investigate the geothermal system associated with the recent volcanism. Two types of fluid were encountered in the well, both derived from seawater. The first fluid, found at 2500 m depth, was a recent mixture of a gaseous phase rich in volcanic CO2 and a geothermal fluid. Flow from this entry was so rich in gas that a CO2H2O froth was formed in the wellbore. The second fluid was water produced by a gas lift from near the bottom of the well at 2957 m. The water appeared to have normal gas concentrations. The geothermometer temperatures of water from the shallow and deep fluid entries were 200 and 245°C, respectively. The geothermometer and the measured temperatures were in close agreement, suggesting low permeability in the geothermal system.

Shallow thermal structure and hydrology of Ascension Island, South Atlantic Ocean

Seven coreholes, 63 to 533 m deep, were drilled on Ascension Island to determine the shallow thermal structure in preparation for siting a deep geothermal exploration well. These holes were used to evaluate specific geothermal exploration targets defined by previously completed geological mapping, aeromagnetic and electrical resistivity surveys. The highest temperature measured was 54.4°C at 203 m below sea-level. Measured temperature gradients reached up to 72°C/km; heat flow, up to 124 m W/m2. The higher heat flow values are associated with the two silicic volcanic centers on the island. The water table in all wells is at approximately sea-level. Groundwater samples from different depths in corehole LDTGH, located in the central part of the island, show a zone of brackish water overlying water with the salinity of seawater. Both waters retain the cation signature of seawater, but show elevated silica concentrations. Mixing calculations and chemical geothermometry indicate temperatures as high as 143°C.

A History of Geothermal Energy Research and Development in the United States. Reservoir Engineering 1976-2006

This report, the third in a four-part series, summarizes significant research projects performed by the U.S. Department of Energy (DOE) over 30 years to overcome challenges in reservoir engineering and to make generation of electricity from geothermal resources more cost-competitive.