Einar Gunnlaugsson

The chemistry of geothermal waters in Iceland. III. Chemical geothermometry in geothermal investigations

Abstract New data from geothermal wells in Iceland have permitted empirical calibration of the chalcedony and Na K geothermometers in the range of 25–180°C and 25–250°C respectively. The temperature functions are: t°C= 1112 4.91− log SiO 2 −273.15 t°C= 933 0.993+ log Na/K −273.15 Concentrations are expressed in ppm. These temperature functions correspond well with the chalcedony solubility data of Fournier (1973) and the thermodynamic data for low-albite/microcline/solution equilibria of Heloeson (1969). A new CO2 geothermometer is proposed which is considered to be useful in estimating underground temperatures in fumarolic geothermal fields. Its application involves analysis of CO2 concentrations in the fumarole steam. The temperature function which applies in the range 180−300°C is: logCO2 = 37.43 + 73192/T- 11829· 103/T2 + 0.18923T- 86.187·logT where T is in °K and CO2 in moles per kg of steam.

The chemistry of geothermal waters in Iceland. II. Mineral equilibria and independent variables controlling water compositions

Abstract The major element chemistry of Icelandic geothermal waters is predictable provided two parameters are known. This follows from an attainment of, or a close approach to, an overall chemical equilibrium in the geothermal systems at temperatures as low as 50°C. It is considered that the geothermal system composition, temperature and kinetic factors determine which alteration minerals form. The system composition is not so much fixed by rock composition as by the rate of leaching of the various constituents from the fresh rock and the composition of inflowing water. The water chemistry is determined by the system composition and the external variables acting on the system. They include temperature and the mobility of chloride. Pressure, which theoretically should be regarded as an external variable, has insignificant effect on water compositions in the range (1–200 bars) occurring in the geothermal systems.

New gas geothermometers for geothermal exploration—calibration and application

Calibration of five gas geothermometers is presented, three of which used CO2, H2S and H2 concentrations in fumarole steam, respectively. The remaining two use CO2H2 and H2SH2 ratios. The calibration is based on the relation between gas content of drillhole discharges and measured aquifer temperatures. After establishing the gas content in the aquifer, gas concentrations were calculated in steam formed by adiabatic boiling of this water to atmospheric pressure to obtain the gas geothermometry functions. It is shown that the concentrations of CO2, H2S and H2 in geothermal reservoir waters are fixed through equilibria with mineral buffers. At temperatures above 230°C epidote + prehnite + calcite + quartz are considered to buffer CO2. Two buffers are involved for H2S and H2 and two functions are, therefore, presented for the geothermometers involving these gases. For waters containing less than about 500 ppm chloride and in the range 230–300°C pyrite + pyrrholite + epidote + prehnite seem to be involved, but pyrite + epidote + prehnite + magnetite or chlorite for waters above 300°C and waters in the range 230–300°C, if containing over about 500 ppm. The gas geothermometers are useful for predicting subsurface temperatures in high-temperature geothermal systems. They are applicable to systems in basaltic to acidic rocks and in sediments with similar composition, but should be used with reservation for systems located in rocks which differ much in composition from the basaltic to acidic ones. The geothermometry results may be used to obtain information on steam condensation in upflow zones, or phase separation at elevated pressures. Measured aquifer temperatures in drillholes and gas geothermometry temperatures, based on data from nearby fumaroles, compare well in the five fields in Iceland considered specifically for the present study as well as in several fields in other countries for which data were inspected. The results of the gas geothermometers also compare well with the results of solute geothermometers and mixing models in three undrilled Icelandic fields.

Rapid carbon mineralization for permanent disposal of anthropogenic carbon dioxide emissions

Inject, baby, inject! Atmospheric CO2 can be sequestered by injecting it into basaltic rocks, providing a potentially valuable way to undo some of the damage done by fossil fuel burning. Matter et al. injected CO2 into wells in Iceland that pass through basaltic lavas and hyaloclastites at depths between 400 and 800 m. Most of the injected CO2 was mineralized in less than 2 years. Carbonate minerals are stable, so this approach should avoid the risk of carbon leakage. Science, this issue p. 1312 Basaltic rocks may be effective sinks for storing carbon dioxide removed from the atmosphere. Carbon capture and storage (CCS) provides a solution toward decarbonization of the global economy. The success of this solution depends on the ability to safely and permanently store CO2. This study demonstrates for the first time the permanent disposal of CO2 as environmentally benign carbonate minerals in basaltic rocks. We find that over 95% of the CO2 injected into the CarbFix site in Iceland was mineralized to carbonate minerals in less than 2 years. This result contrasts with the common view that the immobilization of CO2 as carbonate minerals within geologic reservoirs takes several hundreds to thousands of years. Our results, therefore, demonstrate that the safe long-term storage of anthropogenic CO2 emissions through mineralization can be far faster than previously postulated.

The chemistry of iron in geothermal systems in iceland

The concentration of iron in Icelandic geothermal waters lies in the range of about 0.004–0.3 ppm. Saline waters are highest in iron. At temperatures above about 150°C, the dominant iron species in the water is Fe(OH)4− but Fe2+ and FeOH+ dominate at lower temperatures. Below 180°C the waters equilibrate with pyrrhotite and marcasite. At higher temperatures pyrite and anhydrite equilibrium is attained, but the waters become pyrrhotite-undersaturated. There are indications that the waters also equilibrate with various iron hydroxides depending on temperature. At the lowest temperatures, speciation calculations indicate equilibrium with amorphous ferric hydroxide which is replaced at successively higher temperatures with lepidocrocite, maghemite, geothite and hematite. With the exception of lepidocrocite, these minerals have been identified in hydrothermally altered rocks in Icelandic geothermal systems. Marcasite had not hitherto been reported, but a special search for it during the present study has revealed its presence. The ratio of (Fe2+)12/H+ and the concentration of H2S show a very good correlation with the temperature of the geothermal waters. This results from simultaneous equilibria with the above alteration minerals.

Nesjavellir geothermal co-generation power plant

The Nesjavellir high temperature geothermal field is a part of the Hengill geothermal area in SW-Iceland. The Nesjavellir field has been under exploration and development during the past 25 years. This paper summarizes the main exploration results, i.e. temperature and pressure distribution in the reservoir and the discharge parameters of the production wells. A conceptual model of the Nesjavellir field is described, along with a numerical simulation model. The calculation of the thermal power of district heating schemes in Iceland normally assumes cooling of the network fluid down to 40°C. The numerical model results indicate a power potential large enough to supply a 300 MW thermal power plant at Nesjavellir for 30 years without re-injection into the reservoir. Hitaveita Reykjavikur began the construction of the Nesjavellir geothermal co-generation power plant in 1987. The plant utilizes high pressure steam for electricity generation and low pressure steam and the separated water to heat fresh ground water for district heating in the Reykjavik area. The first stage, a 100 MW, thermal power plant, was commissioned in September 1990. The fully developed plant, using a re-injection system, is planned to deliver 400 MW, for district heating and 80–90 MWc for the national grid.

Magnesium-silicate scaling in mixture of geothermal water and deaerated fresh water in a district heating system

Abstract The low-temperature geothermal fields in Reykjavik utilized by the Reykjavik Municipal District Heating Service are now fully exploited. Additional hot water will be obtained by heating and deaerating fresh water using high temperature geothermal fluid. The heated fresh water will mix with low-temperature geothermal water in the distribution system in Reykjavik. A pilot plant has been set up to investigate magnesium silicate scale formation when mixing of these waters occures. Tests show that the scale formation is dependent on the severity of deaeration of the fresh water and the proportion of geothermal water in the mixture. Increased deaeration and thermal water proportion increase the pH of the mixture, and this promotes scaling. The scale formed is poorly crystalline, near amorphous trioctahedral smectite close to saponite in composition. By using minimum deaeration and traces of geothermal steam to remove the last remaining dissolved oxygen, scaling in the distribution system can by avoided.

Groundwater Contamination Due to Surface Disposal of Geothermal Wastewater at Nesjavellir, Iceland

The Reykjavik District Heating Services operate several low-temperature geothermal fields within the city of Reykjavik. The production is approximately 550 MW and the hot water is used for space heating in Reykjavik and nearby communities. For the future a 400 MW power plant is under construction at Nesjavellir some 25 km from Reykjavik where a high-temperature field will be used for power production. Due to the high concentration of dissolved solids in the produced fluid the water cannot be used directly. Instead, cold water from nearby lava aquifers must be heated in specially designed heat exchangers. The spent geothermal fluid will then be disposed of in surface ponds where it will percolate down to the cold water reservoir.