APPLICATION OF GEOCHEMICAL METHODS IN GEOTHERMAL EXPLORATION Halldór Ármannsson November 2007
Geochemical Exploration Subsurface composition Temperature Origin and flow direction Reservoir location Equilibrium speciation Boiling Deposition Corrosion Environmental effects Contribution to model
CLASSIFICATION OF SUBSURFACE WATERS (White 1986) Meteoric water Ocean water Evolved connate water Metamorphic water Magmatic water Juvenile water 11/26/2007
Classification of geothermal water (Ellis and Mahon 1978) Alkali-chloride water: ph 4-11, least common in young rocks, e.g. Iceland Acid sulphate water: H 2 S SO 4. Constituents dissolved from surface rock Acid sulphate -chloride water: Mixture, H 2 S SO 4 in alkali-chloride water or dissolution of S Bicarbonate water: CO 2 rich steam condenses or mixes with water, excess CO 2 in old high-temperature areas Mostly meteoric water and sea water
ALKALI-CHLORIDE WATER Mostly sodium and potassium chloride In brines Ca concentration often significant ph in a wide range, usually 4-11 Very common but relatively rare in young rocks like the Icelandic ones
ACID SULPHATE WATER Steam at < 400 C condenses and is mixed with surface water H 2 S SO 4 Dissolved solids usually from surface rock Of little use in exploration as little history of subsurface processes
ACID SULPHATE -CHLORIDE WATER Mixture of alkali-chloride water and acid sulphate water Sulphide oxidized to bisulphate in alkalichloride water and ph lowered upon cooling High temperature chloride water in contact with sulphur containing rock causing hydrolysis of sulphur and an acid solution High temperature steam condenses into surface water. F-Cl-SO 4 Cl-SO 4 water. Dissolution of surface rock
BICARBONATE WATER Carbon dioxide rich steam in volcanically active geothermal areas condenses into liquid reservoir Excess CO 2 on the periphery of geothermal systems or in ancient high temperature systems that are cooling down CO 2 from magma at great depth rises to the surface and is mixed with groundwater underway
Examples(mg/kg) 1) El Tatio, Chile; 2) Waiotapu; 3) Ruapheu; 4) Wairakei, New Zealand Type ph Na Ca F Cl SO 4 HCO 3 Alkalichloride 7.32 4340 272 3.1 7922 30 46 1) Acid 2.8 43 27 32 347 0 sulphate 2) Acid sulphatechloríde 1.2 740 1200 260 9450 10950 0 Bicarbonate 8.6 230 12 3.7 2.7 11 680
Legend Title Katwe, cold water, dilute Katwe, cold water, saline Katwe, cold water, brackish Katwe, hot spring water Buranga, cold water, dilute Buranga, hot spring water Kibiro, cold water, dilute Kibiro, cold water, brackish Kibiro, hot spring water SO4 SO4 VOLCANIC WATERS STEAM HEATED WATERS Cl 100 75 50 25 0 Cl MATURE WATERS 100 75. HCO3 0 25 50 75 100 50 PERIPHERAL WATERS 25 0 HCO3
GEOTHERMAL WATER DISSOLVED CONSTITUENTS Water-rock interaction Addition of magmatic constituents Rock forming constituents, e.g. Si, Al, Na, K, Ca, Mg, Fe, Mn Incompatible constituents, e.g. Cl, B, Br
PRODUCTS OF HYDROTHERMAL ALTERATION Controlled by Temperature Pressure Chemical composition of water (CO 2, H 2 S control) Original composition of rock Reaction time Rate of water and steam flow Permeability Type of permeability
ON HYDROTHERMAL ALTERATION Silica concentration dependent on solubility of quartz/chalcedony Temperature dependent Al-silicate ion-exchange equilibria control Na/K, Na/Rb ratios ph controlled by salinity and Al-silicate equilibria involving hydrogen and alkali ions Ca +2, HCO 3- concentrations dependent on ph and CO 2 concentration F -, SO 4-2 concentrations related to that of Ca +2, limited by solubility of fluorite and anhydrite Temperature and salinity dependent silicate equilibria control a very low Mg +2 concentration
RESULTS OF HYDROTHERMAL ALTERATION STUDIES The chemical composition of geothermal fluids originates in controlled reactions dependent on temperature, pressure and rock composition whose reversal may be slow Therefore it is possible to deduce the properties of subsurface water from the chemical composition of water which has been collected at the earth s surface
ORIGIN AND EXTENT OF GEOTHERMAL SYSTEMS Stable isotopes Relationship of major ions, e.g. ternary diagrams Cl - -SO 4-2 -HCO 3 - Conservative constituents Ratios, e.g. Br/Cl, B/Cl Ternary diagrams, e.g. Cl-Li-B
STABLE ISOTOPES D and 18 O MOST USED Analysed for by Mass Spectrometry Difficult to measure absolute concentrations but easy to determine ratios D and 18 O recorded as SMOW (Standard Mean Ocean Water)
GEOGRAPHICAL EFFECTS ON THE ISOTOPE COMPOSITION OF PRECIPITATION Sea water: D and 18 O ~ 0 (SMOW) Evaporation Clouds Precipitation Latitude. Lower isotope ratios at higher latitudes Altitude and distance from sea shore. Lower isotope ratios at higher altitudes and greater distances from shore
TEMPORAL EFFECTS THE ISOTOPE COMPOSITION OF PRECIPITATION Single showers: Origin of cloud, temperature of condensation Seasonal changes: Lower values in winter. More pronounced at high than low latitudes Long-term climatic changes Quantitative effect: Inverse relationship with quantity of precipitation. More pronounced at low than high latitudes
Isotopes in studies of geothermal water After precipitation little change Local annual means for precipitation known Meteoric line (Craig 1961) applies but deviations known and local lines used Geothermal water values suggest origin Mixing, water-rock interaction, condensation and age may have to be accounted for
D 40 Stable isotope ratios 20 5 6 0 Continental African rainline 8 2 3 4 7 1 Kenya rainline (Clarke et al. 1990) -20-40 -60 World meteoric line (Craig 1961) -80-15 -10-5 0 5 10 18 O
Deuterium in Icelandic precipitation Deuterium ratios in precipitation from the three rainwater stations, 657 local groundwater stations and 24 glacier stations
Flow paths to geothermal systems Arrows are drawn from inland hot springs traversing altitude until the deuterium ratio of the precipitation matches that of the springs
B, Cl Sea water: Cl/B = 4350. Rock (Iceland): Cl/B = 50-120. Magmatic steam: Cl/B = 671-4276 Origin: Atmosphere, rock/soil, sea water, magmatic steam S-Iceland ancient seawater; NW-Iceland present day seawater; The Philippines variable steam fraction
Cl Iceland Cl (mg/l) in Icelandic surface water, originating in precipitation Concentration decreases with distance from the sea and altitude
Mixing with seawater Part of chloride originates in seawater Hreppar, Land: Cl, Filled circles >Cl than precipitation water
CHEMICAL GEOTHERMOMETERS Univariant, e.g. SiO 2, CO 2, H 2 S, H 2. Disadvantage: Sensitivity to secondary changes such as dilution, steam loss and condensation Global: Assume that a number of constituents is simultaneously at equilibrium and that their present concentrations can be used to obtain the equilibrium temperature. Depend on analytical reliability and thermodynamic data Equimolar and equicoulombic ratios, e.g. Na/K, CO 2 /H 2. Overcome disadvantage of univariant geothermometers but equilibrium and rate conditions limit their value
Legend Title Na/1000 Kibiro geothermal samples,analyzed Kibiro geothermal component, calculated 100 120 t kn ( C) 160 140 180 200 220 240 260 280 300 Fully equilibrated waters 340 K/100 t km Mg ( C) 340 300 280 260 240 220 200 Immature waters 180 160 140 120 100
FLOW RATE AND DIRECTION Chemical geothermometers. Suggest position of upflow Gas ratios in high temperature steam, e.g. CO 2 /H 2 S. Upon boiling CO 2 is removed from water before H 2 S. If a boiling fluid flows away from upflow CO 2 /H 2 S ratio in steam flow is reduced Tracers: Addition of fluorescent, radioactive or rare constituents
EQUILIBRIUM CALCULATIONS Thermodynamic data Computer programmes Speciation; e.g. WATCH, SOLVEQ Reaction path, e.g. CHILLER Employed to estimate chemical geothermometer temperatures, saturation states for the various minerals, effects of boiling, cooling, condensation, reactions with rock etc.
DATING I RADIOACTIVE ISOTOPES 3 H.T½ = 12.43 years. Tritium units, TU = 1.185 Bq/L Natural cosmogenic level in precipitation a few TU. Rose to 2000 TU from fifties to 1963/1964 but down to 10 TU at present 14 C. T½ = 5730 years. In atmospheric CO 2, living biosphere and hydrosphere after production by cosmic radiation. Underground production negligible. 14 C content often given in % modern carbon (pmc), grown in in 1950 Fallout 14 C (in CO 2 ) date water with mean residence time < 150 years
DATING II. CFC Organics of chlorine and fluorine, man-made, first in 1928. Non-reactive. Non-toxic. CFC-11, CFC- 12 and CFC-113 most common Release of CFC-11 and CFC-12 rose in the 1930s. Deviations noted following 1974, when possible ozone depletion by chlorine-containing species was first announced, and signing of Montreal Protocol in 1987. Release of CFC-113 increased significantly through early- and mid-1980s until Montreal Protocol issued, after which production significantly diminished. Atmospheric lifetime (years) CFC-11 45±7 CFC-12 87±17 CFC-113 100±32
Steam areas No water samples available Steam may have undergone several processes, i.e. condensation, mixing, mixing + boiling Gas concentrations and isotope ratios may be used to find composition of original geothermal steam If chloride concentration substantial saline water Gas geothermometers
Rainwater geothermal steam sample
ORIGIN OF GEOTHERMAL GASES Diverse Magmatic Rock dissolution Organic Atmospheric Radiogenic Isotopes, inert gases, thermodynamic calculations
GAS CONSTITUENTS Major: CO 2, H 2 S, H 2, N 2, Ar, CH 4, O 2 (Low temperature systems) Minor: Higher hydrocarbons (e.g. C 2 H 6 ), CO, other inert gases (e.g. He, Rn), NH 3, Hg, B, As
CO 2 Magmatic: 13 C = -10 - -1 Marine limestone: 13 C= -2 - +2 Organic: 13 C <-20 Atmospheric -5 - -8 Krafla, Iceland: Not all from rock. Magmatic contribution needed to account
H 2 S Leaching from rock: 34 S<0 Magmatic: 34 S 0 Marine: 34 S>0
H 2 Leaching from rock or sediments: D- 300- -450 Magmatic: 2 H<-450
N 2, inert gases Atmospheric: Ne, 36 Ar and Kr, probably most N 2, some He Radiogenic: He, 136 Xe, 222 Rn, 40 Ar. 222 Rn used for soil traverses Organic: Some N 2
HYDROCARBONS Biogenic, C 2+ 0; 13 C CH4,<-55 Thermogenic (wet), C 2+ > 5%, 13 C CH4 higher Thermogenic (dry) C 2+ low, 13 C CH4 terr. high, marine low Magmatic gas, C 2+ 0 13 C CH4 rather high Inorganic,C 2+, 13 C CH4-20 -10
13 C CH4 VS 13 C CO2 IN ICELANDIC GEOTHERMAL AREAS
CO2 concentration in compartment (ppm) 2500 2000 Mælingar á CO 2 flæði 1500 1000 um jarðveg 500 0 0 20 40 60 80 100 time(sec) SOIL GAS Increasing concentration in compartment measurement of CO 2 flux through soil CO 2 gauge Closed compartment CO 2 flux
Soil diffuse CO 2 emissions
CHEMICAL UTILIZATION PROBLEMS Deposition Corrosion Pollution
DEPOSITS Common: Silica, iron oxides, iron silicates, sulphides, calcite, magnesium silicates Also known: Aluminium silicates, anhydrite, barite, apatite, borates, sulphur
DEPOSITION STUDY Thermodynamics. Theory Kinetics: Experiments
CALCITE SCALING Flashing CO 2 stripping and ph increase, causing calcite deposition Ca +2 + 2HCO - 3 CaCO 3 + CO 2 +H 2 O Increasing temperature solubility decreasing Extent of supersaturation can be calculated
MAGNESIUM SILICATES Formed upon heating of silica containing ground water or mixing of cold ground water and geothermal water Form at relatively high ph Well known where geothermal water used to heat groundwater Avoid mixing and keep ph low
Mg-Si deposits. Results of studies Poorly developed antigorite (Gunnarsson et al. 2005) Solubility decreases (deposition increases) with increased temperature and ph. Rate of deposition increases linearly with supersaturation but exponentially with temperature
CORROSION. HALF-REACTIONS Anodic Fe Fe +2 + 2e - Fe Fe +3 + 3e - Cathodic H 2 O + ½O 2 + 2e - 2OH - H 2 O + 2e - H 2 + 2OH - 2H 2 CO 3 + 2e - 2HCO 3- + H 2
CORROSIVE SPECIES O 2 : at low temperatures; H + (ph): Low ph favours cathodic half-reaction; Cl: Fe +2 + Cl - FeCl + favours anodic halfreaction; CO 2 : Controls ph and favours last cathodic half-reaction. H 2 S attacks Cu, Ni, Zn, Pb H 2 S, CO 3-2 and SiO 2 may form protective films on steel Fe +2 + HS - FeS + H + Fe +2 + H 3 SiO 4 - FeSiO 3 + H + + H 2 O Fe +2 + HCO 3 - FeCO 3 + H +
SUMMARY I Natural water classes. Geothermal water groups Conservative constituents origin + flow. Rock-forming constituents temperature and other conditions Stable isotopes in precipitation: Latitude, altitude, distance from coast, age. After that little change except oxygen shift B, Cl: mixing with sea water or volcanic steam
SUMMARY II Log (Q/K): Temperature dependence of minerals geothermometry. Also useful for prediction of deposition. Kinetics of deposition from experiments Isotopes in steam: Origin of fluid and gases. Correction of geothermometers Soil gas: Baselines before production. Traverses fractures Corrosion: Indications from ph, Cl or other potentially corrosive species
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