Stuart F Simmons EGI, U Utah Penrose Conference, Oct, 2013, Park City, Utah A Geochemical Perspective on Assessing/Sustaining Well Productivity

Fluid Compositions Reflect Fluid flow paths (near & far field) Mineral dissolution-precipitation Equilibration temperature Chemical structure of reservoir(s) Extent of the resource Baseline vs production induced effects Other potential resources (e.g., He, metals)

Questions (Where & What?) Resource Fluid pathways inside & outside the reservoir Nature of compositional variability Host rock & mineral influence (siliciclastic vs carbonate units) State, extent & time-span of fluid-mineral equilibria Sources of aqueous/gaseous constituents Proxy Environments: Oil/Gas, Oil Shale, Conv. Geo. Paleo-geothermal reservoirs; Carlin/MVT deposits

extensional fault volcano-intrusion reservoirs < 3 km depth Geothermal Systems: Stored vs Flowing sedimentary basin reservoir ?

Geothermal Wells >$ 5 million 2 to 3 km deep fuel for power station lifetimes >10 yrs 1 or more feed zones Production effects Pressure drop Scaling-corrosion Enthalpy decline Flow decline photo J. Hedenquist

Application Tracers:Cl -, B, HCO 3 -, SO 4 -2 N 2, Ar, He, CO 2, H 2 S, H 2 18 O/ 16 O, D/H, 3 He/ 4 He Indicators: Na +, K +, Ca +2, Mg +2, SiO 2, CO 2, H 2 EngineeringSiO 2, Ca +2, CO 2, HCO 3 -, H 2 S, H 2 (scaling-corrosion) EnvironmentalB, NH 3, As, Hg, H 2 S Species

Sedimentary Basins: Reservoirs In pore spaces where fluid velocity is slow, fluid-mineral equilibria develops controlled by thermodynamically stable minerals. In open fractures where fluid velocity is fast, cooling, mixing, & phase separation control fluid composition. Natural State-Broad Physical Gradients

Sedimentary Basins: Reservoirs springs Exploration Geochemistry Equilibration Temperatures Flow Paths

Sedimentary Basins: Reservoirs exploration Reservoir fluid(s)

Sedimentary Basins: Reservoirs exploration Leaky reservoirs (open vs closed) exploration

Sedimentary Basins: Reservoirs injectorproducer Production induced effects Pressure drawdown Scaling/Injection breakthrough Injectate Treatment/Conditioning Time (>decades)

Geochemical Issues Wide range of TDS ( 100,000 ppm Cl) Carbonate equilibria, CO 2 & pH Rocks & Minerals (lms, ss, evaporites, fldspars, qtz) Thermogenic vs microbial gas production sulfate reduction & H 2 S generation alkalinity change (calcite solubility) Mixing & phase separation Chemical geothermometers

Sedimentary Aquifer Thermal Waters (USA-NZ) Reservoirs hosted in sedimentary rocks (Paleozoic-dolostone, Cenozoic-Ss/Sh, Mesozoic-Meta Ss) Minerals controlling fluid-mineral equilibria are poorly known Preliminary results with the aim of understanding potential chemical geothermometers Water compositions (mg/kg) pHNaKHCO 3 SO 4 Cl Grant Canyon 7GC (115°C) Bacon Flat (122°C) Sen Emedio Nose (149°C) Houston Halls Bayou (150°C) Thermo (177°C) Ngawha (221°C) Hulen et al, 1994; Kharaka & Hanor, 2003; Moore, unpub; Top Energy NZ

Sedimentary Aquifer Thermal Waters (USA-NZ) SiO 2 sat’d with quartz, chalcedony, or cristobalite. All waters also sat’d in calcite & many are sat’d in dolomite.

Sedimentary Aquifer Thermal Waters (USA-NZ) Fluids are out of equilibrium at the reported temperature with respect to feldspars & Na-K ratios Na-Li ratio unreliable indicator of temperature using empirical relationship(Fouilliac & Michard, 1981)

Preliminary Assessments Silica appears to be most reliable Controls on cation ratios inadequately understood Reliability of temperature & analytical data unknown Need fluid analyses of CO 2, HCO 3 -, & pH, other gases too Reaction path modeling suggests no scaling problems in production wells

Conductive Cooling Qtz-supersat’d but unlikely to deposit Extent of heating during injection could bring solution back to saturation in carbonates and sulfates.

Calcite & Carbonate Equilibria Calcite precipitates due to loss of CO 2, generally close to the site of first phase separation. Scaling is exacerbated by high CO 2 concentrations. 2HCO 3 + Ca 2 + = CaCO 3 + H 2 O + CO 2 calcite solubility (Ca 2+ mg/kg) temperature °C In dilute hydrothermal solutions, calcite has reverse solubility, but this does not explain deposition as well scales. Increase CO 2 to dissolve calcite and drive rxn left; remove CO 2 to precipitate calcite. Fresh. Altered. Images left show enhanced porosity through calcite dissolution in Carlin Au deposits. Photos: courtesy of Jean Cline

Exploration Carbonate rocks extend across eastern Great Basin Water compositions from Beowawe & Tuscaroa are HCO3-rich Na-K temperatures indicate ~250 deg C Is it possible that the point of equilibration is beneath the drilled depths of these systems, reflecting a hot laterally extensive resource? Allis et al 2012

Physical: Heat & mass transfer Temperature-pressure gradients Permeability-porosity Hydrology & fluid flow Chemical: Fluid compositions Fluid-mineral equilibria Mineral corrosion/deposition Hydrothermal alteration GEOLOGY Geoscience of Geothermal Energy