Transcript Stuart
A Geochemical Perspective on Assessing/Sustaining Well Productivity Stuart F Simmons EGI, U Utah Penrose Conference, 19-23 Oct, 2013, Park City, Utah 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 Geothermal Systems: Stored vs Flowing reservoirs < 3 km depth volcano-intrusion extensional fault reservoir reservoir sedimentary basin reservoir ? reservoir photo J. Hedenquist 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 Application Species Tracers: Cl-, B, HCO3-, SO4-2 N2, Ar, He, CO2, H2S, H2 18O/16O, D/H, 3He/4He Indicators: Na+, K+, Ca+2, Mg+2, SiO2, CO2, H2 Engineering SiO2, Ca+2 , CO2, HCO3- , H2S, H2 (scaling-corrosion) Environmental B, NH3, As, Hg, H2S Sedimentary Basins: Reservoirs Natural State-Broad Physical Gradients 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. Sedimentary Basins: Reservoirs springs Exploration Geochemistry Equilibration Temperatures Flow Paths Sedimentary Basins: Reservoirs exploration Reservoir fluid(s) Sedimentary Basins: Reservoirs exploration exploration Leaky reservoirs (open vs closed) Sedimentary Basins: Reservoirs producer injector Production induced effects Pressure drawdown Scaling/Injection breakthrough Injectate Treatment/Conditioning Time (>decades) Geochemical Issues Wide range of TDS (<100 to >100,000 ppm Cl) Carbonate equilibria, CO2 & pH Rocks & Minerals (lms, ss, evaporites, fldspars, qtz) Thermogenic vs microbial gas production sulfate reduction & H2S 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) pH Na K HCO3 SO4 Cl Grant Canyon 7GC (115°C) 8.3 2500 251 38 104 4350 Bacon Flat 23-17 (122°C) 8.2 3040 312 33 128 5350 Sen Emedio Nose (149°C) 7.7 4000 620 2870 38 3460 Houston Halls Bayou (150°C) 6.8 20500 180 409 16 34500 Thermo (177°C) 6.4 961 75 330 500 1014 Ngawha (221°C) 7.2 850 82 14450 7 1279 Hulen et al, 1994; Kharaka & Hanor, 2003; Moore, unpub; Top Energy NZ Sedimentary Aquifer Thermal Waters (USA-NZ) SiO2 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 CO2, HCO3-, & 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 In dilute hydrothermal solutions, calcite has reverse solubility, but this does not explain deposition as well scales. Calcite precipitates due to loss of CO2, generally close to the site of first phase separation. Scaling is exacerbated by high CO2 concentrations. 2+ (Ca solubility calcite Anhydrite solubility (Ca2+mg/kg) mg/kg) 3 2 1 0 0 2HCO3 + Ca2+ = CaCO3 + H2O + CO2 100 200 Temperature deg C 300 temperature °C Increase CO2 to dissolve calcite and drive rxn left; remove CO2 to precipitate calcite. Fresh. Altered. Photos: courtesy of Jean Cline Images left show enhanced porosity through calcite dissolution in Carlin Au deposits. 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 Geoscience of Geothermal Energy Physical: Heat & mass transfer GEOLOGY Temperature-pressure gradients Permeability-porosity Hydrology & fluid flow Chemical: Fluid compositions Fluid-mineral equilibria Mineral corrosion/deposition Hydrothermal alteration