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 volcano-intrusion extensional fault

reservoir reservoir

sedimentary basin

reservoir 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

Application

Tracers:

Species

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 Engineering (scaling-corrosion) SiO 2 , Ca +2 , CO 2 , HCO 3 , H 2 S, H 2 Environmental B, NH 3 , As, Hg, H 2 S

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, 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) Grant Canyon 7GC (115

°

C) Bacon Flat 23-17 (122

°

C) Sen Emedio Nose (149

°

C) Houston Halls Bayou (150

°

C) Thermo (177

°

C) Ngawha (221

°

C) pH 8.3

8.2

7.7

6.8

6.4

7.2

Na 2500 3040 4000 20500 961 850 K 251 312 620 180 75 82 HCO 3 38 33 2870 409 330 14450 SO 4 104 128 38 16 500 7 Cl 4350 5350 3460 34500 1014 1279 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 3 In dilute hydrothermal solutions, calcite has reverse solubility, but this does not explain deposition as well scales.

2 Calcite precipitates due to loss of CO exacerbated by high CO 2 2 , generally close to the site of first phase separation. Scaling is concentrations.

1 0 0 100 200 temperature ° C 2HCO 3 + Ca 2+ = CaCO 3 + H 2 O + CO 2 Increase CO 2 to dissolve calcite and drive rxn left; remove CO 2 to precipitate calcite.

300 Fresh.

Altered.

Photos: courtesy of Jean Cline Images left show enhanced porosity through calcite dissolution in Carlin Au deposits.

Allis et al 2012 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?

Geoscience of Geothermal Energy

Physical:

Heat & mass transfer Temperature-pressure gradients Permeability-porosity Hydrology & fluid flow

Chemical:

Fluid compositions Fluid-mineral equilibria Mineral corrosion/deposition Hydrothermal alteration