Stuart F SimmonsEGI, U Utah

Penrose Conference, 19-23 Oct, 2013, Park City, Utah

A Geochemical Perspective on Assessing/Sustaining Well Productivity

Fluid Compositions ReflectFluid flow paths (near & far field)Mineral dissolution-precipitationEquilibration temperatureChemical structure of reservoir(s)Extent of the resource Baseline vs production induced effectsOther potential resources (e.g., He, metals)

Questions (Where & What?)

ResourceFluid pathways inside & outside the reservoirNature of compositional variabilityHost rock & mineral influence (siliciclastic vs

carbonate units)State, extent & time-span of fluid-mineral equilibriaSources of aqueous/gaseous constituentsProxy Environments: Oil/Gas, Oil Shale, Conv. Geo.Paleo-geothermal reservoirs; Carlin/MVT deposits

extensional fault

volcano-intrusion

rese

rvoi

rs <

3 k

m d

epth

Geothermal Systems: Stored vs Flowing

sedimentary basin

reservoir

reservoir

reservoir

reservoir

?

Geothermal Wells>$ 5 million2 to 3 km deepfuel for power stationlifetimes >10 yrs1 or more feed zones

Production effectsPressure dropScaling-corrosionEnthalpy declineFlow decline

phot

o J.

Hed

enqu

ist

Application 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

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 GeochemistryEquilibration 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 effectsPressure drawdown

Scaling/Injection breakthroughInjectate 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 productionsulfate reduction & H2S generationalkalinity 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 ClGrant Canyon 7GC (115°C) 8.3 2500 251 38 104 4350Bacon Flat 23-17 (122°C) 8.2 3040 312 33 128 5350Sen Emedio Nose (149°C) 7.7 4000 620 2870 38 3460Houston Halls Bayou (150°C) 6.8 20500 180 409 16 34500Thermo (177°C) 6.4 961 75 330 500 1014Ngawha (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’din 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

Calcite precipitates due to loss of CO2, generally close to the site of first phase separation. Scaling is exacerbated by high CO2 concentrations.

2HCO3 + Ca2+ = CaCO3 + H2O + CO2

0 100 200 300Te m p era tu re d e g C

0

1

2

3

Anh

ydrit

e so

lubi

lity

(Ca2

+ m

g/kg

)ca

lcite

sol

ubili

ty (C

a2+ m

g/kg

)

temperature °C

In dilute hydrothermal solutions, calcite has reverse solubility, but this does not explain deposition as well scales.

Increase CO2 to dissolve calcite and drive rxn left; remove CO2 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

GEO

LOG

Y

Geoscience of Geothermal Energy