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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