Garg GRC ReservoirEngineering Sept2014

Geothermal Reservoir Engineering

S.K. Garg

©2013 LEIDOS. ALL RIGHTS RESERVED.

What is a Geothermal Reservoir?

A subsurface region where the rocks contain hot water and/or steam that can be withdrawn using wells and put to practical use for direct heating or for generating electricity.

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Essential Elements of a Geothermal ReservoirHigh temperature

Working fluid (water and/or steam)

Permeable flow channels

all at depths which may be economically reached by drilling.

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Geothermal Reservoirs(continued)Geothermal reservoirs are usually found in

fractured volcanic rocks. Permeability of a geothermal reservoir is

usually found in discrete fractures, not intergranular pores.

Vertical dimensions of permeable reservoir zone can be large (comparable to horizontal dimensions).

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Geothermal Reservoirs (continued)Natural condition is dynamic, not static.

Geothermal reservoir is in a continuous state of convective flow, which carries heat from deep underground to exploitable depths.

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Schematic Diagram of a Geothermal Reservoir

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Fraction of heat in reservoir fluid

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Geothermal Reservoirs (continued) Resource is heat, not mass. Brine acts as a

working fluid which can carry energy stored in hot rock to the surface through wells.

In most liquid-dominated reservoirs, over 80 percent of the total heat is stored in rock—vapor-dominated, over 95 percent.

Production/injection wells create artificial circulation system to mine energy from the rock.

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Purpose of Geothermal Reservoir EngineeringTo gain a quantitative understanding of the

flows of fluid mass and of heat which take place within the geothermal reservoir under natural conditions, and how they change due to production and injection operations.

To use this understanding to (1) help in interpreting exploration surveys, and (2) make quantitative appraisals of proposed operational strategies to guide the management of the resource.

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Reservoir Engineering Procedure (1)

Gather and interpret field measurements to help establish geothermal reservoir properties:−Area, volume and depth.

−Hydrological and thermal boundaries.

−Rock physical properties.

−Permeability structure.

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Reservoir Engineering Procedure (2)−Source and state of fluid.

−Natural heat, mass fluxes.

−Natural reservoir pressure distribution.

−Underground temperatures.

−Production/injection history.

−Changes in reservoir from operations

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Reservoir Engineering Procedure (continued)Based on available field data, construct a

deterministic model which describes the known facts about the system, including natural-state and observed response to production/injection operations.

Validated model can be used to make forecasts of field capacity changes and probable drilling requirements, and help in planning and resource management.

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Field Data Useful for Reservoir Engineering (1)Structural interpretations from remote

sensing, air/satellite images, maps, drilling logs.

Surface and downhole resistivity surveys.Surface heat flow surveys.Hot spring mapping and characterization.Stratigraphic and mud loss drilling logs. Laboratory tests of cores.

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Field Data Useful for Reservoir Engineering (2)Geochemical analyses of surface and

reservoir fluids.Shut-in and flowing downhole pressure

logs.Repeat downhole temperature surveys.Downhole spinner logs (shut-in and

flowing).Flow/enthalpy/pressure histories for

production and injection wells.Pressure-transient test results.14


Essential elements of conceptual model Fluid State (Pressure, Temperature, Salinity, Gas

Content) Source Fluids (Meteoric, Sea Water, Magmatic) Permeability Structure and Hydraulic Boundaries

(Major faults and fractures, Permeable formations, Detailed permeability distribution, Impermeable barriers, Recharge and/or Discharge boundaries)

Deep Heat Source (Convective and Conductive heat flux)

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Pressure Distribution with Depth

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Temperature Distribution with Depth (1)

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Temperature Distribution with Depth (2)

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Effect on non-condensable gases

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Faults and Feedzones

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Reservoir Assessment: Two Approaches

USGS Volumetric Assessment together with Monte Carlo Simulation - Method used in early exploration stages.

Numerical Simulation – Method most useful after some wells have been drilled and tested.

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USGS Volumetric Assessment with Monte Carlo Simulation (1) In early exploration stages, often used to provide

estimates of the probable electrical generation capacity.

Methodology consists of combining probability density functions for uncertain estimates of temperature, area, thickness, and thermal recovery factor of a geothermal reservoir.

Used to obtain the probability distribution function for the stored energy (“heat in place”) and the recoverable heat.

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USGS Volumetric Assessment with Monte Carlo Simulation (2) The electrical capacity of a potential geothermal

reservoir is then computed using a conversion (or utilization) efficiency.

The conversion efficiency depends on both the chosen reference temperature as well as the power cycle (steam, flash, or binary).

Because of the large uncertainties in the assumed input parameters, the results of a USGS Volumetric Assessment are often subject to large errors.

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Reservoir Modeling and Simulation Reservoir Simulator

− A computer program which can solve unsteady 3-D governing equations for mass, momentum, and energy conservation in a geothermal reservoir. Subdivides volume into many “grid blocks”; calculates solution in a sequence of “time-steps”.

Reservoir Model− A particular quantitative conceptual picture of a

specific geothermal reservoir. Incorporates locations of boundaries, recharge and discharge areas, distributions of rock properties such as permeability and porosity.

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Reservoir SimulatorsStandard versions of widely used reservoir

simulators (Tetrad, Tough2, STAR) are limited to temperatures below 350 oC or so.

Experimental versions of STAR (Pritchett, 1994) and Tough2 (Croucher and O’Sullivan, 2008) can also treat supercritical conditions for pure water (to ~800°C). These experimental versions do not however allow for dissolved solids (e.g. NaCl) and incondensable gases (e.g. CO2).

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Reservoir Simulators (continued)Temperatures as high as 360 oC

encountered in geothermal wells.Modeling of very high-temperature ( >320

oC) geothermal reservoirs will require a capability to incorporate underlying deeper parts of the system where temperatures may exceed the critical temperature for water.

Requirement for improved Equation-of-State formulations for reservoir fluids.

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Objectives of Reservoir Simulation (1)Before large-scale production/injection

- Help interpret exploration surveys.−Estimate generating potential, lifetime.−Appraise effects of uncertainties on the

estimated capacity.−Define appropriate well spacings and

optimum well locations.

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Objectives of Reservoir Simulation (2)After field exploitation begins−Manage production/injection operations

to optimize energy recovery.− Indicate changes in well locations

needed to avoid premature cold water breakthrough.

−Determine optimum sites for makeup wells.

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Development of a Reservoir Model: The Natural-State If no production/injection wells present, any

acceptable reservoir model MUST yield behavior which is:− (1) steady-state on scales of human lifetimes, AND− (2) consistent with measured pre-production

underground pressures, temperatures, etc.

To develop natural-state model, use simulator iteratively, changing major unknowns (usually boundary conditions at depth and distributions of permeability) from run to run.

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General Procedure for Developing a Reservoir Model

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Exploration Stage & Reservoir Modeling

Reservoir modeling may be used to explore implications of different conceptual models for observables (thermal gradient holes, electrical surveys, etc.)

J.W. Pritchett (2004): Finding hidden geothermal resources in the basin and range using electrical survey techniques, Report prepared for INL.

Modeling used to look for surface signals associated with fault controlled systems (shallow heat flow, surface DC Resistivity, Magnetotelluric, and Self-Potential surveys)

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Hypothetical Earth Structure (Pritchett, 2004)

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©2013 LEIDOS. ALL RIGHTS RESERVED. 33


Computed temperatures (Pritchett, 2004)

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Computed downhole temperatures (Pritchett, 2004)

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Development of the “Natural-State” of a Model of a Small Liquid-Dominated Geothermal Field

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Areal View of Computational Gridfor Oguni Simulation Study

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North-South Vertical Cross-Section Through Oguni Simulation Grid

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Estimating Capacity of Prospects

If field not yet exploited, perform various parametric calculations to appraise probable steam production capacity/field lifetime.

Uncertainty in model parameters may lead to incorrect conclusions.

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Typical Power PlantRepresentationsAvailable with Reservoir Simulators

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Computed Changes in Underground Conditions at Oguni After 30 Years Steam Production at 250 Tons/Hour


Effects of Steam Production Requirements on Makeup Drilling

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History-Matching If field is already producing, impose measured

well flow rates in appropriate grid blocks. Compute resulting changes in reservoir pressure,

well enthalpies, chemical properties, etc. Compare computed data with measurements. Iteratively make further model adjustments as

required to optimize agreement between computed and measured history.

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Typical History-Match of Production Conditions for a Small Geothermal Reservoir

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Reservoir Simulation and Field Management (1)

Reservoir simulation and modeling studies should be a continuing activity throughout the geothermal field lifetime.

As new data become available, models should be updated.


Reservoir Simulation and Field Management (2)

Computer modeling should be correlated with the field monitoring program.

In addition to changes in pressure, temperature and salinity in wells, geophysical survey techniques (e.g., repeat microgravity) may be useful for reservoir monitoring.

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Documents

Garg GRC ReservoirEngineering Sept2014