reservoir assessment

8/19/2019 Reservoir Assessment

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GEOTHERMAL RESERVOIR ASSESSMENT

Benedikt Steingrímsson

Introduction lecture course 2004

May 24th 2004


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Reservoir Assessment Methods(regional or local applications)

• Surface Thermal Flux• Volumetric Methods

• Decline Curve Analysis

• Lumped Parameter Model• Distributed Parameter Models

– Natural State Models

– History Matching and Exploitation Models


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Surface Thermal Flux Method

Estimation of • the continuous thermal flux caused by heat

conduction to the surface.

• The thermal power of geothermal manifestation (hotsprings and fumaroles).


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The Volumetric MethodStored Heat Calculations.

Calculation of the total heat energy stored in avolume of rock as compared to some

reference temperature (i.e. the annual mean

temperature or 0°C).This will be the sum of the thermal energy

stored in the rock matrix and the thermal

energy of the fluid (water, steam) in the rockpores).


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Stored heat in an isothermal reservoir

Eres = Erock + Efluid

Erock = V(1-Φ)ρr Kr (Tres - Tref )

Efluid = VΦρf Kf (Tres - Tref )

or

Efluid = VΦρf (Hres - Href )

where:

Φ is porosity

K is heat capacity

ρ is density

H is enthalpy

Geothermal

reservoir

Volume = V

Temperature = Tres


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Volumetric Resource Assessment(some definitions, see hand-out)

Geothermal resources base:

All the heat in Earth’s crust below a specific area (ref. mean annualtemperature). Commonly to 10 km depth.

Accessible resources base:

Energy at shallow shallow depths that can be tapped by drilling.

Commonly to 3 km depth.

Geothermal resource:

Fraction of accessible resource base that can be extracted

economically and legally at some reasonable future time.

Geothermal reserve:

Identified part of geothermal resource that can be extracted today

at the cost competitive with other energy sources.


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Diagram illustrating subdivision of Geothermal Resources Base

Depth 3 km

Economics at

some future time

Economics at the time

of determination


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Iceland Resource Assessment 1984.

Division into zones


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High temperature fields in Iceland

Proven

Probable


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Iceland Resource Assessment 1984.Estimated crustal temperatures


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High temperature fields. Accessible Resources Base


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Iceland. Geothermal Resource Assessment


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Lumped Models of Geothermal Reservoirs

Simple reservoir models used for matching and predictingchanges in one reservoir parameter caused byproduction from the system. (The Theis model is an example ofa lumped model)

Input parameter:

• Total production from the field as a function of time(Production history).

Parameter matched

• Usually the pressure (water level).Model predictions:

• Future pressures for various production plans.


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A general lumped parameter model used to simulate water

level or pressure changes in geothermal systems(Guðni Axelsson, see hand-out)


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Water level in Hamar LT-system in Iceland

simulated by a lumped model


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Water level changes, past and predictions for two productions

scenarios, calculated by a lumped model


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Example demonstrating the reliability of a lumped model for

Gata geothermal system in S-Iceland


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Distributed Parameter ModelsWhat is numerical modeling??

• Break the reservoir volume into small pieces called

“gridblocks.

• Assign hydrological and thermal properties to each

gridblock.• Assign sinks and sources where there are inflow or

outflow zones in the system or production or injection

wells.

• Use a computer program such as TOUGH2 to

calculate the evaluation of the system


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The Nesjavellir grid 1986-2000

• The extent of the 1986 gridwas 12 km in all directions

but was extended to 100 km

in 1992.

• Each production well has itsown gridblock .

• The model consists of three

400 m thick layers and one

800 m thick reservoir layer.


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General approach to

reservoir numerical

modeling (after Bodvarsson

and Witherspoon)


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Distributed Parameter ModelsWhat is the purpose of numerical modeling??

• To guide the development of the field – estimate generating capacity

– estimate flow rate and enthalpy changes of

production wells

– estimate how many wells need to be drilled

– select proper location of new wells

– evaluate effectiveness of re-injection

– estimate how chemistry will change with time


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Distributed Parameter ModelsWhat is the purpose of numerical modeling??

• To help to evaluate different conceptual models ofthe field

– where are the up-flow and outflow zones?

– how is the permeability structure (barriers, flow

channels)?

– how much is the natural recharge?

– how many different conceptual models can explain

the data?


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Distributed Parameter Models

Main parameter matched:• Temperature distribution in the reservoir

• Pressure distribution in the reservoir

• Flow histories for production wells (Q and H)

• Observed pressure interference between

wells

• The presence of two-phase or vapor zones


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Nesjavellir-Iceland

The 1986 model:

Computed and

observedtemperatures in the

R-layer


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Reasonable match withflow data from mostwells

• Predicted pressure

draw-down similar toobserved

Consequently:Only minor changes in

model parameters inthe 1998 update

Recalibration of the Nesjavellir model in

1998

1980 1984 1988 1992 1996 2000

0

1000

2000

3000

Simulated enthalpy

Observed enthalpy

E n t h a l

p y (

k J / k g )

Well NG-6 Production history 1982-1998

F l ow ( k g / s )

Year

0

20

40

60

Simulated flow

Oberved flow


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Distributed parameter models

Main parameters adjusted during the simulation:• Permeability distribution of the reservoir.

• Porosity distribution in the reservoir.

• Productivity indices for wells (the relation between Qand pressure drop from reservoir into the well).

• Mass recharge to system.

• Heat recharge to the system.


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Permeability and porosity distribution inferred from a well-by-well model for Olkaria, Kenya


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Distributed parameter models

Main parameters predicted.

• Natural recharge to the system.

• Permeability and porosity distribution for thereservoir.

• Location of barriers and flow channels.

• Future pressure draw-down in all observation wells.

• Future flow histories for all production wells (Q andH).

• Expansion of two phase or steam zones due to draw-

down.• Production induced (draw-down) recharge from the

outer boundaries of the reservoir

• Effects of re-injection (pressure maintenance, thermal

breakthrough).


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Nesjavellir

modeling 1986

Main conclusions• Recharge rate in up-flow zone

under Hengill volcano is 65 kg/s

with enthalpy of 1850 kJ/kg

• Permeabilities vary between 1-50 md.

• Porosities vary between 1-10%

• The field can support 300 MW

thermal power plant for at least30 years.


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The end

reservoir assessment

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