Title:

A Numerical Simulation Study of the Performance of Enhanced Geothermal Systems

Authors:

Steve J. Butler, Subir K. Sanyal, Ann Robertson-Tait

Key Words:

enhanced geothermal systems, numerical simulation, hydraulic stimulation

Conference:

Stanford Geothermal Workshop

Year:

2004

Session:

HOT DRY ROCK

Language:

English

File Size:

691KB

View File:

Abstract:

This paper presents the results of a numerical
simulation study of the performance of Enhanced
Geothermal Systems (EGS), specifically, reservoirs
with subcommercial permeability enhanced by
hydraulic stimulation. The performance under
consideration here is the net electrical power
delivered as a function of time and the parameters in
this exercise reflect conditions encountered at the
Desert Peak EGS project in Nevada.
Three well geometries are considered: (a) doublet
(an injection and production well pair), (b) triplet (an
injector flanked by a production well on each side),
and (c) five-spot (an injector at the center and a
production well at each corner of a square). The
injector and producers communicate through a
double-porosity reservoir with a thickness of 4,000
feet and at a temperature of 410?F. After
enhancement by stimulation, the hydraulic
characteristics of the reservoir are assumed to remain
constant. The thickness of the stimulated zone was
varied from 500 to 4,000 feet, and a range of fracture
spacings (from 1 to 1,000 feet) and fracture
permeabilities (from 1 to 100 millidarcy) following
enhancement were considered. The spacing between
the injector and producers was varied over a wide
range.
The injection water temperature was assumed to be
180?F, which is the temperature of the separated
brine available from the existing Desert Peak power
plant. The injection rate was dictated, through
reservoir simulation, by the production rate assigned
to the producers. Production wells were allowed a
maximum drawdown of 500 psi and the injection
well was limited to a maximum pressure buildup of
1,000 psi.
From the forecast of the production rate and
temperature, the gross power available was calculated
as a function of time from the First and Second Laws
of Thermodynamics; from this, the net power
available versus time was calculated for each
by injection and production pumps. For each
combination of assumed geometry, injector-producer
spacing, stimulated thickness, and enhancement level
(fracture spacing and permeability), the net power
generation capacity versus time (ìnet generation
profileî) was calculated.
For each case, the mean and variance of the net
generation over 30 years, net power produced per
unit injection rate, and the fraction of the in-place
heat energy recovered were estimated. The results
indicate that power generation from an enhanced
geothermal system, such as at Desert Peak, should be
technically feasible under a variety of development
scenarios.


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