Fracture network modeling of a Hot Dry Rock geothermal reservoir Page: 4 of 11
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PROCEEDINGS, Thirteenth Workshop on Geothermal Reservoir Engineering
Stanford University, Stanford, California, January 19-21, 1988
SGP-TR-113
FRACTURE NETWORK MODELING OF A HOT DRY ROCK GEOTHERMAL RESERVOIR
Bruce A. Robinson
Los Alamos National Laboratory
Los Alamos, New Mexico 87545ABSTRACT
Fluid flow and tracer transport in a
fractured Hot Dry Rock (HDR) geothermal
reservoir are modeled using fracture network
modeling techniques. The steady state
pressure and flow fields are solved for a
two-dimensional, interconnected network of
fractures with no-flow outer boundaries and
constant-pressure source and sink points to
simulate wellbore-fracture intersections.
The tracer response is simulated by particle
tracking, which follows the progress of a
representative sample of individual tracer
molecules traveling through the network.
Solute retardation due to matrix diffusion
and sorption is handled easily with these
particle tracking methods. Matrix diffusion
is shown to have an important effect in many
fractured geothermal reservoirs, including
those in crystalline formations of relatively
low matrix porosity. Pressure drop and
tracer behavior are matched for a fractured
HDR reservoir tested at Fenton Hill, NM.
INTRODUCTION AND MOTIVATIONS
Reservoir engineers and groundwater hydro-
logists have long recognized the importance
of fractures on fluid flow and solute
transport in underground porous media. Many
analytical and numerical models exist to
predict flow behavior for various fracture
geometries ranging from a single fracture to
multiple, interconnected fractures. Steady
state or pressure transient responses can
often be predicted using these models, which
provide a macroscopic description of the flow
process in terms of parameters suitable for
use by hydrologists and engineers.
Solute transport is not so easily simulated
using these models, however. The typical
approach of employing the convective-
dispersion equation with the adjustable
parameter of dispersion coefficient usually
fails in several important ways. In one
dimension, a good match between model and
field data is often difficult to achieve,
since field data are seldom if ever perfect
Gaussian distributions of residence times
about a mean value. Multi-dimensional formsof the convective-dispersion equation can
provide better fits, but at the expense of
more adjustable parameters of questionable
physical significance.
Fracture network modeling is a different
approach to simulating flow and transport in
fractured rock. The flow system is comprised
of a network of interconnected fractures. A
pressure difference imposed in such a system
due to fluid injection or a natural hydraulic
gradient results in a flow of water through
the fractures. This flow field can be
calculated assuming a fracture geometry,
appropriate boundary conditions, and a
relationship between pressure drop and flow
rate within each fracture. Once the flow
field is determined, the transport of a
conservative, reacting, or adsorbing chemical
component can be calculated using particle
tracking techniques, which follow the
progress of a representative sampling of
tracer molecules through the network.
Fracture network modeling has been used
extensively to model groundwater flow (see,
for example, Castillo et al. (1972), Schwartz
(1977), Smith and Schwartz (1980), Schwartz
et al. (1983), Long et al. (1982), Andersson
and Thunvik (1983), and Hopkirk et al.
(1985), Long and Billaux (1987)). The
primary focus of most previous work has been
to determine the conditions under which a
fractured rock could be treated as an
equivalent porous medium. With the fracture
network approach, one can assess the effect
of fracture size, spacing, aperture, and
orientation on the fluid flow, permeability
distribution, and tracer behavior.
Typically, Monte Carlo techniques are used,
in which a large number of realizations of
different fracture geometries, all with
identical fracture statistics, are performed
to determine the average and variability of
behavior. The latter is a measure of the
inherent uncertainty of flow behavior in the
fracture network, given the measured statis-
tical parameters. In most cases, these
studies have assumed the flow to be within a
rectangular grid in two dimensions, with
constant-head boundary conditions at opposite
ends of the plane and no-flow or linearly--211-
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Robinson, Bruce A. Fracture network modeling of a Hot Dry Rock geothermal reservoir, article, January 1, 1988; Los Alamos, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc885784/m1/4/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.