Hot Dry Rock Geothermal Reservoir Model Development at Los Alamos Page: 4 of 10
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HOT DRY ROCK GEOTHERMAL RESERVOIR MODEL DEVELOPMENT
AT LOS ALAMOS
Bruce A. Robinson
Los Alamos National Laboratory Los Alamos, NM
Stephen A. Birdsell
Mechanical Design Services, Santa Cruz, NM
ABSTRACT by matching steady state field data.Discrete fracture and continuum models are being
developed to simulate Hot Dry Rock (HDR) geothermal
reservoirs. The discrete fracture model is a two-
dimensional steady state simulator of fluid flow
and tracer transport in a fracture network which is
generated from assumed statistical properties of
the fractures. The model's strength lies in its
ability to compute the steady state pressure drop
and tracer response in a realistic network of
interconnected fractures. The continuum approach
models fracture behavior by treating permeability
and porosity as functions of temperature and
effective stress. With this model it is practical
to model transient behavior as well as the coupled
processes of fluid flow, heat transfer, and stress
effects in a three-dimensional system. The model
capabilities being developed will also have
applications in conventional geothermal systems
undergoing reinjection and in fractured geothermal
reservoirs in general.
INTRODUCTION
Reservoir engineers have long recognized the
importance of fractures to fluid flow and tracer
transport in underground porous media. The typical
approach of employing the convective-dispersion
equation with an adjustable dispersion coefficient
is usually an unrealistic simplification.
Multidimensional forms of the convective-dispersion
equation can often provide good fits to the data,
but at the expense of introducing more adjustable
parameters of questionable physical significance.
Furthermore, the fundamental assumption of a
homogeneous porous medium may be incorrect for a
fractured porous medium unless it is highly
fractured (Bear, 1975).
Further complexities arise in modeling HDR
reservoirs. First, since most reservoirs will be
operated at pressures only slightly lower than that
required to induce hydraulic stimulation, fractures
will either be completely open or on the verge of
opening. For a fractured medium in this state, the
porosity and permeability will be strong functions
of pressure. Also, heat extraction will result in
large temperature drops in the rock mass. The
resulting flow patterns will be affected through
the temperature-dependence of viscosity and the
impact of cooling on the effective stress on
joints.
Models currently under development at Los Alamos
are designed to handle these complexities. First,
a fracture network model has been developed to
examine the steady state pressure drop and solute
transport through a fracture network consisting of
a realistic number of fractures. With this model,
we can study the effects of parameters such as mean
fracture spacing, average aperture and aperture
distribution, reservoir size, and rock matrix
properties on the pressure drop and tracer
response. Bounds can be placed on these parametersHowever, when modeling transient behavior in three-
dimensional systems which possess temperature and
pressure dependent reservoir and fluid properties,
the fracture network approach becomes impractical
due to the enormous memory and computational time
requirements. Therefore, we are also developing a
continuum model which assumes an equivalent porous
medium with porosity and permeability relationships
designed to mimic fracture flow and transport.
Using the fracture network simulations to place
bounds on the reservoir properties, we may then
employ the continuum code to simulate pressure and
temperature dependent effects and transient
behavior, thereby further constraining our model of
the reservoir. This paper briefly outlines the
assumptions and development of each model and
presents sample calculations demonstrating the
capabilities of the codes.
FRACTURE NETWORK MODEL
This section summarizes the model assumptions and
capabilities of the fracture network code FRACNET.
A more complete description of the model may be
found in Robinson (1989).
Fracture Network Generator
A fracture network generator has been developed so
that steady-state flow between two wellbores with a
no-flow outer boundary can be simulated. To model
fluid flow, a two-dimensional, interconnected
network of fractures is generated within a circular
region. The diameter of this region is based on an
estimate of the geometry of the fracture network.
The outer boundary is a no-flow boundary, with
wellbores simulated as constant-pressure line
segments within the region.
The technique used to generate the fracture network
is similar to that of Long et al. (1982). The
network consists of two sets of fractures, each
with a preferred orientation. The center of each
fracture is located randomly in space, and then its
direction, length, and aperture are generated from-
the given statistical distributions. .When all
fracture== locations are generated, -the code then
determines the intersection points of each fracture
with, other fractures, the wellbores, and the outer
boundary. Finally, dead-end pathways, which are
nodes or groups of nodes not connected to the rest
of the network or connected through only one node,
are eliminated from the network.
Solution for Fluid Flow and Pressure Field
Assuming that flow in a given fracture can be
modeled as laminar flow between parallel plates,
the fracture aperture, the fluid velocity u is
given by_2
-w AP
l 2iiL(1)
157
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Robinson, Bruce A. & Birdsell, Stephen A. Hot Dry Rock Geothermal Reservoir Model Development at Los Alamos, article, March 21, 1989; Los Alamos, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc881867/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.