Injection plume behavior in fractured, vapor-dominated reservoirs Page: 4 of 11
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PROCEEDINGS, Twenty-First Workshop on Geothermal Reservoir Engineering
Stanford University, Stanford, California, January 22-24, 1996
INJECTION PLUME BEHAVIOR IN FRACTURED,'
Earth Sciences Division, Lawrence Berkeley National Laboratory
University of California, Berkeley, CA 94720
We discuss fluid flow and heat transfer processes
during water injection into hot, fluid-depleted vapor
zones. Numerical simulations of injection plumes in
fractures, modeled as two-dimensional heterogeneous
porous media, indicate complex behavior. Under
certain conditions it is possible to make detailed
quantitative predictions of vaporization behavior.
However, when effects of reservoir heterogeneity are
dominant it will only be possible to predict the
behavior of injection plumes in general terms.
In response to extensive steam production, the vapor-
dominated geothermal reservoirs at The Geysers,
California, and Larderello, Italy, are beginning to run
out of fluid, while heat reserves in place are still
enormous. Injection of water is the primary means by
which dwindling fluid reserves can be replenished, and
recovery of thermal energy be enhanced and
accelerated. Field experience shows that water
injection may have very beneficial effects, increasing
reservoir pressures and flow rates of offset steam
production wells (Beall et al., 1989; Enedy et al.,
1991; Goyal and Box, 1992; Goyal, 1995). Effects of
water injection are not always favorable, however,
because thermal degradation (temperature decline) or
water breakthrough may occur at neighboring wells
(Barker et al., 1992).
Large-scale permeability at The Geysers and
Larderello is provided by networks of interconnected
fractures (Beall and Box, 1989), while the matrix rock
has low permeability typically of order 10-18 m2 (1
microdarcy) or less. The ability of water injection to
sustain steam production then depends on the rate of
heat transfer from the reservoir rocks to water
migrating along fractures. This paper is concerned
with the fluid and heat flow processes that develop
when liquid water enters hot sub-vertical fractures in
rocks of low permeability. Fractures are modeled as
two-dimensional heterogeneous porous media with
rough walls, bounded by semi-infinite rock slabs. The
descent of boiling liquid plumes in such fractures
under the combined action of gravity, capillary, and
pressure forces is analyzed by means of high-
resolution numerical simulations, using our
TOUGH2 general-purpose reservoir simulator
(Pruess, 1991a), enhanced with a set of preconditioned
conjugate gradient solvers (Moridis and Pruess,
1995). We also present a lumped model for plume
migration and vaporization, using simple
approximations for fluid flow in the fracture and for
conductive heat transfer from the wall rocks.
General description of injection plume
Injection of water into hot sub-vertical fractures gives
rise to a complex interplay of multiphase fluid flow
and heat transfer processes. Two-phase flow in the
plume takes place under the combined action of
gravity, capillary, and pressure forces. Liquid water
will also partially imbibe into the rock matrix. In
open sections of fractures water will flow generally
downward, driven by gravity. However, at asperity
contacts (fracture walls touching) water may pond and
be diverted sideways. In regions of small fracture
apertures or strong wall roughness, capillary pressure
effects could also be significant.
Conductive heat transfer from the wall rocks causes
the temperature of the water to increase, and
eventually boiling is initiated when the water reaches
the saturation temperature at prevailing pressure,
Tsat(PO). Thermal diffusivities of rocks are low,
typically of the order of 10-6 m2/s. Thus, heat
conduction to the injection plume is a relatively slow
process, giving rise to very broad two-phase zones
(Calore et al., 1986). This is in contrast to liquid
injection into hot underpressured porous media, where
rock-fluid heat transfer occurs over small spatial
scales (grain sizes), with rapid thermal equilibration
locally and "sharp" two-phase fronts (Schroeder et al.,
1982; Pruess et al., 1987).
The vaporization process is accompanied by an
increase in pressure which drives the steam away from
the injection plume. Two parameter regimes may be
distinguished. When water enters a fracture at a "low"
rate, the rates of vapor generation and flow are also
small, causing an insignificant increase in pressure.
Plume pressure Pp1 remains close to initial fluid
pressure P0 and, because of the one-to-one
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Pruess, Karsten. Injection plume behavior in fractured, vapor-dominated reservoirs, article, January 24, 1996; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc873947/m1/4/: accessed October 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.