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PROCEEDINGS, Eighteenth Workshop on Geothermal Reservoir Engineering
Stanford University, Stanford, California, January 26-28, 1993
EXPERIMENTAL STUDY OF WATER ADSORPTION ON
GEYSERS RESERVOIR ROCKS
Shubo Shang, Roland N. Horne and Henry J. Ramey, Jr.
Department of Petroleum Engineering
Stanford CA 94305-2220
Experimental isotherms of water vapor
adsorption/desorption on three geothermal reservoir rock
samples have been measured at temperatures of 80, 100,
120 and 140 C. Initial surface status of the sample was
found to influence the amount of water adsorbed. At low
relative pressures, adsorption is the dominant process of
water retention onto the rock samples.
Adsorption/desorption hysteresis was observed to exist
over the whole pressure range at all temperatures.
Similar observations were made for all three samples.
The results of this study suggest that adsorption is
important in storing water in geothermal reservoir rocks
not only in itself, but also in inducing capillary
In vapor dominated geothermal systems, it has been
proposed that liquid might exist as adsorbed liquid in
micropores (White, 1973). Evidence from both
laboratory studies (Hsieh, 1980, Herkelrath et al., 1983)
and field data indicates that storage of liquid as
micropore fluid is reasonable. As pointed out by Ramey
(1990) that if the only mechanism for liquid storage is
adsorbed water, then the desorption curve provides
important information for performance matching and
production forecasting. It is therefore obvious that
measurement of adsorption/desorption of water vapor on
reservoir rocks is a crucial step in determining whether
adsorption is the storage mechanism for these systems,
and if so, what would be the appropriate procedure for
performance prediction of vapor dominated geothermal
Water vapor desorption was regarded as a simple and
reliable technique in core analysis by Melrose (1988). In
a comprehensive study to determine capillary pressure in
the low-saturation region, Melrose (1991) compared
results obtained using different techniques, and
concluded that water vapor desorption method gave
capillary pressure in good agreement with those obtained
using the centrifuge and the porous plate methods.
However, it should be pointed out that water vapor
desorption in this case referred to the part of the
desorption curve at relative pressures greater than 0.9,
and the corresponding saturations are higher than 3.3%
for the berea sandstones studied by Melrose (1988). At
high relative pressures, adsorption is accompanied and
complicated by capillary condensation, and it is likely
that water held by capillary force constitutes a large part
of the desorbed water.
Literature on water adsorption has been largely
concerned with pure adsorbents, with carbon and silica
being the most popular (Dubinin, 1980, Stoeckli, et al.,
1983). The majority of the water adsorption studies are
associated with the characterization of porous material.
Willems et al. (1988) reported studies of water adsorption
on hardened cement pastes and observed large
adsorption/desorption hysteresis at 25 C. Carrott et al.
(1991) studied water adsorption on synthetic zeolites in
order to elucidate the relative influence of surface
chemistry, pore size and pore shape upon the form of the
water adsorption isotherm. A common feature of these
studies is that the adsorption tests were all made at low
temperature (between 0 to 25 C) and some special
behaviors of water (Gregg and Sing, 1982) were
Reservoir related water adsorption investigation is
limited, particularly at high temperatures. Hsieh (1981)
constructed a BET type of apparatus for high temperature
water adsorption measurements. He conducted a number
of adsorption measurements on berea sandstone and
unconsolidated silica sand. Conclusions reached in his
study were that adsorbed water is an important source of
steam in vapor dominated geothermal systems and that
water vapor pressure lowering in a rock is dominated by
micropore adsorption (Hsieh and Ramey, 1983).
Luetkehans (1988) continued Hsieh's research by
improving the apparatus. Measurements of N2
adsorption-were made for berea sandstone, cores from the
Geysers, California, and Larderello, Italy. Water
adsorption/desorption tests were also conducted.
However, the true magnitude of the amount adsorbed was
suggested to be in question due to difficulties in
establishing equilibrium. In addition, the long
equilibrium time required made the leakage of high
temperature valves a significant factor in causing the
experimental error. Laboratory studies of water
adsorption on porous media were also conducted by
Herkelrath et al. in association with their work on steam
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Shang, Shubo; Horne, Roland N. & Ramey, Henry J., Jr. Experimental study of water adsorption on Geysers reservoir rocks, article, January 28, 1993; United States. (digital.library.unt.edu/ark:/67531/metadc885243/m1/4/: accessed October 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.