Evidence for the existence of a stable, highly fluidized-pressurized region of deep, jointed crystalline rock from Fenton Hill hot dry rock test data Page: 4 of 17
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Evidence for the existence of a stable, highly fluid-pressurized region of
deep, jointed crystalline rock from Fenton Hill hot dry rock test data
Donald W. Brown
Earth and Environmental Sciences Division
Los Alamos National Laboratory
Abstract
It has been demonstrated several times at Los Alamos National Laboratory's Fenton Hill hot
dry rock (HDR) geothermal test site, that large volumes of naturally jointed Precambrian
crystalline rock can be stably maintained at pressures considerably above the least principal
earth stress in the surrounding rock mass. In particular, for the deeper, larger, and tighter
of the two HDR reservoirs tested at this site in the Jemez Mountains of north-central New
Mexico, testing was carried out for a cumulative period of 11 months without evidence of
fracture extension at the boundaries of the pressure-stimulated region, even though a very
high reservoir inlet circulating pressure of 27.3 MPa (3960 psi) above hydrostatic was
maintained throughout the testing, considerably in excess of the least principal stress in the
surrounding rock mass of about 10 MPa above hydrostatic at a depth of 3500 m.
We review and summarize information concerning the earth stresses at depth and the test
data relative to the containment of pressurized fluid, particularly the data showing the
declining rate of water loss and the absence of microseismicity -- the two principal indicators
of a stable, pressurized reservoir region. We then provide a coherent and concise evaluation
of this and other evidence supporting our assertion that one can indeed maintain large
volumes of jointed rock at pressures considerably in excess of the least principal earth
stress. In addition, a discussion is presented concerning the initial state of stress at depth
beneath Fenton Hill and then possible changes to the stress state resulting from the very
large volumes of injected high-pressure water and the accompanying shear displacements --
and shear dilation -- associated with these pressurizations.
Introduction
Recently, the Laboratory has embarked on a "Cradle-to-Grave" Carbon Management
research program to investigate a number of methods for reducing or eliminating the carbon
dioxide emissions from fossil-fueled power plants. This multi-pronged effort is considering
either upstream carbon separation prior to combustion or carbon dioxide separation from the
power plant effluent stream, and then long-term storage. As one part of the Laboratory's
overall strategy, the sequestering of carbon dioxide by deep earth injection is being
investigated. It is to this end that the data obtained at Fenton Hill is being reviewed, since
this testing represents the most significant data set presently available relating to the deep
earth storage of high-pressure fluids.
From 1972 through 1995, researchers at Los Alamos National Laboratory were engaged in
developing the technology for creating fully engineered geothermal reservoirs in hot,
impermeable, crystalline rock. The two separate hot dry rock (HDR) reservoirs that were
repeatedly tested since the late 1970's were formed by hydraulic fracturing techniques, and
subsequently circulated with water -- at very high pressures -- to mine heat from the hot
rock. The results from this testing have indicated that it is practical and economical to
operate commercial-scale HDR heat mining facilities to produce thermal power on a
sustained basis -- with little or no environmental impact.
However, our colleagues in the international rock mechanics community view with
incredulity the fact that we have been able to maintain, for months at a time, a highly fluid-
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Brown, D. W. Evidence for the existence of a stable, highly fluidized-pressurized region of deep, jointed crystalline rock from Fenton Hill hot dry rock test data, article, June 1, 1999; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc679962/m1/4/: accessed March 28, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.