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Evolution of fracture permeability with respect to fluid/rock interactions under thermohydromechanical conditions: development of experimental reactive percolation tests
Blaisonneau et al. Geotherm Energy
Evolution of fracture permeability with respect to fluid/rock interactions under thermohydromechanical conditions: development of experimental reactive percolation tests
A. Blaisonneau
M. Peter‑Borie
S. Gentier
The evolution of fracture permeability is crucial as regards the lifespan of the deep fractured geothermal exchanger of an Enhanced Geothermal System site. The objective in developing reactive percolation tests in fractures under thermohydromechanical conditions is to improve our understanding of fluid/rock interactions and their role in the evolution of fracture permeability. This article describes the test apparatus and the experimental protocol developed to meet this objective. The data from a test on a sample of fractured granite are interpreted with a view to characterising the phenomena that occurred during the reactive percolation and their impact on the behaviour of the fracture and its permeability. The test showed that the free face type dissolution of some minerals led, through a deepening of existing channels on the fracture walls, to an increase of the fracture's permeability by one order of magnitude and to a change in its hydromechanical behaviour.
Fracture; Permeability; Fluid/rock interactions; Laboratory testing; Fracture characterisation; Engineered/Enhanced Geothermal System (EGS)
Background
The need to diversify the energy mix has, among other things, led to attempts to
generalise the use of the Earth’s heat for the production of heat and/or electricity. Moving
beyond the “classic” geothermal energy stage associated with active volcanic areas and
specific aquifers whose hydraulic and thermal characteristics are directly and
economically exploitable is a new field of geothermal energy known as the Engineered/Enhanced
Geothermal Systems (EGS). These systems use deep underground (high-temperature)
rock formations as heat exchangers through promoting the circulation of a natural fluid
(MIT 2006)
. In the absence of a typical aquifer (permeable porous medium), natural
fluids circulate in complex hydraulic systems made up of fracture and fault networks
that are more or less well connected, depending on the considered scale. The aim of
the new exploitation techniques is to provoke fluid circulation between an injection
© 2016 Blaisonneau et al. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License
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well and one or more production wells, thus modifying the local circulation dynamics
in order to obtain an economically viable flow rate and temperature. Given their weak
injectivity and initial productivity, the wells in these environments commonly require a
development phase based on hydraulic and/or chemical stimulation through
overpressurised injection of a cold fluid into the hot fractured medium. During this development
phase, predominant physical processes in the fracture network depend on the
stimulation scenario: hydromechanical processes are of first order during hydraulic
stimulation, whereas hydrochemical processes drive the behaviour during chemical stimulation.
Following the development phase, the cost effectiveness of these systems lies in their
sustainability over time, i.e. at least 20 years of operation without any substantial
reduction in well injectivity and/or productivity or any thermal short circuit due to a localised
increase in the permeability of the deep fractured rock mass. During this exploitation
phase of the EGS, the problem of permeability evolution in a natural fracture (basic
element of the hydraulic system in question) due to fluid–rock interactions, within a
varying thermal and mechanical context depending on the distance from the well, is a crucial
issue.
For over 20 years, the Soultz-sous-Forêts experimental site in Alsace (France) has
been dedicated to the scientific study of these new geothermal systems
(Genter et al.
2010)
. The various research projects carried out at this site have shown the importance
of understanding both the natural and the induced circulation of fluids in the fractured
and/or faulted granitic basement and its evolution in situations of specific thermal
and mechanical stress. This problem, which is relatively new in the field of geothermal
energy, is similar to that posed over the past 30 years in connection with the
underground storage of radioactive waste
(Rutqvist and Stephansson 2003)
and more recently
in the oil industry for the exploitation of fractured reservoirs.
The evolution of a fracture’s hydraulic behaviour under both normal stress and shear
has already been widely studied in relation to the storage of nuclear waste in the 1980s
and 1 (...truncated)