Hydraulic fracturing during the ISC experiment
| Project responsable | Benoît Valley |
| Team member | Nathan Dutler |
| Project partner | Florian Amann |
| Abstract |
Successfully creating permeability in deep-seated rock masses to
economically tap the heat to generate electricity remains a
challenge for society. If solution can be found to unlock these
resources a large amount of clean, local and renewable energy can
be produced. Indeed, the Swiss Energy Strategy 2050 (ES2050)
propose scenarios with 7% of national electricity supply from Deep
Geothermal Energy (DGE), which corresponds to over 500 MWel
installed capacity. In Switzerland, temperatures between
170-190°C are found at 4-6 km depths, but the natural
permeability of deep crystalline basement rocks is too low to allow
large volume fluid circulation for sufficient heat extraction.
Therefore, permeability must be enhanced using high-pressure fluid
injection to exploit DGE in an Enhanced Geothermal System (EGS).Two
different paradigms are commonly referred to when discussing
permeability creation processes through hydraulic injections: 1)
hydraulic fracturing as the initiation and propagation of mode I
fractures and 2) hydraulic stimulation, i.e. the mobilization of
existing discontinuities in shear with associated dilation leading
to a self-propping mechanism. The former is the common concept used
in reservoir enhancement in the oil and gas industry, while the
latter is typically the case in enhanced geothermal reservoirs. If
it is currently accepted that both mechanisms can occur
concomitantly, it is not well-understood how these processes
interact and what rock mass characteristics and injection metrics
controls which mechanisms dominate.To address these questions,
relevant datasets from well-controlled hydraulic injection
experiments are required. We propose to perform such experiments in
an underground laboratory where rock mass conditions are
representative of target formations for EGS in Switzerland, and
with excellent access to the rock mass. Experimental work on small
samples in loading frames suffers issues of scale because the
samples tested are often homogenous and therefore unrealistic at
larger scales. Well-controlled experiments at larger scales are
often impeded by insufficient access to the rock mass. Funding to
execute a well-controlled experiment at the Grimsel test site has
secured under the umbrella of the Swiss Competence Center for
Energy Research - Supply of Electricity (SCCER-SoE), and an
experimental plan has been developed. This experiment is referred
as the In-situ Stimulation and Circulation experiment
(ISC-experiment). The ISC experimental plan focusses on the
stimulation of existing shear zones, and includes an extensive pre-
and post- stimulation rock mass characterization program and a
comprehensive monitoring package. This unique setting will allow
observations and measurements in a coupled manner of all parameters
relevant to geomechanics (i.e. stress state, deformation,
fracturing, pressure propagation during injection and following
shut-in, etc.), hydrogeology (i.e. fluid flow, permeability,
fracture connectivity, etc.) and seismology (i.e. a micro-seismic
monitoring system at multiple scales).The objective of this
proposal is to expand the ISC experimental plan by adding a
research component dedicated to understanding and modeling the
initiation and propagation of hydraulic fractures. This research
will address unresolved questions regarding the initiation,
propagation, and interaction of hydraulic fracturing in tough
crystalline rocks. Multiple hydraulic fractures will be created
using a variety of injection strategies to determine their behavior
and compare their impact. The characterization and monitoring
systems in place will allow precise mapping of hydraulic fracture
evolution, and their impact on the pore pressure field and the rock
mass. Essential parameters controlling hydraulics, such as fracture
aperture changes during injection, will be measured. The seismic
response to hydraulic fracturing will be measured and compared with
shear zone stimulation. The research plan is composed of three main
components: 1) execution of the hydraulic fracturing tests, 2)
characterization of the fractured volume and the analyses of the
collected data, and 3) benchmarking of hydraulic fracturing
simulation tools against the field data set. Results of this
research will significantly contribute to the understanding of
injection strategies to truly engineer permeability creation in the
rock mass at depth for deep geothermal systems. |
| Keywords |
Hydromechanical rockmass response; hydraulic fracturing; Induced seismicity; Enhanced Geothermal Energy; In situ experiments; Grimsel test site |
| Type of project | Fundamental research project |
| Research area | Geothermics |
| Method of financing | SNF |
| Status | Completed |
| Start of project | 1-11-2016 |
| End of project | 31-8-2020 |
| Contact | Benoît Valley |