This paper provides a review of the geomechanics and modeling of geomechanics associated with geologic carbon storage (GCS), focusing on storage in deep sedimentary formations, in particular saline aquifers. The paper first introduces the concept of storage in deep sedimentary formations, the geomechanical processes and issues related with such an operation, and the relevant geomechanical modeling tools. This is followed by a more detailed review of geomechanical aspects, including reservoir stress-strain and microseismicity, well integrity, caprock sealing performance, and the potential for fault reactivation and notable (felt) seismic events. Geomechanical observations at current GCS field deployments, mainly at the In Salah CO2 storage project in Algeria, are also integrated into the review. The In Salah project, with its injection into a relatively thin, low-permeability sandstone is an excellent analogue to the saline aquifers that might be used for large scale GCS in parts of Northwest Europe, the U.S. Midwest, and China. Some of the lessons learned at In Salah related to geomechanics are discussed, including how monitoring of geomechanical responses is used for detecting subsurface geomechanical changes and tracking fluid movements, and how such monitoring and geomechanical analyses have led to preventative changes in the injection parameters. Recently, the importance of geomechanics has become more widely recognized among GCS stakeholders, especially with respect to the potential for triggering notable (felt) seismic events and how such events could impact the long-term integrity of a CO2 repository (as well as how it could impact the public perception of GCS). As described in the paper, to date, no notable seismic event has been reported from any of the current CO2 storage projects, although some unfelt microseismic activities have been detected by geophones. However, potential future commercial GCS operations from large power plants will require injection at a much larger scale. For such large-scale injections, a staged, learn-as-you-go approach is recommended, involving a gradual increase of injection rates combined with continuous monitoring of geomechanical changes, as well as siting beneath a multiple layered overburden for multiple flow barrier protection, should an unexpected deep fault reactivation occur.

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The geomechanics of CO2 storage in deep sedimentary formations

Geothermal energy is a renewable energy source that can be found in abundance on our planet. Only a small fraction of it is currently converted to electrical power, though in recent years installed geothermal capacity has increased considerably all over the world. This review focuses on Enhanced Geothermal Systems (EGS), which represent a path for turning the enormous resources provided by geothermal energy into electricity for human consumption efficiently and on a large scale. The paper presents a general overview of this ever-expanding technology from its origins to the current state of the art. The Geodynamics plant in Habanero (Australia), which started up on 2 May 2013, is the first privately-run commercial EGS plant to produce electricity on a large scale. Thanks to the technological development of EGS in recent years, the future looks bright for such plants in the decades to come.

Enhanced geothermal systems (EGS): A review

To stimulate the reservoir for a “hot dry rock” geothermal project initiated by a private/public consortium in the city of Basel, Switzerland, approximately 11,500 m3 of water were injected at high pressures between 2 December and 8 December 2006 into a 5-km-deep well below Kleinhuningen (Haring et al. 2008). A six-sensor borehole array, installed by the operators of the project at depths between 317 and 2,740 meters around the well to monitor the induced seismicity recorded more than 10,500 seismic events during the injection phase. Hypocentral locations could be calculated for more than 3,000 of these events. The gradual increase in flow rate and wellhead pressure was accompanied by a steady increase in seismicity, both in terms of event rates and magnitudes. In the early morning hours of 8 December, after water had been injected at maximum rates in excess of 50 l/s and at wellhead pressures of up to 29.6 MPa for about 16 hours (Haring et al. 2008), a magnitude ML 2.6 event occurred within the reservoir. This exceeded the safety threshold for continued stimulation, so that injection was stopped prematurely. In the afternoon and evening of the same day, two additional events of magnitude ML 2.7 and 3.4 occurred within the same source volume. As a consequence, the well was opened and the water allowed to flow back. In the following days about one third of the injected water volume flowed back out of the well (Haring et al. 2008). Though the seismic activity declined rapidly thereafter, even more than two years later sporadic microseismicity was being detected in the stimulated rock volume by the downhole-instruments.

The mainshock was felt distinctly in the urban area of Basel. People reported short, high-frequency shaking lasting 1–3 seconds, often accompanied by a loud bang similar to an …

Earthquakes Induced by the Stimulation of an Enhanced Geothermal System below Basel (Switzerland)

Global review of human-induced earthquakes

Induced seismicity and hydraulic fracturing for the recovery of hydrocarbons

http://home.ustc.edu.cn/~ly2014/Advances_in_Geosciences/Reading%20materials%20for%20Week%202/Group6-Characterisation%20of%20the%20Basel%201%20enhanced%20geothermal%20system.pdf

Characterisation of the Basel 1 enhanced geothermal system

The Bedretto Underground Laboratory for Geosciences and Geoenergies (BULGG) is located in central Switzerland and serves as a test bed for geothermal energy research. Several boreholes were drilled from the laboratory section (ca. 1.1 km overburden) to serve as injection boreholes for stimulation and geophysical monitoring boreholes. During a hydraulic stimulation injection in winter 2020 into injection borehole ST2 interval ranging from 313 to 320 m, we observe a thermal perturbation using distributed fiber optic temperature sensing in a neighboring open borehole (MB1) at a depth of 275 m to 295 m. Prior to injection, there is a thermal anomaly in MB1 at about 289 m due to natural fracture fluid flow. Below this depth the temperature is approximately 1.5 °C higher than above. During injection there is a gradual upward movement of the thermal anomaly to ca. 278 m depth. After injection is stopped, the thermal signal gradually recovers to the original depth. The cause for such a temperature change is potentially due to increased warm water flow reaching the base of MB1 from deeper ST2 or poro-elastic fracture closure of the cold-water conducting fractures at 278 and 289 m depth in MB1 during stimulation.

Ben C. Dyer

Papers

Characterisation of the Basel 1 enhanced geothermal system

Heat propagation through fractures during hydraulic stimulation in crystalline rock