Energy 101 for LinkedIn Professionals: Geothermal Energy — Challenges (2–2)
In the last article, I have identified two of four technical challenges associated with geothermal energy development, including expensive upfront cost and its geographical dependency. In this article, I’m addressing one of the major challenges for Enhanced Geothermal Systems (EGS), induced seismicity.
3) Induced seismicity
In simple terms, induced seismicity is an earthquake caused by human activity, such as mining, waste disposal wells, hydrocarbon extraction and storage, groundwater extraction, hydraulic fracturing (a.k.a. fracking) and carbon capture and storage (CCS). As you may have guessed, the activities often alter the underground conditions (e.g. pressure and temperature) by injecting or extracting fluids from the subsurface which consequently trigger minor earthquakes. Several factors contribute to induced seismicity, such as fluid injection volume, depth, rock type, pore pressure change, nearness to critically pre-stressed and extended faults (i.e. the faults that are already prone to cause earthquakes), stress and fracture heterogeneity, etc. All the changes in the underground conditions may lead to a change in the stress state which may cause reactivation of existing faults or fractures.
Figure 1: Various mechanisms of induced seismicity
Figure 2: Various human activities causing induced seismicity
For a conventional geothermal energy development, induced seismicity has not been a major issue. However, it could be one of the major concerns for Enhanced Geothermal Systems (EGS). As I mentioned in the last article, EGS basically relies on the same fluid injection process as fracking. For this process, fluid is injected into the subsurface geothermal reservoir at very high pressure in order to crack open the rock mass. This is conducted to create underground paths within the low-permeability geothermal reservoir so that when cool water is injected into the reservoir via an injection well, it can raise its temperature as it travels toward a production well. During the rock-cracking process, two effects come into place. The first effect is the so-called “pressure effect.” As the water is injected at a very high-pressure during fracking, it changes the natural pore fluid pressure and a change in the stress state. When the rock is cracking, there are minor tremors directly from the rock. However, typically the magnitude of these tremors is typically low (<M 2.0) and can be hardly felt from the surface. The bigger issue is that when there is already a fault near the geothermal reservoir which is already at a critical state (i.e. the fault is under relatively unstable condition). If the influence of the injected fluid is large enough, it can cause the fault to activate and cause an earthquake.
The “temperature effect” can also play a role in causing induced seismicity. Injected cold water reduces the temperature of a hot geothermal reservoir over time. This produces zones of different temperatures. The reduction of the reservoir temperature induces contraction of the affected rock mass. The contraction of the rock mass can cause induced seismicity.
For EGS, induced seismicity is caused by the combination of pressure effect and temperature effect. As of 2020, the magnitude of the largest earthquake caused by an EGS is M 5.5, which happened in the city of Pohang, South Korea in November 2017. So, you may be asking, “Is it really safe to construct EGS systems?” In my opinion, I think the induced seismicity from EGS is manageable. The reason for this statement is, first, the magnitude of the earthquake from EGS has been relatively low. For humans on the surface to feel vibrations, the magnitude of the earthquake has to be greater than 3.0. However, most of the earthquakes produced from EGS have been below 3.0, except for a few notable ones. Also, generally, the EGS plants are constructed distant from our habitats.
Figure 3: Largest Events at EGS Sites Worldwide
Figure 4: Earthquake frequency and destructive power
To prevent these unwanted outcomes, the reservoir properties should be well-defined, as the reservoir contains the greatest uncertainties regarding the design of the geothermal power plant.
The first process that can be taken prior to the construction of the plant is a literature review and data collection. This allows building reservoir models based on geological, structural, petrophysical, thermal and geophysical data. Preliminary seismic risk evaluation is completed with the available data at a relatively low cost. More data is obtained from field assessment. By using some geophysical exploration methods such as 2D/3D seismic method, gravimetry method, electrical/electromagnetic method, and by drilling shallow geothermal gradient boreholes, some of the reservoir properties are inferred. These properties include the geothermal gradient, existence, and location of faults and joints, rock type and etc.
Test drilling and well testing are conducted to further characterize the properties of the well and the geothermal reservoir. For well testing, fluid is injected into the ground to evaluate the quantity of water/gas/oil that can be extracted/injected from/to the reservoir. Tracer testing and well logging are completed with the purposes of determining the hydrodynamic properties of the reservoir and the properties of the well and the rock at or close to the well’s wall, respectively. Based on the collected data, it is the best practice to avoid constructing EGS plants near geologically high-risk areas and any human habitats.
The best way to manage seismic events after the operation of an EGS system is commenced is by continuous monitoring of seismic events. For instance, the Basel Deep Heat Mining Project, located in Basel, Switzerland, employs a real-time short-term earthquake prediction (STEP) concept. This STEP system allows estimating the probability of a future earthquake occurrence with an estimated magnitude within a time period of the order of several hours based on the statistical analysis with the data collected from the site. The goal of this system is to adjust injection parameters in real-time so that the damaging seismic events do not occur. This system is possible because the seismic data and injection parameters are well monitored and collected at the Basel operation since the start of the first injection.
I’ve written a few articles related to this issue before. For the readers who are curious to learn more about the induced seismicity, please refer to the articles below:
To be continued…
References
Academic sources:
Douglas, J., & Aochi, H. (2014). Using Estimated Risk to Develop Stimulation Strategies for Enhanced Geothermal Systems. Pure and Applied Geophysics,171(8), 1847–1858. doi:10.1007/s00024–013–0765–8
Majer, E. L., Baria, R., Stark, M., Oates, S., Bommer, J., Smith, B., & Asanuma, H. (2007). Induced seismicity associated with Enhanced Geothermal Systems. Geothermics,36(3), 185–222. doi:10.1016/j.geothermics.2007.03.003
Kraft, T., Mai, P. M., Wiemer, S., Deichmann, N., Ripperger, J., Kästli, P., . . . Giardini, D. (2009). Enhanced Geothermal Systems: Mitigating Risk in Urban Areas. Eos, Transactions American Geophysical Union,90(32), 273. doi:10.1029/2009eo320001
Bromley, C.J. & Mongillo, M.A. (February 2007), “All Geothermal Energy from Fractured Reservoirs — Dealing with Induced Seismicity” (PDF), IEA Open Journal, 48 (7): 5, archived from the original (PDF) on 2012–06–09, retrieved 2010–01–07
Atkins (2013). Deep Geothermal Review Study. Department of Energy & Climate Change (DECC). London, United Kingdom.
Web sources: http://www.seismo.ethz.ch/en/knowledge/things-to-know/geothermal-energy-earthquakes/geothermal-energy-and-induced-earthquakes/ Image sources
- https://earthquake.usgs.gov/research/induced/modeling.php
- https://eos.org/editors-vox/the-challenges-posed-by-induced-seismicity
- http://www.geologyin.com/2015/01/using-richter-scale-to-measure.html
- Bromley, C.J. & Mongillo, M.A. (February 2007), “All Geothermal Energy from Fractured Reservoirs — Dealing with Induced Seismicity” (PDF), IEA Open Journal, 48 (7): 5, archived from the original (PDF) on 2012–06–09, retrieved 2010–01–07
Originally published at https://www.linkedin.com.
