In our ongoing series of articles on the 'Geomechanics of Superhot Rocks', we've been on a captivating journey into the depths of geothermal science, and today, we continue our exploration. In the previous post, we learned about the permeability variations within the Earth's crust, discussing prevailing theories that posited relatively low permeability in superhot rock conditions due to their ductile nature.
We also looked at modeling results that employed economic thresholds to identify potentially fruitful reservoirs, teasing the tantalizing prospect of extracting economic sources of superhot rocks hosting supercritical fluids. These analyses unveiled conditions for select resources with the ideal combination of permeability and supercritical fluid presence, making them economically viable targets.
But now, we start a new chapter in our quest for knowledge, one that dares to challenge existing assumptions. We turn our attention to the intriguing hypotheses suggesting that superhot rocks might possess higher permeabilities than we've believed, accompanied by a surprising brittleness factor that defies convention.
Why is Higher Permeability Crucial?
The concept of elevated permeability holds immense significance, particularly when we consider the context of Brittle Ductile Transition (BDT) rocks. Not only does it open doors to naturally permeable superhot rock resources, but it also hints at the potential for artificial enhancements through stimulation techniques. This revelation carries the promise of expanding our pool of exploitable Superhot Rock (SHR) resources, especially in the case of rock types like granite, which exhibit low BDT temperatures—aligning perfectly with the conditions conducive to supercritical fluid occurrences.
The Ripple Effect of Higher Permeability
So, what unfolds when permeability is pushed to new heights? Figure 1 presents a visual representation of potentially exploitable SHR geothermal resources, symbolized by the distinctive yellow triangles.
This graph charts the territory of opportunity, taking into account variables such as temperature, depth or effective confining stress, and, of course, permeability. Through this, we discern the boundaries of brittle, BDT, and ductile rock behaviors, painting a vivid picture of the geothermal landscape.
Additionally, we spot the locations of five wells strategically drilled within supercritical fluid systems, underscoring the practical implications of our understanding. It becomes abundantly clear that an augmentation of permeability, particularly within the BDT and sub-BDT domains, has the potential to unlock a treasure trove of new resources.
Figure 1. Potentially exploitable supercritical geothermal resources (yellow triangle) as a function of rock temperature, depth/confining pressure, and rock BDT (Source: Watanabe et al., 2017).
Mechanism: How is Higher Permeability Possible in Superhot Rocks?
Higher permeability in superhot rocks hinges on several factors. Firstly, it demands that natural fractures within the rocks remain open, allowing for the free flow of fluids. This behavior necessitates a certain degree of brittleness in the rock, as opposed to the ductile tendencies that tend to close these fractures. Alternatively, hydraulic fracturing can be employed to artificially enhance permeability, but even in this scenario, the rock's inherent ductility must be limited to ensure the fractures stay open.
Challenging the Status Quo: Possible Brittle Behavior of BDT Rocks
Contrary to the prevailing belief that BDT rocks are inherently devoid of open natural fractures and fracability potential, there have been arguments suggesting a latent brittle nature in these formations as discussed below:
Effect of Strain Rate
Rock behavior demonstrates a clear dependence on strain rate, with elevated rates of strain pushing it toward increased brittleness. This intriguing phenomenon suggests that even BDT and sub-BDT rocks could potentially display brittle characteristics, particularly in scenarios involving rapid strain, such as fault displacement or hydraulic fracturing. Scholz (2019) asserts that even within the BDT range and below, rocks may exhibit brittle behavior when subjected to exceedingly high strain rates, possibly associated with abrupt fault movements or hydraulic fracturing events.
Elevated Permeability Amidst Earthquakes
A related line of reasoning spotlights the presence of increased permeability within ductile crust during specific seismic events, as demonstrated by the 2011 Tohoku-Oki earthquake in northeastern Japan, , where an increased permeability of approximately ~10-15 m^2 was noted (Okada et al., 2015). Nevertheless, it's essential to recognize that this heightened permeability could be a temporary phenomenon, manifesting solely during the earthquake event and not necessarily a lasting characteristic, as it may be the case for hydraulic fracturing.
Difference Between Compaction and Expansion
An interesting phenomenon arises when we consider the differing responses of rock during compaction and expansion (Watanabe et al., 2017). While rocks may showcase ductile behavior under compressive stresses, they may paradoxically reveal brittle characteristics when subjected to tensile forces (Engvik et al., 2005). This suggests that the creation of shear fractures may seem less likely in ductile rock, but the potential for the formation of tensile fractures through stimulation remains.
Stay Tuned!
As we dive deeper into these intriguing possibilities, we continue to unravel the sizzling secrets concealed beneath the Earth's surface. Stay tuned for the next post in our series as we further investigate the potential for enhanced permeability induced by hydraulic fracturing in SHR and its far-reaching implications in the realm of geothermal energy resources.
References
Engvik, A. K., Bertram, A., Kalthoff, J. F., Stöckhert, B., Austrheim, H., & Elvevold, S. (2005). Magma-driven Hydraulic Fracturing and Infiltration of Fluids into the Damaged Host Rock, an example from Dronning Maud Land, Antarctica. Journal of Structural Geology, 27, 839–854.
Okada, T., Matsuzawa, T., Umino, N., Yoshida, K., Hasegawa, A., Takahashi, H., Yamada, T., Kosuga, M. Takeda, T., Kato, A., Igarashi, T., Obara, K., Sakai, S., Saiga, A., Iidaka, T., Iwasaki, T., Hirata, N., Tsumura, N., Yamanaka, Y., Terakawa, T., Nakamichi, H., Okuda, T., Horikawa, S., Katao, H., Miura, T., Kubo, A., Matsushima, T., Goto, K., & Miyamachi, H. (2015). Hypocenter Migration and Crustal Seismic Velocity Distribution Observed for the Inland Earthquake Swarms Induced by the 2011 Tohoku-Oki Earthquake in NE Japan: Implications for Crustal Fluid Distribution and Crustal Permeability. Geofluids, 15, 293–309.
Watanabe, N., Egawa, M., Sakaguchi, K., Ishibashi, T., & Tsuchiya, N. (2017). Hydraulic Fracturing and Permeability Enhancement in Granite from Subcritical/Brittle to Supercritical/Ductile Conditions. Geophysical Research Letters, May. DOI: 10.1002/2017GL073898.