In my last post, I talked about the tremendous energy stored within supercritical fluids trapped in superhot rocks, highlighting the crucial requirement for sufficient permeability to render Subsurface Hot Rocks (SHR) resources financially feasible.
This permeability is contingent upon the existence of either inherent natural fractures or fractures intentionally created by human intervention. Nonetheless, the presence or creation of both fracture types might face potential challenges due to the ductile mechanical response displayed by these rocks when exposed to elevated temperatures and high confining pressures.
In this article, my primary emphasis will be on discussing the impact of temperature, while the examination of confining pressure effects will be reserved for a future discussion.
Brittle Rocks in Conventional Geothermal Resources
Conventional geothermal reservoirs have long been associated with brittle behavior, allowing only limited deformation before abrupt failure. These rocks, prone to fractures, are manipulated through natural and induced fissures to create pathways for hydrothermal fluid convection. But what happens when temperatures rise, altering their fundamental nature?
A 'Glassic' Example
A classic example of a material transitioning from brittle to ductile behavior is glass. At lower temperatures, glass behaves in a relatively brittle manner. When subjected to stress, it tends to fracture with little deformation and without significant warning. However, when glass is heated to higher temperatures, it undergoes a transition to a more ductile behavior. This behavior is evident in processes like glassblowing, where glass is heated to a high temperature and then shaped into various forms through manipulation.
Figure 1. The brittle glass (Source: news.yale.edu)
Forging Ductility in the Inferno
As we venture closer to the Earth's core and encounter superheated rocks, similar to glass, extreme temperatures reshape the game. Once brittle, now ductile, these rocks exhibit remarkable endurance against deformation. As plastic flow becomes predominant, fractures gradually close, transforming the landscape of permeability. Consequently, these rocks exhibit reduced permeability in contrast to their brittle counterparts with open fracture networks.
While the topic of permeability in these rock formations will be thoroughly examined in an upcoming article, let's currently talk about the shift in their mechanical characteristics from being brittle to adopting a ductile behavior with a case study.
Heating Up Japansese Stones
The graphs presented in Figure 2 capture the experimental journey undertaken as part of the Japan Beyond Brittle Project (JBBP), where granite samples were tested across temperatures ranging from 600°C to 1000°C. These results unveil the rock's transition from brittle to ductile behavior, evident in distinct stress responses.
Lower temperatures induce abrupt stress drops, reflecting shear failure, while higher temperatures sustain ductile deformation with minimal stress fluctuations. See as lower temperature brittle samples shatter in shear failure, while higher temperatures prompt graceful deformation in ductile samples.
Figure 2. Experimental results of brittle to ductile deformation for granite (Source: Acosta et al., 2021).
Threshold for Brittle-Ductile Transition
What temperature triggers the magic? The temperatures needed to induce a brittle-ductile transition vary among different lithologies, spanning from approximately 360°C for silicic rocks to 800°C for non-glassy basaltic rocks (Scott et al., 2015).
Brittle-Ductile Transition (BDT) Zone
The region where the mechanical properties of rocks shift from brittle to ductile behavior is known as the Zone of Brittle-Ductile Transition (BDT). This transitional area is influenced by the geothermal gradient and is not confined to a specific depth, but rather spans a range of depths due to factors such as variations in lithology, the strain-dependent response of the rock, and the magnitude of in-situ stress.
While under low geothermal gradients, the extent of this zone might extend beyond 10 km, it can reduce to as shallow as 3 km when the geothermal gradient is higher (Tsuchiya, 2017).
A Real-world Case
Figure 3 shows the evolution from brittle to ductile behavior along the WD-1a well within the Japan's Kakkonda geothermal field. The remarkably elevated temperature gradient, a common feature in Japan, combined with the lower BDT temperature requisite for the rock, leads to the Brittle-Ductile Transition manifesting at exceptionally shallow depths in this location. This figure demonstrates the shift from brittle rock to fully ductile/plastic rock occurring across a span of over one kilometer.
Such a metamorphosis in mechanical traits brings about significant alterations in rock attributes, such as deformation and failure, the presence and aperture of natural fractures, rock permeability, fracability, stress anisotropy, and seismic behavior, as will be elaborated in the coming posts.
Figure 3. Variations of (a) temperature and (b) total horizontal stress by depth along the WD-1a well at the Kakkonda geothermal field, Japan. (Source: Suzuki et al., 2014)
Stay Tuned for More!
If you are a geomechanics enthusiast who finds such subjects intriguing, keep an eye out for the upcoming post. I'll be revisiting the topic of permeability, exploring it through the lens of BDT as we've discussed in this article."
References
Acosta, M., Gibert, B., Violay, M., 2021, Evolution from Brittle to Ductile Deformation in the Continental Crust: Mechanics of Crystalline Reservoirs and Implications for Hydrothermal Circulation, Proceedings of the World Geothermal Congress 2020+1, Reykjavik, Iceland, April - October.
Scott, S., Driensner, T., Weis, P., 2015, Geological Influences on Supercritical Geothermal Resources above Magmatic Intrusions, Nature Communications, DOI: 10.1038/ncomms8837.
Suzuki, Y., Ioka, S., Muraoka, H., 2014, Determining the Upper Limit of Hydrothermal Circulation Using Geothermal Mapping and Seismic Activity to Define the Depth to the Brittle-Plastic Transition in Northern Honshu, Japan, Energies, 7, 3503-3511; doi:10.3390/en7053503.
Tsuchiya, N., 2017, Promising Prospects for Supercritical Geothermal Reservoirs, GRC Transactions, Vol. 41.