If you've been following our ongoing series on the 'Geomechanics of Superhot Rocks,' you're probably familiar with our previous articles that discussed the fundamental principles governing the behavior of superhot rocks as potential geothermal resources:
1. The extraordinary potential of superhot rocks arises from their fluid, which exists in a state referred to as 'supercritical fluid state.' The formation of these fluids depends on specific depths, pressures, and temperatures within superhot rock formations.
2. In order for these fluids to flow and be harnessed effectively, the permeability within these rock formations must exceed a practical threshold.
3. Superhot rocks, exposed to extreme temperatures and pressures, can undergo ductile deformation, leading to a reduction in their permeability. Consequently, understanding the mechanical properties of these rocks within the brittle-ductile zone (BDT) becomes of paramount importance.
In this article, we will explore the variation of permeability within the superhot rocks, all within the context of these three crucial conditions.
Permeability Variation Within the Earth's Crust
It's well estabilished that rock permeability tends to decrease with greater depth, primarily due to variations in porosity, fracture size, increased in-situ stresses, and the influence of mineral deposition and hydrothermal alterations. Ingebrigtsen and Manning (2010) have shed light on the patterns of crustal permeability changes. As shown in Figure 1, their suggested trends for permeability variation are grounded in the assumption of convective hydrothermal flow within the brittle crust. Nevertheless, these researchers contend that even in the presence of ductility beneath the Brittle-Ductile Transition (BDT) zone, rock permeability should not fall below a critical threshold (Figure 1). This threshold, ranging from 10^-16 to 10^-18 m², signifies the minimum permeability necessary to sustain convective heat transfer."
Figure 1. Changes in crustal permeability with increasing depth according to Ingebrigtsen and Manning (2010).
Adequate Permeability for Geothermal Exploitation
But what level of permeability qualifies as sufficiently high for economically harnessing superhot rocks? As Scott et al. (2015) pointed out, a permeability exceeding 10^-16 m² is a critical threshold for the potential feasibility of geothermal resource utilization. When we compare the anticipated permeability of rocks below the Brittle-Ductile Transition (BDT) with the minimum permeability necessary for geothermal exploitation, it becomes evident that a crucial aspect of assessing the feasibility of superhot geothermal projects is the examination of how ductility affects permeability. Consequently, it is imperative that we explore deeper into this matter.
Three Burning Questions
In summary, thus far, in this series, we have explored the concepts of supercritical fluids, the brittle-ductile transition, and variations in crustal permeability. Based on these foundational principles, the assessment of feasibility for any superhot geothermal project requires addressing the following important questions:
1. Does a supercritical fluid system exist?
2. Does the targeted rock possess a minimum economically viable permeability (e.g., 10^-16 m²)? Alternatively, can such permeability levels be achieved through stimulation?
3. Is the supercritical fluid system susceptible to alterations in permeability within the BDT zone?
Let's continue our discussion with addressing the potential answers to these questions.
Impact of Permeability on Supercritical Fluid Formation
Research conducted by Scott et al. (2015) indicated that rocks with high permeability (>10^-14 m²) are less inclined to yield substantial supercritical resources within superhot rocks (SHR) (See Figure 2b, 2d, and 2f). In such instances, the rate of convective water circulation is fast, preventing water from lingering in the transition zone for sufficient time to attain supercritical temperatures. The authors conclude that permeability values falling between these two extremes (i.e., 10^-14 m² and 10^-16 m²) are expected to yield more extensive supercritical resources suitable for exploitation (See Figures 2e and 2g). Recall that the lower limit corresponds to the highest permeability of BDT as proposed by Ingebrigtsen and Manning (2010), and it also represents the minimum permeability necessary for the economic utilization of these resources, as outlined by Scott et al. (2015).
Figure 2. Formation of supercritical water resources depends on geological controls. Potentially exploitable supercritical water regions are identified by red. TBDT is the rock’s BDT temperature and k0 is rock permeability (Source: Scott et al., 2015).
Overlap of Supercritical Fluid and BDT Zones
With a substantial decrease in rock permeability occurring within the brittle-ductile transition (BDT) zone, it becomes imperative to explore the potential overlap of this zone with the region where supercritical fluids are present. Assuming that the BDT zone's temperature is lower than 450°C, this could result in the formation of limited supercritical resources due to the diminished permeability, as shown in Figures 6c. Consequently, higher BDT temperatures are required to create exploitable superhot rock (SHR) resources, as demonstrated in Figures 2e and 2g.
All-in-one Superhot Graph
The findings discussed above are visually summarized in Figure 3. This graph maps out the prospects for harnessing supercritical resources by considering fluid pressure and temperature, the resource's depth in relation to the magmatic intrusion's upper boundary, and the temperature of the BDT zone. According to this graph, superhot resources are more likely to form in rocks with higher BDT temperatures such as basalt. Additionally, the graph within Figure 3 provides specific enthalpy values for these resources.
The yellow star * on this graph signifies the temperature and enthalpy data pertaining to the famous IDDP-1 well in Iceland, which was drilled into a superhot geothermal resource.
Figure 3. The thermal structure of high-enthalpy resources. The area of exploitable supercritical water is identified by red. The yellow star shows the temperature and specific enthalpy for the IDDP-1 well in Iceland (Source: Scott et al., 2015).
But What If ...
But what if the permeability at the Brittle-Ductile Transition (BDT) zone surpasses the estimates provided by Ingebrigtsen and Manning (i.e., 10^-16 m² or 10^-18 m²)? What if it can be enhanced through the stimulation of superhot rocks? The industry is actively deliberating the potential for higher-than-anticipated permeabilities, within the superhot rocks.
Stay tuned for our upcoming posts as we talk more about these possibilities.
References
Ingebritsen, S.E., Manning, C.E., 1999, Geological Implications of a Permeability-depth Curve for the Continental Crust, Geology, 27(1), 107–110.
Ingebritsen, S.E., Manning, C.E., 2010, Permeability of the Continental Crust: Dynamic Variations Inferred from Seismicity and Metamorphism, Geofluids, 10, 193–205.
Scott, S., Driensner, T., Weis, P., 2015, Geologic Controls on Supercritical Geothermal Resources above Magmatic Intrusions, Nature Communications, DOI: 10.1038/ncomms8837.