Welcome to the second post ofthe PetroGem's SHR series, where we learn more about the significance of fluid characteristics in unlocking the immense energy reserves within superhot rocks. Join me as we take a brief glimpse into the behavior of supercritical fluids, paving the way for a deeper understanding of the rock mechanics driving these extraordinary geothermal resources.
The Source of SHR’s Power
In the previous blog post, we embarked on an exploration of superhot rocks (SHR) and their tremendous potential as geothermal resources. The capacity of these rocks to generate energy is truly remarkable, exceeding that of conventional geothermal sources by a factor of ten or even more.
This exceptional power stems from their distinctive fluid content, existing in a state known as the 'supercritical fluid state'. Indeed, the remarkable value of superhot rocks as geothermal resources is intrinsically linked to their abundant supercritical fluid content. This connection is so significant that the industry widely adopts the term 'supercritical geothermal resources' to specifically refer to these rocks.
Transforming Fluids into Supercritical!
Supercritical fluid is an extraordinary state of matter, sometimes referred to as the "fourth state," separate from solids, liquids, and gases. While it shares some properties with vapors, it possesses unique characteristics that set it apart from traditional forms of matter. This special state occurs when a substance, such as water, is subjected to heating and pressurization beyond a critical point, causing it to lose its distinct liquid or gas behavior (Figure 1). In this supercritical state, the material becomes a single-phase fluid with exceptional energy content (Figure 1).
Figure 1. How does water become supercritical (source: renmatix.com)
SHR: The Ideal Environment for Supercritical Fluids
Supercritical water forms when the temperature and pressure exceed a critical point. For pure water, this critical point is at 374°C and 22.1 MPa, but it varies considerably for saline solutions (Figure 2). Superhot rocks buried deep in earth create the ideal conditions of high pressure and temperature necessary for water to exist in its supercritical state.
Super Energy of Supercritical Fluid
Supercritical fluids, like water, possess a high specific enthalpy, making them incredibly attractive as geothermal resources. An excellent example is the exploratory well IDDP-1 drilled by the Iceland Deep Drilling Project (IDDP), which accessed a reservoir with supercritical water at an impressive 450°C and an enthalpy of 3.2 MJKg-1. This reservoir has the capacity to generate 35 MW of electrical power from a single well, far surpassing the typical 3-5 MW output of conventional wells (Scott et al., 2015).
Figure 2. Supercritical state on temperature and pressure diagram for water. Critical points for pure water, saline with NaCl and water with dissolved CO2 are shown on the graph. (source: Tsuchiya, 2017).
The Crucial Role of Open Fractures
While enthalpy is a crucial factor, it alone is not sufficient for a supercritical rock system to become an economically exploitable SHR geothermal resource. The presence of a minimum permeability is equally vital. Permeability, largely influenced by the mechanical behavior of SHR, allows the efficient flow of fluids through the rock.
The permeability of SHR is greatly influenced by the fractures it contains. These rocks may naturally contain open fractures, or fractures can be induced through stimulation, creating pathways for the convection of hydrothermal fluids. The presence of conductive fractures heavily relies on the rock's mechanical response, as a certain level of brittleness is required for the formation and preservation of open fractures.
Moving Deeper: The Transition from Brittle to Ductile Rock
Ductile rocks exhibit the remarkable ability to withstand significant deformations without experiencing sudden failure. In these rocks, fractures tend to close due to plastic flow within the rock matrix. Consequently, ductile rocks possess less permeability and fewer open fractures when compared to brittle rocks with extensive open fracture networks.
Temperature and stress play a crucial role in determining rock brittleness, especially in the context of superhot rock (SHR) resources (Figure 3). The elevated confining pressure and temperatures within SHR can trigger a transformation from brittle to ductile mechanical behavior, directly influencing the presence and the conductivity of open fractures.
Figure 3. Rock Behavior Transition from Brittle to Ductile with Depth (source: instruct.uwo.ca)
Stay Tuned for the Next Adventure!
The captivating tale of temperature's profound influence on rock mechanical response awaits in our upcoming post. Join me for the next post as we venture deeper into the Earth, exploring the interesting world of rock behavior in superhot rock environments.
References
Scott, S., Driensner, T., Weis, P., 2015, Geologic Controls on Supercritical Geothermal Resources above Magmatic Intrusions, Nature Communications, DOI: 10.1038/ncomms8837.
Tsuchiya, N., 2017, Potential Candidates of Supercritical Geothermal Reservoir, GRC Transactions, Vol. 41.
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