Modelling a Novel Scheme of Mining Geothermal Energy from Hot Dry Rocks

Abstract

On the basis of a conceptual model for an Excavation based Enhanced Geothermal System (EGS-E), which proposed to extract heat from Hot Dry Rock at depth through dominantly adopting shaft, roadways, and caved rock failure techniques but not depending on either wellbore drilling or fracturing stimulation, a novel extensive version of heat extraction is proposed in this paper. Considering its mechanical stability issues, the new scheme contains two fields apart away: the ones are near-field by piping flow to touch the tunnel wall; the others are far-field through filling and driving fluid within the voids of collapsed rock. The former is represented as a tunnel unit being installed hollow linear, which can extract and produce heat precisely according to structural design and accurate operative prediction. The latter is represented as interconnective fissures being induced by stope excavation due to gravitational weight and unloading of a deep-buried squeeze. The EGS-E uses a two-stage heat exchange system of “fluid-rock” and “fluid-fluid.” Then, a 3D transient thermal-hydraulic model is established to demonstrate the heat extraction performance. The temperature field and accumulated heat energy are investigated. The modeling work provides a tentative workflow to simulate an EGS-E system and, most probably for the first time, demonstrated that the deep underground Hot Dry Rock heat mining turns out to be preliminarily studied in a quantitative way.

1. Introduction

The utilization of fossil energy sustains the rapid development of the world economy but also brings about an increasingly serious world energy crisis and environmental pollution [1,2]. Therefore, searching for renewable and clean energy that can replace fossil one has become an important task for governments and research institutions.

Renewable energy sources mainly include wind, solar, hydraulic power, and geothermal energy. As for the aforementioned renewable energy sources, their reserves are huge. However, the large-scale application of solar energy is limited by natural conditions such as diurnal, latitude, and altitude. And wind energy is heavily dependent on climatic conditions, where unstable wind speed will result in a discontinuous generation of power. Additionally, the development of hydraulic power is restricted by geographical and seasonal conditions and is not suitable for regions with water shortages. Due to the independence of seasonal, climatic, and diurnal as well as other factors, geothermal energy turns out to be a stable and uninterrupted energy supplies [3]. Considering its many advantages, geothermal energy has received widespread attention at home and abroad [4]. According to their depth in the earth’s strata and production conditions, geothermal resources can be divided into three categories: shallow geothermal energy, hydrothermal resource, and hot, dry rocks (HDR) [5]. In particular, due to its huge reserves, HDR is expected to play a key role in solving the aforementioned societal problems [6]. According to conservative estimation, in China, the amount of geothermal energy reserved inside HDR reaches over 2.09 × 10⁷ EJ, which is about 100,000 times more than other categories [7]. Consequently, how to achieve large-scale exploitation with regards to HDR is becoming a prospective but promising direction among the studies of geothermal developments.

HDR is characterized by its extremely low natural permeability, so developing HDR requires establishing Enhanced Geothermal System (EGS) [8]. In 1970, the concept of EGS was first introduced by Los Alamos National Laboratory. It typically consists of injection wells, production wells, and heat reservoirs created by artificial enhancement techniques [9,10]. Generally, hydraulic fracturing techniques are used to create fracture networks to connect these wells and make more heat transfer areas. However, hydraulic fracturing is still immature, and its many inherent flaws are seriously bottlenecking heat energy reserved inside HDR from being extracted at a large industrial scale. It is so far improbable to credibly control fracture extension and precisely predict the structural distribution of reservoir morphology. This results in the reservoir volumes being limited up to pilot testing scales, and the field designs are tentative [11,12]. Although project Fenton Hill had achieved satisfactory fracturing outcomes, its reservoir volume was only 0.035 km³ as estimated, which is far from the required 0.2 km^3, which is the minimum for commercial development [13]. Furthermore, in order to improve the stimulating effects of fracturing, the fracturing fluids to be adopted usually contain chemicals that can seriously contaminate groundwater environments [14]. In addition, fracturing activity disturbs the crude balance existing among in-situ stresses, which might trigger an accidental earthquake. In Basel, Switzerland, 3500 micro seismic events occurred between 2006 and 2007 as a result of hydraulic fracturing activity [15]. In 2007, an earthquake of magnitude 5.4, the highest magnitude estimated to be related to EGS, struck the northern part of Pohang city. Then the local project of geothermal power generation was suspended, causing a direct economic loss of more than USD $47 M [16]. Similar lessons have told many times that stability and safety need pre-requisitely guaranteed. Enough experiences from field testing have implied that, as much as possible precise controllability is essentially demanded during implementing or operating heat extraction from HDR.

Closed-loop geothermal systems (CLGSs) also become an essential approach in developing geothermal energy from deep, including dry rocks. Numerous closed-loop configurations have been proposed, mainly including coaxial closed-loop geothermal systems and U-tube systems [17,18]. During the heat extraction process, fluid circulation is looping convection inside the borehole closure, transporting heat exchanged from the wellbore inward wall by its conduction radially through the external wall from the contacted surrounding rocks. Since the flow rate of injection fluid inside closure tubes has been facilitated for flexible adjustment, the two-level heat exchange not only relies on the formation of original permeability and stimulation effect of hydraulic fracturing. Hence, the controllability during heat production is substantially improved than that during conventional EGS with regard to HDR. As well, instability hazards such as induced earthquakes are effectively avoided. In 2019, near Sylvan Lake, Alberta, Canada, Eavor established a full-scale demonstration project by adopting CLGSs. With a bottom-hole temperature of 78 °C and well depth of approximately 2400 m, about 800 kw/h was achieved as thermal output [19]. However, as CLGSs collect heat energy mainly along radial heat conduction, a crucial challenge becomes how to revise the minute heat conductivity of surrounding rocks and provide sufficient contacting areas of the heat exchange interface between fluid and solid [20,21]. In conclusion, it is hardly expectable to satisfy the scale of power generation by exploiting HDR only by adopting either conventional EGS or cutting-edged CLGSs. Therefore, in terms of fundamental schemes for the sake of large-scale deep geothermal exploitation, an innovative breakthrough looks urgently needed.

Aiming at a solution to the bottleneck existing in EGS and CLGSs, a novel scheme is proposed in this paper turning geothermal energy extraction using dominant techniques from petroleum over to mineral mining, i.e., an extensive study on Excavation based Enhanced Geothermal System (EGS-E), to contribute to realizing extremely large-scale heat production from HDR. In addition, to introduce the essence inside this EGS-E, the representative heat exchange mechanism with regard to a core engineering unit is analyzed through numerical simulation. In addition, its intrinsic advantages, as well as scientific and technical challenges being brought about, are discussed.

2. Progress in Enhanced Geothermal System Based on Excavating

According to the shortcomings with regards to the conventional EGS, the prospective features should be recognized to implement HDR geothermal energy extracting system, i.e., requiring not only on a large-scale but also with small randomness. For almost all technological realization of geothermal energy development, researchers are mainly concerned about two aspects: creating geothermal reservoirs; to achieve efficient heat production, which is closely related to the effectiveness of heat extraction systems. Under the traditional EGS with regards to HDR geothermal energy extraction, borehole drilling, and fracturing stimulation were used to artificially create heat reservoirs. In addition, a direct “fluid-rock” heat exchange system was adopted in traditional EGS and CLGSs.

However, having fundamentally moved away from the reliance on these two major oil and gas industry-associated techniques, under the scheme of EGS-E, mining technology such as shaft and tunnel systems is adopted as well as other structures including so-called heat reservoirs being created inside intact Hot Dry Rocks. Moreover, in contrast, a two-stage heat exchange system of “fluid-rock” and “fluid-fluid” will be adopted in EGS-E. The working fluid that is injected from the surface circulates driven by external power in the basic units. During the primary “fluid-rock” heat exchange process, the working fluid, which belongs to the first level of heat exchange, extracts heat through contact with the boundary surfaces of tunnel walls, blasted blocks, or collapse fissures. As for the secondary “fluid-fluid” heat exchange process, the heat contained and brought in the first level working fluid will be collected by the second level working fluid inside closed looping spiral pipes, which are installed inside along the corresponding tunnels.

2.1. Physical Components of EGS-E

The structure of this extensive EGS-E scheme consists of a shaft, tunnel, lined piping system, a powered drive system, and other supporting accessories, as shown in Figure 1, Figure 2 and Figure 3.

In the above diagrams:

1—Shaft; 2—Connecting tunnel; 3—Maintenance tunnel; 4—Heat exchange tunnel; 5—Near-field tunnel being lined for the secondary stage of heat extraction; 6—Pre-introducing (to fail together for primary stage of heat exchange) and post-connecting (to be retained for facilitating heat transport) sections of far-field roadway; 7—Annulus formed due to liner installation; 8—Caved/collapsed area; 9—Gate of Maintenance tunnel; 10—Gate of heat extraction tunnel and heat exchange tunnel; 11—1# injection pipe; 12—pump; 13—2# injection pipe; 14—Production pipe; 15—Heat exchange pipe;16—Lifting equipment. The dashed arrow represents the flow direction of the 2# working fluid, and the solid arrow represents the flow direction of the 1# working fluid.

At least one shaft with a large diameter is constructed from the surface to the targeted depth by excavating implementation. It will perform the main access from or existing to the ground. Various functional facilities are installed inside in a separate and safety-guaranteed way. The hoisting, ventilating, conveying, and communicating devices are partitioned there and protected well.

The network of roadways consists of multiple tunnels for connecting, heat exchange and extraction, local maintenance, etc. The connecting tunnels play as the main passages which represent the frameworks of development corresponding to horizons. The tunnels of the EGS-E function as a framework for the horizontal stratification of the geothermal reservoirs. Multiple layers of connection tunnels can be arranged longitudinally along the shaft as required. The heat extraction tunnels are the core structure of the first-stage heat exchange of “fluid-rock,” which forms the near-field heat extraction zone and the far-field heat extraction zone. A near-field heat extraction zone consists of rigorously regular tunnels constructed by boring machines as well as installed pipelines, whose operation will be precisely predictable and controllable. Notably, during heat extraction, a low-temperature zone is formed in the center of the tunnel, which reduces the average production temperature, and then affects the heat exchange efficiency. Therefore, an annular structure is introduced to reduce the influence of this phenomenon, as shown in Figure 4. Inner concrete tubes are installed in the tunnel, thus creating an annulus between the tunnel walls and inner tubes. The concrete inner pipe is fixed in the tunnel through the support. The near-field heat extraction zone can provide a stable, predictable, and high-quality heat supply to the system. As for the far-field heat extraction zone, blasting or caving is carried out in an orderly manner, causing the rocks around the tunnel to collapse and break, forming a rough but large-volume compensatory reservoir. The far-field heat extraction zone provides sufficient heat for the system to compensate for the small amount of heat in the near-field heat extraction zone. The near-field and far-field heat extraction zone should be far apart from each other. Where there are multiple tunnel system levels in geothermal reservoirs, the locations of the heat extraction tunnels at the upper and lower levels are staggered to avoid mutual interference.

The function of the heat exchange tunnels is to gather the high-temperature fluid flowing out of the heat extraction zones and provide a place for the second-stage heat exchange of “fluid-fluid.” The maintenance tunnels connect the connection tunnels with the heat extraction and heat exchange tunnels to facilitate access for maintenance. The entrance gates of tunnels are provided with multiple holes for various types of pipes to pass through, facilitating connections to the outside. The piping system refers to all kinds of pipelines located in the shaft, connection tunnels, and heat exchange tunnels, mainly including 1# injection pipeline, 2# injection pipeline, collection pipeline, and heat exchange pipeline. The drive system mainly includes fluid pumps and other equipment, and its function is to promote the working fluid to circulate in the heat extraction zone and heat exchange tunnels.

According to needs, the basic unit geothermal energy extraction can be arranged around the shaft and the connecting tunnel in any combination to form a larger-scale geothermal reservoir.

2.2. Physical Components of EGS-E

The system operation process reflects the heat exchange process. The extraction of deep geothermal energy is accomplished through a two-stage heat exchange: “fluid-rock” and “fluid-fluid.”

For the first-stage heat exchange process, the 1# working fluid is transported through the 1# injection pipeline to heat extraction zones and heat exchange tunnels. When the 1# working fluid fills the above spaces, then the 1# working fluid injection is stopped. The drive devices outside the gates are opened to drive the 1# working fluid to start circulating in the above spaces and harvest heat from the hot tunnel walls and gravel. After full heat exchange, the 1# working fluid converges into the heat exchange tunnels, thus completing the first-stage heat exchange.

For the second-stage heat exchange process, the 2# working fluid is injected into the spiral heat exchange pipes through the 2# injection pipeline, and the 2# working fluid in the spiral heat exchange pipe exchanges heat with the 1# working fluid filled in the heat exchange tunnels and vaporizes. Due to the thermosiphon effect, the vaporized 2# working fluid floats to the ground through the collection pipe under the action of thermal buoyancy and enters the heat utilization equipment. After utilization is completed, the 2# working fluid is cooled and liquefied and flows into the spiral heat exchange pipe through the 2# injection pipe again under the action of gravity to perform self-circulation and reciprocate in this way to realize the second-stage heat exchange.

In summary, during the geothermal energy extraction process of EGS-E, the heat is first transferred to the 1# working fluid through the hot tunnel walls and gravel. Secondly, the 1# working fluid acts as a carrier medium to collect heat to the heat exchange tunnels. Finally, the heat is transferred to the 2# working fluid in the spiral heat exchange pipe and then to the utilization equipment located on the ground. The flowchart of heat transfer in EGS-E is shown in Figure 5.

3. Numerical Simulations

A novel EGS-E model has been proposed to achieve HDR geothermal development, and it is necessary to analyze the representative mechanism of heat extraction and the technical performance of the system. Through the layer-by-layer analysis of the flow and heat transfer processes, it is found that the single zone of near- and far-field heat extraction, as the basic and core heat extraction structure, should be selected as the study subject. Then, a representative and applicable 3D thermal-hydraulic transient process model is established.

3.1. Computational Models

Figure 6 and Figure 7 display the schematic of the computational models according to the design. The near-field heat extraction zone is composed of surrounding rock and a tunnel with an annular structure located at the center of it. The cold working fluid flows in the annulus of the tunnel, and it absorbs geothermal energy from the surrounding rock. In order to efficiently represent the long tunnels, the 1D non-isothermal pipe flow model was adopted [22,23]. The accuracy and efficiency of the 1D model for long tunnels were verified by an experiment in a longitudinally ventilated high-temperature tunnel [24,25]. The computational model of the near-field heat extraction zone has a radius of 200 m and a length of 2000 m.

The model of the far-field heat extraction zone consists of a stimulated reservoir volume (SRV) and its further surrounding rock. Notably, the modified HDR is known as SRV, which has a high porosity and permeability [26,27]. The SRV is created by blasting or caving instead of hydraulic fracturing. The SRV is simplified as a cylinder with a length of 2000 m and a diameter of 60 m, located in the center of the surrounding rock. The computational models are sufficiently large to reduce the boundary effects during the geothermal energy extraction, which are demonstrated in subsequent studies. Moreover, other detailed parameters of the two computational models are shown in

Table 1. Input parameters of the models.

Items	Value	Items	Value
Density of rock(kg/m3)	2700	Inner radius of annulus(m)	2
Thermal capacity of rock(J/(kg·°C))	1000	Outer radius of annulus(m)	3
Thermal conductivity of rock(W/(m·°C))	3.5	Outer radius of tunnel (m)	3
Porosity w.r.t. SRV	0.23	Radius of shattered rock zone	30
Permeability w.r.t. SRV (m2)	10−10	Outer radius of models(m)	200
Length of models(m)	2000

.

3.2. Mathematical Model

3.2.1. Model Assumption

It is necessary to introduce some appropriate assumptions to improve the efficiency of numerical computation. The assumptions are as follows [28,29,30,31,32]:

• The properties of the intact rock are assumed to be isotropic, homogeneous, and constant.
• Since the specific flow process is not concerned within the SRV where a porous medium with homogeneous and isotropic properties is regarded as equivalent.
• The chemical and mechanical interactions between fluid and rock are ignored.
• Fluid flow is laminar within porous media and obeys Darcy’s law.
• Local thermal equilibrium is maintained between rock and fluid.
• Water is adopted as the working fluid.

The properties changing with temperature with regard to water follow the following formulas [33]:

$$\mu =\{\begin{array}{l}1.379-0.021\cdot T+1.360\cdot {10}^{-4}\cdot T^2-4.645\cdot {10}^{-7}\cdot T^3+8.904\cdot {10}^{-10}\cdot T^4-9.079\cdot {10}^{-13}\cdot T^5+3.845\cdot {10}^{-16}\cdot T^{6}273.15 \mathrm{K}\le T\le 413.15 \mathrm{K}\\ \\ 0.004-2.107\cdot {10}^{-5}\cdot T+3.857\cdot {10}^{-8}\cdot T^2-2.397\cdot {10}^{-11}\cdot T^{3}413.15 \mathrm{K}\le T\le 553.75 \mathrm{K}\end{array}$$

(1)

$$c_p=12010.147-80.407\cdot T+0.309\cdot T^2-5.381\cdot {10}^{-4}\cdot T^3+3.625\cdot {10}^{-7}\cdot T^{4}273.15 \mathrm{K}\le T\le 553.75 \mathrm{K}$$

(2)

$$\rho =\{\begin{array}{l}1000\cdot \left(1-\frac{{\left(T-277.13\right)}^2}{503570}\cdot \frac{T+9.85}{T-205.89}\right)273.15 \mathrm{K}\le T\le 293.15 \mathrm{K}\\ 996.9\cdot \left(1-3.17\cdot {10}^{-4}\cdot \left(T-298.15\right)-2.56\cdot {10}^{-6}\cdot {\left(T-298.15\right)}^2\right)\hspace{1em} 293.15 \mathrm{K}\le T\le 523.15 \mathrm{K}\end{array}$$

(3)

where μ is viscosity, c_p is the isobaric heat capacity, ρ is density, and T is Kelvin temperature.

3.2.2. Governing Equations

(1)

Water flow and heat transfer in the far-field heat extraction zone

For the far-field heat extraction zone, the water flow in the porous medium follows Darcy’s law, and the mass conservation equation can be expressed as [34]:

$$\frac{\partial \left({\rho }_{wf}\phi \right)}{\partial t}+\nabla ·\left({\rho }_{wf}{u}_f\right)=0$$

(4)

where ρ_wf is the density of water and φ is the porosity of the thermal reservoir. The subscripts w and f denote the working fluid as water and the far-field heat extraction zone, respectively. t is the time. u_f is Darcy velocity, which can be expressed as:

$$u_f=-\frac{k}{{\mu }_f}\nabla p$$

(5)

where k is the permeability, p is the pressure, μ_f is the viscosity of water.

The heat transfer process in porous media is described by the following energy conservation equation [35],

$${\left(\rho c\right)}_{eff}\frac{\partial T_{f2}}{\partial t}+{\rho }_{wf}{c}_{p,wf}{u}_f·\nabla T_{f2}-\nabla ·\left({\lambda }_{eff}\nabla T_{f2}\right)=0$$

(6)

where T_f2 is the fluid temperature, c_p,wf is the heat capacity of water, (ρc)_eff and ${\lambda }_{eff}$ is the equivalent heat capacity and thermal conductivity, respectively,

$${(\rho c)}_{eff}=\phi {\rho }_{wf}{c}_{p,wf}+(1-\phi ){\rho }_r{c}_r$$

(7)

$${\lambda }_{eff}=\phi {\lambda }_{wf}+(1-\phi ){\lambda }_r$$

(8)

where ρ_r is the density of rock, and the subscript r stands for rock. c_r is the heat capacity of the rock, ${\lambda }_{wf}$ is the thermal conductivity of water, and ${\lambda }_r$ is the thermal conductivity of the rock.

The heat transfer process in the surrounding rock is mainly heat conduction, which can be expressed as,

$${\rho }_r{c}_r\frac{\partial T_{f1}}{\partial t}=\nabla ·\left({\lambda }_r\nabla T_{f1}\right)$$

(9)

where T_f1 is the temperature of the surrounding rock.

Due to the coupling heat transfer between the surrounding rock and the porous medium, the conditions of equal interface temperature and conservation of heat flow should be satisfied,

$$T_{f1}=T_{f2}$$

(10)

$$-{\lambda }_r\nabla T_{f1}=-{\lambda }_{eff}\nabla T_{f2}$$

(11)

(2)

Water flow and heat transfer in the near-field heat extraction zone

For the near-field heat extraction zone, the water flow and heat transfer process in the tunnel are described by a 1D non-isothermal pipeline model. The momentum and mass equations can be expressed as [36],

$$\frac{\partial \left(A{\rho }_{wn}\right)}{\partial t}+\nabla ·\left(A{\rho }_{wn}{u}_n\right)=0$$

(12)

$${\rho }_{wn}\frac{\partial \left(u_n\right)}{\partial t}=-\nabla p-\frac{1}{2}{f}_D\frac{{\rho }_{wn}}{d_p}\left|u_n\right|u_n$$

(13)

where ρ_wn is the water density, and the subscript n denotes the near-field heat extraction zone. A is the cross-sectional area, u_n is the flow rate, p is the pressure, d_p is the hydraulic diameter, given by d_p = 4A/Z, Z is the wetted perimeter, f_D is the friction factor, calculated using the following model [37],

$$f_D=8{\left[\left(\frac{8}{Re}\right)+{\left(k_a+k_b\right)}^{-1.5}\right]}^{1/12}$$

(14)

$$k_a={\left[-2.457\ln \left({\left(\frac{7}{Re}\right)}^{0.9}+0.27\left(\frac{e}{d_p}\right)\right)\right]}^{16}$$

(15)

$$k_b={\left(\frac{37530}{Re}\right)}^{16}$$

(16)

where Re is the Reynolds number and e is the tunnel surface roughness.

The heat transfer equation of the fluid in the tunnel is [38],

$${\rho }_{wn}A{c}_{p,wn}\frac{\partial T_{n2}}{\partial t}+{\rho }_{wn}A{c}_{p,wn}{u}_n·\nabla T_{f2}-\nabla ·\left(A{\lambda }_{wn}\nabla T_{f2}\right)=\frac{1}{2}{f}_D\frac{{\rho }_{wn}A}{d_p}\left|u_n\right|u_n^2+Q_{\mathrm{wall},\mathrm{n}}$$

(17)

where c_p,wn is the heat capacity of the water, T_n2 is the temperature of the water, and Q_wall,n is the convective heat transfer between the rock wall and the water.

The formula for Q_wall,n is,

$$Q_{\mathrm{wall},\mathrm{n}}=hZ\left(T_{n1}-T_{n2}\right)$$

(18)

where T_n1 is the temperature of the rock, h is the heat transfer coefficient,

$$h=Nu\frac{{\lambda }_{wn}}{d_{\mathrm{h}}}$$

(19)

where Nu is the Nusselt number, it can be determined by,

$$Nu=\frac{\left(f_D/8\right)\cdot (\mathrm{Re}-1000)\cdot \mathrm{Pr}}{1+12.7\sqrt{f_D/8}\left({\mathrm{Pr}}^{2/3}-1\right)}$$

(20)

The energy conservation equation in the surrounding rock is,

$${\rho }_r{c}_r\frac{\partial T_{n1}}{\partial t}=\nabla ·\left({\lambda }_r\nabla T_{n1}\right)$$

(21)

3.2.3. Initial and Boundary Conditions

For the near- and far-field heat extraction zone, the initial temperature of the rock is 200 °C. The initial fluid temperature in the annulus of the tunnel and SRV is the same as the initial temperature of the surrounding rock. As both models are large enough, the outer surfaces of the models are set to thermally insulated conditions. The temperature and flow rate of water injected into the SRV were set to 30 °C and 500 m³/h, respectively. Additionally, water was injected into the annulus at a temperature and flow rate of 30 °C and 50 m³/h, respectively. The production time was set as 20 years.

3.2.4. Model Solution

The commercial software COMSOL is used to conduct the finite element discretization of the models and to solve the above partial differential equations. Figure 6 and Figure 7 show the meshing scheme for the models of near- and far-field heat extraction zone. Triangular elements are generated on the sides of the cylinder, and then the meshes are swept along the axis of the cylinder to the opposite face to generate triangular prismatic elements. In order to improve the efficiency of the simulation, the meshes of the SRV and the area near the tunnels were refined. Furthermore, edge meshes are generated for 1D tunnels based on the sweep meshes distribution characteristics of the 3D surrounding rock. After discretizing the models with COMSOL, the number of finite elements mesh for the near- and far-field heat extraction zone model is 80,300 and 150,200, respectively.

3.2.5. Definitions of Characteristic Parameters

For the convenience of analysis, the production temperature and accumulated heat energy were used to characterize the heat extraction performance of EGS-E. The temperature at the end of the tunnel and the SRV can be considered as the production temperature of the working fluid [39]. The accumulated thermal energy refers to the cumulative amount of heat extracted by the working fluid during operation [40], which is calculated as:

$$Q={\displaystyle {\int }_0^t(q_{\mathrm{out} }{\rho }_{w,}{}_{\mathrm{out} }{c}_{p,w,\mathrm{out} }{T}_{\mathrm{out} }-q_{\mathrm{in} }{\rho }_{w,\mathrm{in} }{c}_{p,w,\mathrm{in} }{T}_{in})}dt$$

(22)

where Q represents the accumulative thermal energy, q is the flow rate, and T is the production temperature. The subscripts in and out represent the inlet and outlet.

3.3. Results and Analysis

3.3.1. Analysis of Temperature Field

Figure 8 indicates that there are obvious low-temperature zones around the tunnel, and with the increase in time, the low-temperature zones are gradually expanding. This demonstrates that the heat exchange between water and the surrounding rock primarily occurs in the radial direction and that the heat conduction rate in the surrounding rock is lower than the heat exchange rate between the water and surrounding rock. The temperature distribution of the working fluid in SRV is shown in Figure 9. With the increase in production time, the cold front advances continuously along the direction of fluid injection. The low-temperature zone gradually increases and shows a clear temperature gradient along the radial direction. At the same time, the influence range of the cold front is the largest at the injection point and decreases along the injection direction.

The thermal influence area radius is used to determine the influence scope and provide guidance for the spacing design of the structure. This value refers to the radial distance when the temperature closes to the initial temperature in the surrounding rock. In this paper, line ab (see Figure 8 and Figure 9) is selected to investigate the temperature distribution along the radial direction and to determine the maximum influence radius. Figure 10 illustrates that the thermal influence area radius increases with time, while the increase rate of the thermal influence area radius decreases with the production time. For the near-field model, the thermal influence area radius increased by 82.12 m from year 0.1 to year 5, but only increased by 15.42 m from year 15 to year 20. Meanwhile, for the far-field model, the value increases by 60.79 m in the first 4.9 years but only by 23.82 m in the last 5 years.

The maximum thermal influence areas that the near- and far-field models determined are 156.60 m and 172.92 m in radius, respectively. This indicates that a distance of at least 172.92 m should be maintained between the near-field and far-field zones to avoid thermal interference, which also shows that the size of the computational model is sufficient.

3.3.2. Analysis of Heat Extraction Performance

Figure 11 shows that the production temperature of the near-field zone decreased rapidly in the first 0.2 years due to the obvious change in the rock temperature around the tunnel. The production temperature curve stabilizes after 5 years, with a temperature drop of only 3.5 °C during this stabilization period. In addition, this figure indicates that the cumulative thermal energy presents a linear growth trend. At the same time, the share of thermal energy extracted in the stable region is approximately 67.5%.

For the far-field zone, the temperature curve can be divided into three phases: stable region (0–0.8 years), decline region (0.8–3 years), and stable region (3–20 years). This phenomenon exhibits both the temperature variation characteristic of EGS and CLGSs. The production temperature of EGS often remains constant during the initial stage, while that of CLGSs often shows a sharp drop in this stage [41]. Due to the high production temperature in the first stage, thermal energy accumulates rapidly, accounting for about 39.5% of the total. Along with extracting time going by, both efficiency and production temperature attenuate mostly after running for 0.8 years because the scale of the high-permeability zone is limited in this model.

As shown in Figure 12, the cumulative heat recovery of the far-field zone is about 8.3 times that of the near-field zone, which can make up for the small heat recovery of the near-field zone, and then this system can provide sufficient energy for utilization. Although the heat extraction capacity of the near-field zone is relatively low, its operation mode is controllable, and its performance parameters are highly predictable. Meanwhile, despite the poor controllability of the far-field zone, it has large heat production, simple construction, and low investment. Consequently, the EGS-E combines the characteristics of both in an organic way, allowing for scalable and controllable geothermal energy extraction.

4. Superiority and Challenges

This study proposed a new geothermal extraction method of EGS-E and investigated the performance of its core heat extraction structure over a 20-year operating cycle using numerical simulation. In summary, the advantages of the proposed system are as follows.

The volume of the geothermal reservoir is customizable. The volume of geothermal reservoirs of EGS is affected by hydraulic fracturing, making it difficult to form geothermal reservoirs in scale and difficult to accurately control the volume of geothermal reservoirs. The EGS-E uses a tunnel system to delineate the geothermal reservoir. Adjusting the structural parameters of the tunnel system, such as length, diameter, quantity, etc., to establish the different volumes of geothermal reservoirs more precisely to meet different requirements.

The heat extraction rate can be adjusted. The circulation rate of the working fluid plays an important role in the rate of heat extraction. EGS cannot even guarantee a stable output flow rate, so the controllable circulation speed is more difficult to achieve, and thus it is impossible to realize the adjustment of the heat extraction rate. For EGS-E, the heat extraction rate can be adjusted in a controlled manner according to changes in external demand. The heat extraction rate is regulated by adjusting the driving device to control the circulation rate of 1# working fluid or by adjusting the 2# working fluid injection rate.

Low energy is consumed to operate the system. For EGS, the working fluid must overcome huge resistance when flowing in hydraulic fracturing fractures and HDR with low permeability. In contrast, for EGS-E, 1# working fluid flows in the tunnels and the large fractures formed by gravel accumulation with minimal flow resistance, thus significantly reducing frictional energy consumption in operation.

Good safety and environmental friendliness can be manifested. The high safety of EGS-E is reflected in the high level of its own safety and the low risk to the outside environment. In terms of its own safety, considering the convenience of tunnel construction, tunnel support methods such as wet shotcrete support, bolt support, etc., can be used to reinforce tunnels and shafts in the EGS-E structure. In terms of low risk to the outside environment, EGS-E does not induce earthquakes as it does not require hydraulic fracturing. The mining industry has triggered small micro-earthquake events during its development, but large-scale earthquakes have never been reported. The tunnel walls are sealed by a low permeability coating so that working fluids do not leak and therefore do not pollute the groundwater.

As with other new technologies, EGS-E also faces many challenges. In terms of technical feasibility, how to implement it under the current technology is a problem, such as excavating tunnels in high temperatures and ultra-deep rock mass and maintaining their long-term stability. The Mponig gold mine reaches a depth of over 4350 m, which shows us that it is possible to build tunnel structures kilometers underground [42]. However, the rock temperature at this mine is only 60 °C, whereas HDR often exceeds 200 °C. Consequently, deep excavation remains a challenge, especially with the presence of ultra-high temperatures. Recently, the German company of Herren Knecht has developed a shaft boring machine that can drill tunnels with a diameter of 7–12 m in hard rock formations with the in-situ stress level of up to 120 MPa [43]. This brings us inspiration. Intelligent mining robots are rapidly iterating and evolving and can be expected to gradually replace humans for deep, high-temperature underground work as their accuracy and power increase. In addition, the efficient transfer of heat from the ground to the surface is also a problem that EGS-E needs to face. In 2019, Eavor’s project demonstrated the effectiveness of heat transfer based on the thermosiphon principle [44]. The depth of the borehole reached 2400 m, and the working fluid was water. However, EGS-E projects are often built up to 3000 m below the ground and finding a better-working fluid will be of paramount importance in order to achieve heat transfer at ultra-high differences. Finally, the high cost of construction has become an important factor restricting its development. Nowadays, the exploitation of mineral resources has been fully advanced from shallow to deep. Therefore, Tang et al. proposed a collaborative development model of minerals and geothermal resources to reduce the investment in deep geothermal energy extraction [45].

It is clear from the above challenges that the problems faced by EGS-E are not essentially associated with EGS or rock engineering but have more to do with materials, mechanics, artificial intelligence, etc. Therefore, the future development of EGS-E requires the integration of multidisciplinary knowledge systems and the development of much scientific research and technological innovation.

5. Conclusions

This paper proposes a novel geothermal energy extraction method of Excavation based Enhanced Geothermal System (EGS-E). The EGS-E uses mining technology to complete the construction of a geothermal reservoir instead of using drilling and fracking methods. Accordingly, a huge but customized scale of the project could be designed as a structural system, which mainly consists of two zones: near-field heat extraction zone and far-field heat extraction zone. A near-field zone consists of rigorously regular tunnels whose operation will be precisely predictable and controllable. As for the far-field heat extraction zone, blasting or caving is carried out in an orderly manner, causing the rocks around the tunnel to collapse and break, forming a rough but scalable compensatory reservoir. The EGS-E uses a two-stage heat exchange system of “fluid-rock” and “fluid-fluid.” As for the first-stage heat exchange of “fluid-rock,” the 1# working fluid flows in the annulus of the tunnel and SRV zone, and absorbs heat from the hot tunnel walls and gravel. As for the second-stage heat exchange of “fluid-fluid,” the 2# working fluid is injected into the spiral heat exchange pipes to exchange heat with the 1# working fluid filled in the heat exchange tunnels. To demonstrate the heat extraction performance of EGS-E, we have established a representative and applicable 3D transient thermal–hydraulic numerical calculation model. The 1D non-isothermal pipe flow model and equivalent porous medium model were adopted to efficiently represent the tunnels and SRV, respectively. The temperature field results show that there are obvious low-temperature zones around the tunnel and SRV and that the zones gradually expand with time. The maximum thermal influence area radius of the near- and far-field models are 156.60 m and 172.92 m, respectively, which provide an important reference for the optimal spacing design of the structure. The production temperature of the near-field zone decreased rapidly in the first 0.2 years and then gradually remained stable from the fifth year. In contrast, the temperature change in the far field area can be divided into three stages: stable region (0–0.8 years), decline region (0.8–3 years), and stable region (3–20 years). The cumulative heat extraction of the near-field zone and the far-field zone is 1.017 × 10¹⁵ J and 8.421 × 10¹⁵ J, respectively. In an orderly regulatory manner, the latter will provide the former with sufficient compensatory heat transport, and then this system can provide sufficient energy for utilization. By combining the characteristics of these two zones, the EGS-E can realize scalable and controllable geothermal energy extraction.