Advancements and Future Prospects in the Hydraulic Fracturing of Geothermal Reservoirs

Abstract

Reservoir reconstruction is a critical challenge in many significant underground energy projects, such as enhanced geothermal systems, oil shale extraction, and shale gas development. Effectively reconstructing geothermal reservoirs can significantly enhance the exploitation and production capacity of geothermal resources. However, this process requires stringent technical standards and varies with different geological conditions across regions, necessitating tailored reconstruction strategies. This review offers a comprehensive examination of hydraulic fracturing within geothermal reservoirs, covering the geological and physical characteristics inherent to these systems, the effects of injection methods and thermal stimulation on hydraulic fracturing processes, and the assessment and optimization of transformation effects, as well as environmental implications and risk management considerations. We explore the influence of various injection modes on hydraulic fracturing dynamics. Moreover, we compare the differences between hydraulic fracture propagation with and without thermal effects. Additionally, we summarize optimization strategies for reservoir reconstruction. Finally, we discuss several challenges and potential future directions for development, offering insights into possible advancements. This review is of substantial significance for both research and commercial applications related to hydraulic fracturing in geothermal reservoirs.

1. Introduction

Currently, global environmental and energy supply pressures are increasing, and there is an urgent need to supplement some green and sustainable energy sources [1]. Geothermal energy is a renewable resource because the Earth’s mantle is continuously replenished with heat through conduction. Unlike other renewable energy sources, geothermal energy offers the benefits of a stable supply, lack of climate constraints, and a high average utilization factor [2]. Its use positively impacts the environment by reducing emissions of pollutants associated with conventional energy sources like coal and natural gas. Beyond energy and spa treatments, geothermal water resources also foster the growth of tourism, cosmetics, food production, and other related economic sectors [3].

Stimulation geothermal reservoirs can significantly boost the extraction and production capacity of geothermal resources. However, this process is technically demanding and involves overcoming challenges such as improving the circulation of subsurface fluids and increasing recharge to extract more thermal energy. Balancing extraction and recharge during geothermal resource development is crucial to prevent water depletion and pressure reduction in reservoirs, necessitating innovative technologies and methods. Additionally, geological complexity varies by region, requiring tailored stimulation strategies [4]. The Enhanced Geothermal System (EGS), depicted in Figure 1, is a primary method for developing geothermal resources in dry hot rocks. Yet, it faces critical issues like irrational hydraulic fracture networks in high-temperature reservoirs, unclear multi-scale and multi-field coupling laws, low heat extraction efficiency due to flash flow in geothermal wells, and inefficient thermoelectric conversion of geothermal fluids. These issues hinder large-scale commercial exploitation of geothermal resources [5,6].

Hydraulic fracturing, a technique that involves injecting high-pressure fluids to create a network of fractures, is used to release gases or liquids trapped within rocks. It has broad applications in oil and gas well stimulation, shale gas extraction, coal bed methane development, and geothermal energy exploitation [7,8,9,10,11,12,13]. The process entails injecting a mixture of water, chemical additives, and proppant (such as sand) into a subsurface reservoir at high pressure. This fluid creates fractures, and the proppant keeps them open after the pressure is released, facilitating the flow of gas or liquid to the wellbore [14]. Recently, research on hydraulic fracturing for reservoir remodeling has gained prominence due to the increasing focus on geothermal energy development. Studies indicate that hydraulic fracturing can effectively create fracture networks in hard rocks like granite and enhance their permeability, which is vital for geothermal energy extraction [15,16,17]. The enhanced connectivity of reservoirs achieved through hydraulic fracturing enables more efficient heat extraction from deeper geological formations [18], which is a crucial aspect of an enhanced geothermal system (EGS). Moreover, innovative injection methods, including cyclic and fatigue hydraulic fracturing, have demonstrated benefits in forming fracture networks in granite and potential in controlling seismic activity triggered by fluid injections [19]. By employing a modelling approach to elucidate the fluid–solid interactions occurring during the fracturing process, it is possible to predict the initiation, extension, and ultimate morphology of the resulting fractures. Recent studies have demonstrated that a complex fracture model, designated a cloud fracture network, can be generated by hydraulic fracturing in superheated geothermal environments. The formation of this fracture network can increase the fluid-rock contact area by creating dense, permeable fractures within the rock mass. Furthermore, real-time monitoring of fracture development during the fracturing process is possible using fibre-optic distributed acoustic sensing (DAS) and microseismic monitoring techniques. This can assist in enhancing comprehension and control of the fracturing process, thereby optimising the efficiency of thermal energy extraction [20,21,22]. This review aims to investigate the role of hydraulic fracturing in geothermal reservoir modification, assess its effects on fracture network formation, permeability improvement, and environmental impacts, and explore how optimizing injection strategies can enhance the economic and sustainable development of geothermal resources.

2. A Comprehensive Study of Classic Cases in Geothermal Reservoir

In 1973, the U.S. Department of Energy established the world’s first demonstration site for the development and utilization of dry-heat rock in the Fenton Hill area of New Mexico. Although the project did not realize commercial-scale geothermal power generation, it has achieved a series of scientific research results and innovative understanding, and has laid an important foundation for the subsequent emergence of the wave of dry-heat rock research on a global scale. Subsequently, countries such as the UK, France, Japan and Australia have all invested in dry-heat rock research. In recent years, China’s geothermal industry has been booming, and several enhanced geothermal demonstration projects have been carried out one after another.

Geothermal reservoirs have several important characteristics that are the focus of extensive discussions:

2.1. Temperature

High-temperature advantage: High-temperature geothermal reservoirs (generally above 150–200 °C are more valuable. For example, in the Geysers geothermal field in California, the reservoir temperature can reach up to 300 °C. At such high temperatures, the steam or hot water extracted can be used more efficiently to generate electricity through steam turbines.

Temperature distribution: The temperature of geothermal reservoirs usually increases with depth. In a normal geothermal gradient area, the temperature rises about 2–3 °C per 100 m of depth. However, in areas with active magma chambers or intense tectonic activity close to the surface, the temperature increase rate may be much higher.

2.2. Porosity and Permeability

Porosity significance: Porosity refers to the proportion of voids in the rock. High-porosity rocks can store more geothermal fluids (steam or hot water). For example, porous sandstone and limestone can act as good geothermal reservoirs. In some sedimentary basins, the porosity of sandstone reservoirs can reach 10–30%.

Permeability importance: Permeability is related to the ability of fluids to flow through the rock. Good permeability allows the geothermal fluid to move easily between injection wells and production wells. Fractured rocks, such as fractured granite, often have high permeability. Hydraulic fracturing techniques can also be used to improve the permeability of low-permeability reservoirs to enhance fluid flow.

2.3. Fluid Properties

Composition: Geothermal reservoir fluids usually contain a variety of dissolved substances. The most common components are water, along with dissolved salts such as sodium chloride, potassium chloride, and calcium carbonate. In some high-temperature geothermal areas, the fluid may also contain gases such as carbon dioxide, hydrogen sulfide, and methane. For example, in areas with a lot of volcanic activity, the content of sulfur-containing gases in geothermal fluids is relatively high.

Density and viscosity: The density and viscosity of geothermal fluids affect their flow characteristics. Hot water generally has a lower density and viscosity than cold water, which is beneficial for fluid extraction and circulation. The density and viscosity of the fluid also change with temperature and the content of dissolved substances. As the temperature rises, the viscosity of the fluid decreases, which is more conducive to the flow of the fluid in the reservoir and wellbore.

2.4. Geological Structure

Tectonic setting: Geothermal reservoirs are often related to tectonic activity. Areas near plate boundaries, such as the Pacific Ring of Fire, are rich in geothermal resources. Tectonic movements can create fractures and faults in the crust, providing channels for geothermal fluids to migrate and accumulate. For example, in Iceland, which is located on the Mid-Atlantic Ridge, the extensive rifting and volcanic activity provide favorable conditions for the formation of geothermal reservoirs.

Cap rock: The existence of cap rock is crucial for the preservation of geothermal reservoirs. The cap rock is usually a layer of low-permeability rock (such as shale) above the reservoir, which can prevent the geothermal fluid from escaping upward and maintain the pressure and temperature of the reservoir.

In this paper, some classic geothermal projects are selected as the object of analysis, aiming to put forward some rough suggestions and opinions on the future geothermal reservoir modification on this basis. As shown in

Table 1. Typical geothermal projects worldwide.

Project Name	Country	Running Time	Development Methodology	Development Period	Primary Purpose	Earnings
Fenton Hill	America	1973–2000	By drilling deep wells and performing hydraulic fracturing tests to create or activate large-scale fracture networks.	Phase I (1973–1980): Thermal storage depth of about 3 km, temperature 200 °C. Phase II (1979–2000): thermal storage depth of about 4 km, temperature of 300 °C.	Development of methods for the economic extraction of thermal energy from high-temperature crystalline or metamorphic rock bodies, as well as validation of the technical feasibility of the dry-heat rock concept.	Failed to give a complete description of the performance required at a commercial scale from a commercial development perspective, but the project provided valuable experience and data.
Rosemanowes	England	1978–1991	Microseismic monitoring and tracing techniques to evaluate reservoir fracturing effectiveness and fluid flow paths during development.	Phase I (1977–1980): drilling of wells to a depth of 300 m and establishment of water circulation. Phase II (1980–1988): reservoir development at 2 km depth. Phase III (1988–1991): development of a prototype commercial system	Development of relevant equipment and technology for use in deep geothermal mining projects.	The technical feasibility of establishing water circulation and reservoir development in granite was verified, and high-temperature instrumentation was developed to provide the necessary technical support for the construction of a commercial HDR system.
Hijiori	Japan	1985–2002	Drilling deep wells and testing hydraulic fracturing to create artificial thermal storage systems in high-temperature rocks.	Phase I (1985–1991): Shallow reservoirs were created and multiple HDR technology developments were made. Phase II (1992–2002): Larger, higher temperature reservoirs were created and a two-layer reservoir HDR system was established in 1994.	Verification of the technical feasibility of establishing an artificial thermal storage system in a high-temperature rock body for long-term heat extraction tests.	A data integration methodology was developed that enables the integration of geologic structures, core data, and logging data into the model.
Soultz	France	1987–Present	Artificially constructed deep heat exchangers to extract thermal energy from deep thermal reservoirs and used it to generate electricity.	Phase I (1987–2007): Work such as hydraulic and chemical excitation between wells was completed. Phase II (2007–2009): A demonstration power plant of 1.5 MW enhanced geothermal system was built. Phase III (2009–present): Grid-connected power generation was carried out.	Thermal energy in deep thermal reserves is mined and used to generate electricity by artificially constructing deep heat exchangers.	Successfully created a commercial-scale artificially stimulated reservoir.
Cooper Basin	Australia	2003–Present	Adoption of the “one injection, one mining” model for development and testing.	Started in 2003; connected cycle realized in 2009; trial power generation realized in 2013: trial power generation realized after installation of 1 MW pilot duplex genset.	Validation of the feasibility of an enhanced geothermal system (EGS) in a high-temperature granite substrate for commercial power generation.	The project estimates that the well group has the potential to generate up to 2.5 MW of electricity.
Gonghe	China	2013–Present	Based on the characteristics of dry-heat rocks in the Republican Basin, the EGS model was established to analyze the influence of key parameters.	/	Promote new breakthroughs in the development of dry-heat rock geothermal heat, and form a number of dry-heat rock development demonstration areas.	Supporting the restructuring of the national energy structure and helping to realize the country’s “dual-carbon” goal.
Huangshadong	China	2017–Present	Modeling four types of geothermal power plants suitable for the Huangshadong geothermal field.	/	Provide reference for the subsequent dry hot rock resource exploration and target area selection in the same type of area along the southeast coast.	Provide a demonstration for the commercial development of geothermal energy.

, the characteristics of seven domestic and international classic geothermal projects are analyzed and summarized, and their geographical distribution is shown in Figure 2.

3. Influencing Factors on Hydraulic Fracturing of Geothermal Reservoirs

In conducting hydraulic fracturing simulation tests, scholars employ acoustic emission technology to monitor microfracture activity within the rock in real time, thereby ensuring that various parameters, including Poisson’s ratio, Young’s modulus, tensile strength, injection displacement, injection pressure, temperature, pore pressure, fracture length, fracture number, and fracture spacing, which influence fracture expansion behaviour, are aligned with the actual geothermal reservoir conditions. In numerical simulation, scholars typically utilise fixed parameters. However, due to the unfeasibility of continuous injection of fracturing fluid in actual field operations, incorporating dynamic changes in physical parameters (velocity), and correlating them with the fracture cracking and extension process, can enhance the precision of the simulation test to a certain extent [23].

3.1. Impact of Geologic and Physical Properties of Geothermal Reservoirs on Hydraulic Fracturing

The purpose of hydraulic fracturing is to improve the reservoir’s conductivity, thereby enhancing the efficiency of geothermal energy extraction. Hydraulic fracturing technology achieves this by creating an artificial network of fractures. It does so by injecting high-pressure fluids into the wellbore, which facilitates fluid flow and heat exchange [24]. However, when compared to the hydraulic fracturing of sedimentary rocks in conventional oil and gas fields, the process of hydraulic fracturing in hard, dry, and hot rock formations presents significant challenges.

The geological characteristics of geothermal reservoirs encompass their structural features, lithology, tectonics, and the presence of fissures and fractures. Utilizing geological surveys and drilling techniques, a geological model of the geothermal reservoir can be constructed to analyze its lithological attributes, tectonics related to thermal storage, and tectonics influencing hydraulic conductivity. This facilitates an understanding of the circulation mechanisms of geothermal fluids and lays a theoretical foundation for geothermal energy development [25,26,27]. However, geological heterogeneity has an impact on the performance of geothermal reservoirs, which are typically connected to more permeable rocks or natural fractures. In order to quantify the impact of heterogeneity on geothermal reservoir performance, a new methodology is proposed, which is assessed by introducing a new parameter. The methodology is evaluated through the integration of well test data and fracturing operation data, with the objective of diagnosing and assessing the performance of hydraulic fracturing [28,29,30].

The physical properties of geothermal reservoirs include porosity, permeability, thermal conductivity, specific heat capacity, and density (Figure 3). Research into these physical properties is vital for the scientific advancement and efficient utilization of geothermal energy. They dictate the heat capacity, fluid flow dynamics, and the feasibility of energy extraction from geothermal reservoirs [31]. The researchers evaluated thermal conductivity variations at varying saturation levels in 101 samples of diverse rock types from the Songliao, Gonghe, and Ordos basins in China. They proposed an enhanced model and a calibrated model, which were developed through a process of integrating insights from three established models and the interrelationship between porosity, saturation, and thermal conductivity. By comparing the improved and fitted models, the range of applicability of each model was determined, and a detailed error analysis was conducted. The results of the experiments demonstrate that there is a potential for thermal conductivity to increase by up to 50% when the rock transitions from an unsaturated state to a saturated state. This effect is observed to increase exponentially with rising porosity [32].

For porosity and thermal conductivity, scientists conducting hydraulic fracturing tests on artificial samples (with porosities ranging from 1.8 to 2% [33]) found that fracture extension varied with pre-existing fractures of different dips, and that hydraulic fracturing could easily cause pre-existing fractures with large dips to open further, while pre-existing fractures with small dips would act as a barrier to hydraulic fracturing(Figure 4).

In conclusion, studying the geological and physical properties of geothermal reservoirs is essential for the development and utilization of geothermal energy. It aids in enhancing energy utilization efficiency, achieving energy conservation and emission reduction, and aligns with the goals of the “dual carbon” emission reduction targets.

3.2. Impact of Injection Patterns on Hydraulic Fracturing

In the deep granite stimulation phase, some geothermal projects have been terminated due to the occurrence of seismic events induced by a fluid injection of a larger magnitude. These events not only incited public panic but also resulted in significant economic losses. They have substantially impeded the technological progress and widespread application of geothermal energy development. Consequently, there is an urgent need to explore new hydraulic fracturing methods that can minimize the risk of induced seismicity, making this a pressing issue at the forefront of international scientific research.

In the current scenario, there is significant scope for optimizing the thixotropic control and permeability enhancement in hydraulic fracturing technology for EGS reservoir modification. During the hydraulic fracturing process, the fluid injection pattern and the percolation rate within the rock mass generate varying fracture pressures, ultimately leading to seismic activities of differing magnitudes. The Swedish Äspö Hard Rock Laboratory conducted experiments on hard rock hydraulic fracturing using conventional hydraulic fracturing (HF), progressively increasing cyclic injection, and superimposed pressure pulse injection with increasing flow rates to explore ways to mitigate seismic risks and enhance rock permeability during hydraulic fracturing [34]. The National Local Joint Engineering Research Center for Shale Gas Exploration and Development, Chongqing Geology and Mineral Resources Research Institute, China, has analyzed the fracture extension, fluid pressure distribution and acoustic emission (AE) energy under four different hydraulic fracturing methods (constant pressure, sinusoidal pulse, triangular pulse, and rectangular pulse) through laboratory experiments and numerical simulations (Figure 5). The experimental results show that pulsed hydraulic fracturing is capable of forming complex fractures in a short period of time and can reduce the risk of induced earthquakes by reducing the energy released when the shale is completely destroyed [35]. The Massachusetts Institute of Technology (MIT), USA, applied cyclic injections to very tight and strong sandstones to reduce fracture pressure through laboratory hydraulic fracturing experiments [36]. The School of Mechanical Science and Engineering at Northeast Petroleum University, China, explored the application of pulse hydraulic fracturing in tight oil and gas reservoirs, focusing on its dynamic damage mechanism and parameter optimization. The study indicated that pulse hydraulic fracturing can effectively lower fracture initiation pressure, enhance fracturing efficiency, and create a complex fracture network in coalbed methane extraction [37]. The simulation of different geological conditions and injection strategies is achieved through the application of cyclical increases and decreases in injection pressure or rate, as well as through the introduction of brief pulses of high pressure or flow during the injection process. By regulating the periodic alterations in injection pressure, the likelihood of fault activation and seismic activity resulting from excessive pressure increases can be diminished, thereby reducing the seismic risk associated with stress concentrations [38]. The School of Resource and Safety Engineering at Central South University, China, examined the impact of fatigue loading on the hydraulic fracturing behavior of granite through a series of experiments. The results demonstrated that the fracture initiation pressure and rupture pressure of the rock vary with the amplitude of cyclic loading, resulting in the formation of non-planar and narrower fractures [39]. The Department of Civil and Environmental Engineering at the University of Strathclyde, UK, developed a numerical method to simulate rock fractures induced by hydraulic pulses, incorporating rock fatigue. This method, based on S-N curves and implemented through FORTRAN scripts and ABAQUS solvers, employs a cohesive crack model to simulate discrete crack extensions in rock [40]. The State Key Laboratory of Deep Geotechnics and Underground Engineering at China University of Mining and Technology studied the effects of pulsed hydraulic fracturing on the strength deterioration mechanism of rock-like specimens and established a crack damage deterioration model [41].

3.3. Effects of Thermal Stimulation on Hydraulic Fracturing

Currently, the majority of studies on hard rock hydraulic fracture extension focus solely on fluid–solid interactions, neglecting thermal effects. In dry-rock hydraulic fracturing as part of reservoir modification, the thermal seism that occurs when cold water encounters high-temperature geothermal reservoirs can significantly expedite the emergence and growth of reservoir fractures. This enhancement of the fracturing effect aids in achieving rapid production capacity in dry-rock hydraulically fractured reservoirs.

Lin et al. [42] developed a numerical model for the non-planar propagation of multiple clusters of hydraulic fractures in deep shale by incorporating thermal effects. This model is based on the displacement discontinuity method, finite volume method, and finite difference method, and integrates equations for fluid viscosity, fluid filtration coefficient, and thermal stress as functions of temperature. The study analyzed the differences in the propagation of multiple hydraulic fractures with and without considering thermal effects (Figure 6). The thermal effects in hydraulic fracturing, as described in the paper, are formulated as follows.

Equation for fluid viscosity versus temperature:

$$\mu \left(T\right)={\mu }_0\exp (-CT(T-T_0\left)\right)$$

(1)

where ${\mu }_0$ is the fluid viscosity, Pa∙s, at the reference temperature, $T_0$, and $CT$ is the temperature sensitivity coefficient, 1/°C.

Equation for fluid filtration coefficient versus temperature:

$$C_{leak}=C_{0leak}\exp (-CT(T-T_0))\sqrt{{\displaystyle \frac{1}{\mu }}}$$

(2)

where $C_{0leak}$ is the fluid filtration coefficient at the reference temperature $T_0$, ${\mathrm{m}·\mathrm{s}}^{1/2}$.

Thermal tensile stress equation:

$${\sigma }_{\Delta T}=E{\alpha }_e{\displaystyle \frac{(T_{f_{near}}-T_0)f\left(\beta \right)}{(1-v)}}$$

(3)

where ${\alpha }_e$ is the coefficient of thermal expansion of the rock, °C⁻¹; $T_{f_{near}}$ is the center temperature of the near-fracture unit, °C; $T_0$ is the reference temperature, °C, and $f\left(\beta \right)$ is a coefficient related to the cooling area.

The results indicate that the higher the stratum temperature, the more pronounced the thermal effect becomes. This leads to a greater variation in the morphology of the multiple clusters of cracks. Furthermore, the thermal effect becomes particularly significant when the formation temperature surpasses 90 °C.

Therefore, when selecting sites for EGS projects, it is often preferable to choose dry-rock reservoirs with higher initial temperatures, greater moduli of elasticity, higher initial permeability, and lower homogeneity indices. These characteristics facilitate reservoir modification and enhance the effectiveness of fracturing processes.

4. Evaluation and Optimization of the Effects of Stimulation

Evaluating and optimizing the effects of reservoir stimulation is crucial for ensuring economic viability. In both geothermal and conventional oil and gas stimulation, pressure and flow rate are crucial parameters. For example, during hydraulic fracturing operations, in both cases, pressure gauges are installed in the wellbore to monitor the injection pressure. In a hydraulic fracturing treatment for a shale gas well and a geothermal well, the injection pressure is measured to understand the resistance of the formation and the propagation of fractures. Seismic techniques are used in both geothermal and conventional oil and gas stimulation to understand the subsurface structure and the effect of stimulation. Microseismic monitoring is a common method. When fractures are created or existing fractures are extended during stimulation, small-scale seismic events are generated. However, unlike traditional hydraulic fracturing in the oil and gas industry, geothermal fluid parameters, temperature stress, and duration of stimulation are different, which can result in differences in evaluation effectiveness [43]. This process necessitates a comprehensive assessment of geological properties, fracture propagation patterns, technical parameters, economic factors, and environmental impacts. Continuous technological innovation and research can enhance the efficiency and effectiveness of reservoir stimulation, thereby promoting the sustainable growth of the industry.

4.1. Methods for Assessing Reservoir Performance After Stimulation

The assessment of reservoir performance after stimulation primarily encompasses several key aspects (Figure 7): Utilizing fiber optic distributed acoustic sensing (DAS) and distributed strain sensing (DSS) technologies to monitor dynamic changes during the fracturing process; employing microseismic monitoring and imaging technologies to evaluate the effectiveness of fracturing and the propagation of fractures; capturing the morphology of fractures and monitoring fluid flow; and assessing the degree of fracture compression and opening, as well as the output effect of fracturing. The extent of fracture opening and the output effect are evaluated [44,45]. By establishing models for fracture initiation and expansion theory, the non-equilibrium and network expansion patterns of fractures are simulated, providing a theoretical foundation for the optimal design of reservoir modification [46,47,48]. Modern numerical analysis and machine learning techniques, integrated with a multi-parameter comprehensive evaluation approach, are employed to quantitatively assess reservoir performance [49,50].

4.2. Optimization Strategies for Reservoir Stimulation

Zhou investigated the hydraulic fracturing technology for reservoir modification in the Deng 4 Member of the Lower Permian Lampshade Formation in the Sichuan Basin. Using a large-scale true triaxial acid pressure model, experiments were conducted to simulate the reservoir reconstruction process. The study integrated the evaluation of reservoir geological conditions with indoor physical simulation results for field application optimization [51]. Qi et al. proposed a denoised autoencoder-based migration learning framework to optimize well placement in hydraulically fractured reservoirs. This migration learning framework effectively leverages experience from previous tasks, accelerating the optimization process, particularly for complex hydraulic fracturing reservoir modification challenges [52]. Wang et al. employed temporary plugging and steering fracturing techniques to enhance the complexity of reservoir reconstruction fracturing. They developed a three-stage temporary plugging and steering evaluation device capable of simulating three-stage steering fractures under 3–15 mm fracture openings and varying roughness conditions, as well as steering simulations at 5–30 MPa by adjusting rupture disc pressure levels [53]. Zhang et al. explored the use of bi-directional long and short-term memory networks and particle swarm optimization (PSO) algorithms for logging data reconstruction in hydraulic fracturing reservoir modification. They confirmed that the PSO-BiLSTM model offers superior accuracy and stability in predicting missing logging data compared to the standard LSTM and BiLSTM neural networks. This provides valuable insights for logging data reconstruction in complex formations and aids in the optimization of reservoir modification [54].

5. Environmental Impact and Risk Management

The environmental impact and risk management associated with hydraulic fracturing for reservoir modification is a complex issue that necessitates a holistic approach, considering various factors including geology, engineering, and environmental aspects. It also requires the implementation of suitable technical methods and the establishment of preventive and control measures to guarantee the safety and environmental sustainability of hydraulic fracturing operations.

5.1. Environmental Impacts of Hydraulic Fracturing

Casing deformation and fault slip pose a significant challenge during hydraulic fracturing for shale gas extraction in Sichuan. Research indicates that fault slip induced by hydraulic fracturing could be a primary cause of casing deformation [55]. While hydraulic fracturing can trigger microseismic events, these typically do not escalate into catastrophic seismic events. The tectonic stress state significantly influences the magnitude of induced seismic events; a reverse fault stress state tends to induce larger events, whereas a strike-slip or normal fault stress state generally results in smaller events [56]. The environmental impact of shale hydraulic fracturing is greater than that of conventional oil and gas fields, and the latter is on a larger scale. Toxicological tests were conducted on hydraulic fracturing fluids and return fluids, with a focus on the diversity of the two types of fracturing fluids, particularly in terms of pH, mineralisation and organic matter content. Additionally, He conducted physico-chemical analyses of samples of return fluids from wells A and B. He posited that the rational management of return fluids, particularly through pre-treatment and reuse, could markedly diminish the detrimental impact of the oil industry on the natural environment [57]. Additionally, hydraulic fracturing technology can lead to environmental pollution, particularly affecting the contamination of groundwater resources and environmental pollution from returned discharges, even as it boosts production (Figure 8) [58].

Studies have demonstrated that hydraulic fracturing induces cracks in rock formations through the injection of fluids at high pressure, which inevitably triggers microseismic activities. Further research into the mechanisms underlying hydraulic fracturing-induced seismicity and strategies for mitigating earthquake risks can offer a foundation for risk prevention and control. This, in turn, can ensure the sustainable advancement of hydraulic fracturing technology.

He et al. conducted a study on pulsed hydraulic fracturing (PHF), demonstrating that the PHF damping mechanism primarily manifests in the propagation and attenuation of pressure waves. The PHF process is bifurcated into two phases: intense pressure fluctuations during the initial fracturing stage and a subsequent steady state. Compared to conventional hydraulic fracturing (HF), PHF significantly amplifies the pressure gradient of the pulse waveform, an important parameter affecting pulse pressure [59]. He et al. also proposed an advanced stepped rectangular pulse hydraulic fracturing technique, which achieves higher pressures and more complex fractures by pumping low-energy fluids. This technique’s pressure propagation can be divided into stable and unstable phases, leading to longer unstable propagation times, higher pressures, and more complex pressure distributions compared to conventional methods [60]. Xie et al. showed that pulsed hydraulic fracturing can reduce rock fracture pressure by generating a seismic wave, which also mitigates seismicity during the fracturing process. By leveraging the “water hammer effect”, this technique creates a more complex fracture network at lower hydraulic energy, thereby reducing the risk of induced earthquakes [61]. Liang et al. investigated the stress field inversion of microseismic events in shale gas development through experiments and numerical simulations. They found that the joint source scanning algorithm can be used to better understand hydraulic fracturing-induced seismicity, providing a foundation for risk management [62].

The key formulation of the pulse hydraulic fracturing dampening mechanism is as follows [59]:

Fluid equations of motion and continuity equations:

$$\left\{\begin{array}{c}{\displaystyle \frac{\partial \overrightarrow{u}}{\partial x}}+\left(\overrightarrow{u}·\nabla \right)\overrightarrow{u}+{\displaystyle \frac{1}{\rho }}\nabla p=\overrightarrow{f}\\ {\displaystyle \frac{\partial \rho }{\partial t}}+\nabla \left(\rho \overrightarrow{u}\right)=0\end{array}\right.$$

(4)

where $\overrightarrow{u}$ is the velocity vector of the PHF fluid, $\rho$ is the fluid density, p is the pressure, $\overrightarrow{f}$ is the mass force and frictional resistance, $\nabla$ is the differential operator, and $t$ is time.

Fluid elastic stress wave equation:

$${\displaystyle \frac{d\rho }{dt}}={\displaystyle \frac{1}{{\alpha }^2}}{\displaystyle \frac{dp}{dt}}$$

(5)

where $\alpha =\sqrt{{\displaystyle \frac{k}{\rho }}}$, $\alpha$ is the velocity of the fluid pressure wave in water, and k is the volume elasticity coefficient of the fluid. $\alpha$ is generally 1000 to 1400 m/s.

Two-dimensional pulsed transient flow equations considering attenuation:

$${\displaystyle \frac{{\partial }^{2}p}{\partial t^2}}={\alpha }^2\left({\displaystyle \frac{{\partial }^{2}p}{\partial x^2}}+{\displaystyle \frac{{\partial }^{2}p}{\partial y^2}}\right)-{\displaystyle \frac{\lambda }{2\delta }}{\displaystyle \frac{\partial p}{\partial t}}$$

(6)

where $p$ is the pressure, $t$ is the time, $\alpha$ is the velocity of the fluid pressure wave in the water, $x$ and $y$ coordinates, $\delta$ is the width of the fissure, and $\lambda$ is the integrated damping coefficient.

Pulse hydrodynamic energy equation:

$$W=W_k+W_p={\displaystyle \frac{1}{2}}\left(∆m\right){\left({\displaystyle \frac{\partial y}{\partial t}}\right)}^2+{\displaystyle \frac{1}{2}}\left(k\right){\left({\displaystyle \frac{\partial y}{\partial x}}\right)}^2$$

(7)

where $W$ is energy per unit mass, $W_k$ is kinetic energy per unit mass, and $W_p$ is elastic potential energy per unit mass.

5.2. Risk Assessment and Security Measures

Chen et al. explored the correlation between hydraulic fracturing and induced earthquakes by analyzing hydraulic fracturing tests and associated microseismic activities. They quantified the evolution of several parameters, including statistical predictions of hydraulic parameters, microseismic events, b-values, and event sizes. Their investigation focused on hydraulic fracturing-induced earthquakes in the development of dry-rock geothermal energy in the Republican Basin of Qinghai Province, China [63]. Some scholars have employed unsupervised machine learning algorithms for the purpose of classifying microseismic events, thereby facilitating the monitoring of subsurface fracture flow. By analyzing the time-frequency characteristics of long duration (LD) events, it is possible to monitor subsurface fluid flow dynamics, such as injection-induced non-seismic sliding in fractures or faults, in order to optimise hydraulic fracturing stimulation [64]. In contrast, the Alford rotation and linear transformation technique (LTT) has been employed for the processing of seismic data, enabling the separation of fast and slow shear wave modes and the assessment of shear anisotropy and fracture strength [65]. In the context of induced seismicity, researchers have utilised a dense array of temporary earthquakes and a deep learning workflow to construct a high-precision seismic catalogue, which allows for the tracking of temporal variations during and after hydraulic fracturing through the analysis of the distribution, frequency, magnitude, and focal mechanisms of induced earthquakes [66]. The selection of red and yellow traffic light thresholds is based on the current understanding of induced seismicity. The risk of damage or nuisance is calculated using probabilistic maximum magnitudes, magnitude in relation to ground shaking, population density, statistical distributions of site amplification, and thresholds of felt or damaging ground shaking. This approach enables regulators to design traffic light protocols in a risk-informed manner, thereby facilitating a more effective balancing of the consequences of their decisions [67,68]. Xu et al. developed a risk assessment methodology for hydraulic fracturing-induced earthquakes, employing a combination of rough set theory and Bayesian networks, along with geospatial correction. They introduced “potential factors” to achieve a quantitative assessment of the seismic risk level in the study area. A case study was conducted in the Changning Shale Gas Development Zone in Yibin City, Sichuan Province, China [69]. Thompson et al. examined the risk of groundwater contamination due to well integrity failure during hydraulic fracturing. They combined event tree analysis (ETA) and fuzzy fault tree analysis (FFTA), using fuzzy logic to assess risk pathways that are challenging to quantify because of limited data. They proposed a fuzzy logic-based risk assessment methodology [70]. Badjadi et al. investigated the risk management of hydraulic fracturing, particularly in shale gas reservoirs. The study developed a comprehensive risk assessment model that considered multiple condition indicators and extreme working conditions by integrating various data sources, including quantitative and qualitative data, observational records, expert judgments, and global sensitivity analyses using Sobol’s method [71]. Liu et al. investigated the risk of casing rupture during hydraulic fracturing leading to aquifer contamination. The study quantified the risk of aquifer contamination by accounting for the uncertainty of hydrogeological parameters and the non-homogeneity of the formation. They developed a new conceptual model, specified it, and derived the necessary data through numerical simulation. A sensitivity analysis was performed to identify key factors affecting groundwater contamination [72].

6. Conclusions and Recommendations

This paper outlines the technological advancements in geothermal reservoir modification. It reviews the current state of research on hydraulic fracturing for geothermal reservoir enhancement, examining the impact of injection techniques and thermal stimulation on the fracturing process. This paper also assesses and optimizes the effects of these modifications, evaluates their environmental impacts, and addresses risk management strategies. Furthermore, it discusses the prevailing challenges and prospective directions for future development in the field.

(1) The intricate and variable geological conditions within geothermal reservoirs, coupled with the reliance of existing modification technologies on the specific in situ geological environment, pose challenges in developing a universally “replicable” model for thermal reservoir modification. Hydraulic fracturing, a pivotal technology in this field, substantially enhances reservoir permeability by creating an artificial fracture network beneath the surface. This enables more efficient extraction of thermal energy to the surface. Nonetheless, the technology remains in an advanced developmental stage for geothermal reservoir modification, necessitating further research and optimization.

(2) Pulse injection during hydraulic fracturing effectively promotes the expansion of fractures within rock formations while simultaneously reducing the magnitude of induced earthquakes. This technique leverages the thermal seism phenomenon, which accelerates the emergence and expansion of reservoir fractures, thereby enhancing the fracturing effect in dry-heat rock reservoirs. Nonetheless, geothermal reservoir modification technology confronts challenges posed by extreme conditions, such as high temperatures and pressures. This necessitates the development of equipment and materials that possess greater resistance to these harsh environmental factors.

(3) While progress has been made in geothermal reservoir modification technology, further in-depth research is required, particularly in the areas of hydraulic fracturing modes and real-time microseismic monitoring. These can commence at the early stage of fracture network formation and analyse the real-time monitoring data through cross-disciplinary data integration using today’s machine learning and emerging artificial intelligence technologies to predict the fracture extension paths, thus enabling more accurate real-time monitoring.

(4) The reduction in environmental pollution caused by contamination of groundwater resources and return emissions is of particular importance for the sustainable development and utilisation of geothermal resources. In order to assess the potential impact of hydraulic fracturing fluid leakage on groundwater resources, it is possible to carry out simulation studies of groundwater contamination in parallel. Furthermore, environmentally friendly fracturing fluids can be researched and developed with the objective of reducing the use of chemicals in the hydraulic fracturing process and the potential negative impact on the environment.