Numerical Study on Permeability Evolution of a Natural Fracture in Granite During Shearing

Frictional process, AE characteristics, and permeability evolution of rough fractures on Longmaxi shale with specific roughness under water injection

ABSTRACT: The long-term success of an Enhanced Geothermal System (EGS) project requires distributed fluid flow in created fractures, ideally each with uniform and moderate permeability to avoid early thermal breakthrough. Yet, thermal depletion causes fracture opening, increasing the likelihood of flow channeling in areas with high fracture permeability. Furthermore, the effective reservoir rock stiffness (including natural fracture compliance) has a first-order impact on thermally induced stress changes, and thus fracture permeability. The objective of this work is to explore the role of thermal depletion on hydraulic fracture permeability considering a non-linear elastoplastic geothermal reservoir response. We utilize three-dimensional numerical simulations based on effective medium theory of fractured rocks to implicitly account for natural fracture compressibility and strength. Results demonstrate that a portion of the thermal strain-induced by cooling- is absorbed by natural fracture compressibility, which reduces the overall stress change, and tends to attenuate hydraulic fracture opening. Critically stressed natural fractures can yield during operation and decrease the likelihood of flow channeling. Lastly, the modeling results indicate that linear elastic models tend to overpredict fracture opening compared to models that account for effective properties of fractured rock masses. 1 INTRODUCTION Predictions of recoverable heat energy from Enhanced Geothermal Systems (EGS) reservoirs with models that neglect stress-dependent and non-linear fracture permeability are conservative estimates (Kohl et al., 1995). Flow channeling and thermal short-circuiting caused by thermo-poroelastic coupled feedback is often observed early on in field tests and would limit the installed capacity unless efforts were made to improve the flow distribution (MIT, 2006). Localized fracture opening increases injectivity and decreases the geothermal effective reservoir volume by localizing injected flow (Hicks et al., 1996). Hence, reservoir stresses (in-situ and any changes during operation) play a significant role in the distribution of EGS circulation fluid and evolution of fracture permeability (McLean and Espinoza, 2023). Spatial heterogeneity in the initial fracture aperture may further decrease reservoir performance from the beginning because preferential flow paths may exist prior to injection (Guo et al., 2016).

The dynamic permeability evolution of a fracture is a key scientific problem for fluid flows in rock masses within engineering systems. Understanding the dynamic permeability evolution and its mechanism is conducive to design and operation engineering. The dynamic permeability evolution of a rough granite fracture was revealed by laboratory experiments and numerical models. The permeability evolution of six fractured samples with rough fractures were monitored under 1.9–20 MPa effective normal stresses. The results show that the shearing process significantly affects the permeability and that the variation trend of the permeability depends on the magnitude of the effective normal stress. Under effective normal stresses of 1.9–5 MPa, the permeability is first significantly enhanced and then decreased by shearing. When effective normal stresses of more than 5 MPa are applied, the permeability only shows a decreasing trend. A high effective normal stress not only limits the dilatancy of a fracture but also enhances the formation of fault gouges. The mechanism of the dynamic permeability evolution was revealed by numerical simulations based on the discrete element method. The shearing mechanism includes the sliding mechanism and shearing mechanism. Under a low normal stress, first, the sliding mechanism is dominant and decreases the contact area, which is conducive to establishing a flow channel and increases the permeability. Then, the shearing mechanism becomes increasingly impactful, causing the contact area to increase and the permeability to decrease. Under a high normal stress, the sliding and shearing mechanisms are always engaged, which generates many wear products and reduces the permeability.

Hydraulic-fracturing treatments have become an essential technology for the development of deep hot dry rocks (HDRs). The deep rock formation often contains natural fractures (NFs) at micro and macroscales. In the presence of the NF, the hydraulic-fracturing process may form a complex fracture network caused by the interaction between hydraulic fractures and NF. In this study, analysis of carbon dioxide (CO2)-based enhanced geothermal system (EGS) and water-based EGS in complex fracture network was performed based on the thermo-hydro-mechanical (THM) coupling method, with various rock constitutive models. The complexity of the fracture geometry influences the fluid flow path and heat transfer efficiency of the thermal reservoir. Compared with CO2-based EGS, water-based EGS had an earlier thermal breakthrough with a rapid decline in production temperature. CO2 can easily gain heat rising its temperature thus reducing the effect of a premature thermal breakthrough. Both CO2-based EGS and water-based EGS are affected by in-situ stress; the increase in stress ratio improved the fracture permeability but resulted in an early cold thermal breakthrough. When the same injection rate is applied to water-based EGS and CO2-based EGS, water-based EGS displayed higher injection pressure buildup. Water-based EGS had higher reservoir deformation area than CO2-based EGS, and thermoelastic constitutive model for water-based EGS showed larger deformed area ratio than thermo-poroelastic rock model. Furthermore, higher values of rock modulus accelerated the reservoir deformation for water-based EGS. This study established a novel discussion investigating the performance of CO2-based EGS and water-based EGS in a complex fractured reservoir. The findings from this study will help in deepening the understanding of the mechanisms involved when using CO2 or water as a working fluid in EGS.

Performance optimization of pinnate horizontal well in geothermal energy utilization with orthogonal test

Geothermal energy is renewable, sustainable and available in large amounts. The hot dry rock (HDR) systems, in particular, have the largest potential for long-term sustainability and therefore draw a lot of attention. The combination of horizontal wells and the technology of multiple transverse fractures technology is an attractive approach of such HDRs, which is called in this paper as the specific EGS (enhanced geothermal system). The main objective of this paper is to study the heat extraction over a period of 20 years by water circulating in a deep geothermal reservoir using this specific EGS. The wellbore flow module, T2WELL/ECO2N, is implemented in the parallelized simulator TOUGH2MP as a new code TOUGH2MP-WELL/EOS3, which enables coupled wellbore–reservoir simulations. Using this newly developed code, the sensitivity of heat extraction against various parameters of the formation and fractures is assessed. The influences of the fracture geometry and the space of two neighborhood fractures on the geothermal energy performance are analyzed. According to the results in this paper, the injection rate, the wellbore radius and the fracture permeability are three main influence factors for the distribution of total flow through individual fractures. A large injection rate, a small wellbore radius and a large fracture width will cause the short-circuit effect, which reduces the performance of this specific EGS. Compared with the classic HDR system (doublet or triplet vertical wells + single fracture), this specific EGS has a much higher performance and a longer duration of the economic production. Furthermore, the horizontal wells can also be drilled in both directions of the minimum horizontal stress instead of in just one direction, so that the energy performance is doubled and the investment cost per kW energy production is decreased and the advantage of this specific EGS is significantly enlarged.

The EGS (enhanced geothermal system), particularly the multi-fracture EGS, is expected to be a key method to effectively develop the deep geothermal resource. The accurate modeling of flow and heat transfer in an EGS reservoir, particularly the fracture, is challenging. Therefore, an improved model considering the LTNE (local thermal non-equilibrium) and non-Darcy flow in a rough fracture is proposed. Then, the flow and temperature fields in the matrix are comprehensively investigated. The relationship between matrix and fracture is analyzed. The LTNE and LTE (local thermal equilibrium) are compared to determine the specific thermal process in the fracture. The difference between non-Darcy and Darcy flow in the fracture is studied to evaluate the actual flow behavior. The influences of the rough and smooth fractures are discussed. The results show that the direct contribution of the fracture to production is close to 100%, while the matrix mainly plays an indirect role as a heat source. There is a noticeable local low-pressure region induced by fluid density change in the matrix, while the heat conduction, especially rock conduction, dominates the heat transfer of the matrix. The intense heat convection in the fracture leads to a heat transfer rate difference of up to 5000 times compared with the heat conduction of the matrix, causing a marked thermal breakthrough. The non-Darcy flow represents the actual flow in the fracture, and the velocity is significantly lower than the Darcy flow velocity when a high injection flow rate is employed in the EGS production. The total extracted heat difference between rough and smooth fractures reaches 7.51%.

Thermal and hydraulic characteristics of a new proposed flyover-crossing fracture configuration for the enhanced geothermal system

The Enhanced Geothermal System (EGS) constructs an artificial thermal reservoir by hydraulic fracturing to extract heat economically from hot dry rock. As the core element of the EGS heat recovery process, mass and heat transfer of working fluid mainly occurs in fractures. Since the direction of the natural and induced fractures are generally perpendicular to the minimum principal stress in the formation, as an effective stimulation approach, horizontal well production could increase the contact area with the thermal reservoir significantly. In this paper, the thermal reservoir is developed by a dual horizontal well system and treated as a fractured porous medium composed of matrix rock and discrete fracture network. Using the local thermal non-equilibrium theory, a coupled THM mathematical model and an ideal 3D numerical model are established for the EGS heat extraction process. EGS heat extraction capacity is evaluated in the light of thermal recovery lifespan, average outlet temperature, heat production, electricity generation, energy efficiency and thermal recovery rate. The results show that with certain reservoir and production parameters, the heat production, electricity generation and thermal recovery lifespan can achieve the commercial goal of the dual horizontal well system, but the energy efficiency and overall thermal recovery rate are still at low levels. At last, this paper puts forward a series of optimizations to improve the heat extraction capacity, including production conditions and thermal reservoir construction design.

To improve the heat recovery of hot dry rock (HDR) and realize the efficient and sustainable development of geothermal energy, it is necessary to analyse the factors affecting the heat recovery performance of HDR fractured reservoir. In this paper, a numerical model of heat recovery in fractured reservoirs with HDR is established. The effects of injection parameters on the heat recovery performance of the Enhanced Geothermal System (EGS) are compared and analysed by means of the average temperature, heat extraction rate, production temperature and heat recovery power of matrix rocks at different locations. The influence degree of injection flow rate and injection temperature on production temperature was analysed by grey correlation analysis method. The results showed that reducing the injection temperature is beneficial to fully extract the heat energy in the rock mass. The injection flow rate is positively correlated with the heat extraction rate and negatively correlated with the extraction temperature. In this simulation scheme, the optimal results are as follows: injection temperature is 298.15K, injection flow is 12 kg/s, and the influence of injection flow on production temperature is greater than that of injection temperature. This study provides a theoretical basis for the development and utilization of geothermal energy in HDR and provides a reference for the design of engineering operation parameters.

Uncertainty analysis for heat extraction performance from a stimulated geothermal reservoir with the diminishing feature of permeability enhancement

This paper presents a numerical study on thermal energy mining from hot dry rock (HDR) using an enhanced geothermal system (EGS). In these simulations, the thermal–hydraulic–mechanical (THM) coupling model is employed on the basis of the embedded discrete fracture model. The evolution of physical fields of the fractured reservoir, including temperature field, pressure field, and stress field is studied over time, and the effects of different controllable factors, such as fracture morphology, fluid injection rate, and the distances between the injection well and producing well on the heat recovery capacity are investigated. The results show that the fracture morphology significantly influences heat extraction performance. The working fluid mainly flows along with the fracture networks, which causes locally low temperatures and low mean effective stress near fractures. The porosity and permeability increase due to the decrease in mean effective stress. For reservoir models with inclined fractures, there will be a significant decrease in the extraction temperature. In the 30th year, the decline in the heat recovery rate is 46.6%, which is much higher than the model without inclined fractures. Moreover, the increasing injection temperature barely influences the production temperature, while it significantly decreases the heat recovery of the EGS. When the injection and production well spacing is small, increasing the well spacing is an effective way to improve the thermal extraction performance of the EGS. In the model in the paper, the heat production increases up to 13.7% when the injection-production well spacing is increased from 150 m to 450 m. The results of this work could provide guidance for the optimization and operation of EGS.

Understanding working fluids flow and heat transfer in Enhanced Geothermal System (EGS) is critically important for assessing production capabilities and heat extraction rates from hot dry rock (HDR) effectively. Water is a conventional working fluid of EGS; therefore, it is necessary to study water–rock coupling heat transfer within HDR fractures. A single fracture is a basic element of complex fracture systems of EGS, so there has been many researches investigating water flow and heat transfer through a single fracture within a cylindrical HDR specimen for preliminary study, and heat transfer coefficient (HTC) is often adopted to evaluate heat transfer performance between water and rock, but there is a lack of knowledge about the inner surface temperature (Ti), which is a key parameter to calculate HTCs, due to limitations of current experimental methods and analytical solutions. Numerical modeling can be an alternative way to help us have a more comprehensive understanding of the distributions of Ti, but the 2D numerical models only reveal the distribution of Ti along the flow direction without its distribution perpendicular to the flow direction, namely, the distribution along radius. Therefore, it is essential to conduct the corresponding 3D numerical modeling to obtain the radial distribution of Ti. In this paper, a 3D numerical model accounting for surface morphology is proposed to simulate water flow and heat transfer through a single fracture within a cylindrical HDR specimen and verified by experimental data. The results reveal that the radial distribution of Ti can be described by a quadratic function mostly, and neither the surface morphology nor the phase state has influence on the tendency of Ti. Finally, four relatively stable formulas to calculate HTCs so far are compared and discussed.

Evaluation of a Greener Fracturing Fluid for Geothermal Energy Recovery: An Experimental and Simulation Study

<p>The long-term sustainability of fractures in Variscan metamorphic rocks will determine whether it is reasonable to utilize such formations as potential unconventional EGS reservoirs. During long lasting fluid flow within fractures, dissolution, precipitation, and chemical reactions between the fluid and the rock matrix may alter the flow pathway structure and flow properties. Within the framework of the European Union's Horizon 2020 initiative "MEET (Multi-Sites EGS Demonstration)", we performed long-term fracture permeability experiments on saw-cut slate samples from the Hahnenklee drill site, Harz Mountains, Germany, under constant pressure and temperature conditions. Two experiments were performed using deionized water as pore fluid with intermittent flow for more than one month at 10 MPa confining pressure and 1 MPa pore pressure. Three sequential investigations were performed, including (1) an initial continuous flow tests at room temperature, (2) temperature cycles between room temperature and up to 70 °C or 90 °C, and (3) measurement of the time-dependent permeability evolution at 70 °C or 90 °C. During stage (3), the effluents were sampled in time intervals of 6 days and analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES). The results show that (1) sample permeability first continuously decreases, but progressively converges within about three days, (2) increasing temperature leads to an additional permeability decline that is irreversible, and (3) the time-dependent permeability reduction is much more pronounced at 90 °C in comparison to that at 70 °C. The effluents are enriched with Na, Fe, K, Ca, Si, where the Na concentration is always an order of magnitude higher than the others. Except for Si, concentrations are progressively decreasing with time. During the entire experimental period, sample permeability was reduced by approximately 90% at 90 °C and 60% at 70 °C compared to their initial values. In contrast, both samples showed a negligible permeability decline with time at room temperature after cooling. Our results demonstrate that thermally-enhanced fluid-rock interactions lead to a permanent and at least partial closure of fracture aperture, which is unfavorable for geothermal exploitation. However, the degree of permeability reduction may strongly depend on initial fracture roughness, which remains to be investigated.</p>