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[Display omitted] • The low-velocity layer in the thickened crust provides the most important heat source in Tibetan Plateau. • Heat production of highly radiogenic rock is the main heat source of shallow intracrustal heat in southeastern China. • Heat upwelling along fractures is the main heat source of EGS in the north China sedimentary basin. A heat source is the basis for Enhanced Geothermal Systems (EGS) and is the most important factor for the site selection for EGS development. In this paper, we analyze regional variation patterns of mantle heat flow, study crustal thermal states, construct models of crustal steady-state temperature fields, and discuss the heat source mechanism of EGS in different zones, including the strongly tectonic activity zone of the Tibetan Plateau, North China sedimentary basin, and the highly radiogenic rock distribution zone of southeastern China. The results show that differences in the crustal thermal structure of different regions control their deep thermal conditions and EGS heat sources. The intense tectonic activity zone of the Tibetan Plateau, represented by the Gonghe Basin, shows an anomalously heated crustal thermal structure, and the low-velocity layer in its thickened crust provides the most important intracrustal heat source in the region. The local area in southeastern China is influenced by both highly radiogenic rocks and good sedimentary cover insulation, indicating a hot crust-cold mantle type of crustal thermal structure, thus reflecting the role of heat production from highly radiogenic rocks in controlling the shallow crustal temperature field in the area. The North China Plain has a typical cold crust-hot mantle type of crustal thermal structure, and the development of EGS in the area must consider the undulating bedrock pattern in the sedimentary basin and whether there is a deep fracture that connects the nearby mantle heat source. The results are of scientific significance for guiding future target site selection and EGS development in different regions. [ABSTRACT FROM AUTHOR]

The beginning of 2022 was a time of major changes in the perception of energy availability and security in European countries. The aggression of Russia against Ukraine destabilizing the European energy economy, combined with the withdrawal from fossil fuels which has been going on for a dozen years, has strengthened activities to introduce new energy technologies based on renewable energy sources. One of the most promising and stable sources of renewable energy is geothermal energy, in particular enhanced geothermal systems (EGS) in hot dry rocks (HDR). These deposits occur at a great depth in almost every place on Earth, but due to their low permeability, they require hydraulic fracturing, which results in high investment costs. This technology has been developed for several decades. The current situation in Europe seems to confirm that its rapid development to a level that guarantees stable and profitable operation is crucial. This is of particular importance in the case of former member states of the economic zone of the Council for Mutual Economic Assistance, which until recently were heavily dependent on Russian energy. This review, based on the latest available data, covers potential HDR prospective areas in the countries of the south-eastern Baltic basin, including Lithuania, Latvia, Estonia and Poland. It is specific to this region that the original heat flux density is lower as a result of the paleoclimatic effect associated with the youngest ice age; however, thermal conditions do not deviate too much compared to western Europe, especially Rhine Graben, and significantly exceed the conditions of Finland, where an EGS project is currently being operated. In Lithuania, the most prospective area is the ZNI intrusion (south of Klaipeda), characterized by a geothermal gradient of up to 40 °C/1000 m. In addition, the Precambrian batholith south of Liepāja (Latvia) and the Rapakivi granites in the north and center of Estonia are promising EGS sites. Poland has relatively the most explored EGS potential, in both volcanic, crystalline and sedimentary rocks, especially in the area of the Szczecin Trough, Gorzów Block, Moglino-Łódź Trough and Karkonosze Mountains. Unfortunately, local tectonic conditions, in particular the development of faults and natural fracture zones that affect the directions of fracture propagation during hydraulic fracturing, have not been sufficiently recognized, which is one of the main barriers to the expansion of EGS pilot projects in these countries. These issues present challenges for the researchers, especially in terms of petrophysical analyses of rocks in target zones and local stress conditions, which have a key impact on fracturing operations and profitability of the systems. Despite high investment costs on the one hand and a significant slowdown in the global economy in 2022 on the other, it remains hopeful that the authorities of individual countries will decide to accelerate research work, leading to the implementation of pilot projects of EGS installations, and that this technology will be further improved to ensure a stable clean energy supply. [ABSTRACT FROM AUTHOR]

The beginning of 2022 was a time of major changes in the perception of energy availability and security in European countries. The aggression of Russia against Ukraine destabilizing the European energy economy, combined with the withdrawal from fossil fuels which has been going on for a dozen years, has strengthened activities to introduce new energy technologies based on renewable energy sources. One of the most promising and stable sources of renewable energy is geothermal energy, in particular enhanced geothermal systems (EGS) in hot dry rocks (HDR). These deposits occur at a great depth in almost every place on Earth, but due to their low permeability, they require hydraulic fracturing, which results in high investment costs. This technology has been developed for several decades. The current situation in Europe seems to confirm that its rapid development to a level that guarantees stable and profitable operation is crucial. This is of particular importance in the case of former member states of the economic zone of the Council for Mutual Economic Assistance, which until recently were heavily dependent on Russian energy. This review, based on the latest available data, covers potential HDR prospective areas in the countries of the south-eastern Baltic basin, including Lithuania, Latvia, Estonia and Poland. It is specific to this region that the original heat flux density is lower as a result of the paleoclimatic effect associated with the youngest ice age; however, thermal conditions do not deviate too much compared to western Europe, especially Rhine Graben, and significantly exceed the conditions of Finland, where an EGS project is currently being operated. In Lithuania, the most prospective area is the ZNI intrusion (south of Klaipeda), characterized by a geothermal gradient of up to 40 °C/1000 m. In addition, the Precambrian batholith south of Liepāja (Latvia) and the Rapakivi granites in the north and center of Estonia are promising EGS sites. Poland has relatively the most explored EGS potential, in both volcanic, crystalline and sedimentary rocks, especially in the area of the Szczecin Trough, Gorzów Block, Moglino-Łódź Trough and Karkonosze Mountains. Unfortunately, local tectonic conditions, in particular the development of faults and natural fracture zones that affect the directions of fracture propagation during hydraulic fracturing, have not been sufficiently recognized, which is one of the main barriers to the expansion of EGS pilot projects in these countries. These issues present challenges for the researchers, especially in terms of petrophysical analyses of rocks in target zones and local stress conditions, which have a key impact on fracturing operations and profitability of the systems. Despite high investment costs on the one hand and a significant slowdown in the global economy in 2022 on the other, it remains hopeful that the authorities of individual countries will decide to accelerate research work, leading to the implementation of pilot projects of EGS installations, and that this technology will be further improved to ensure a stable clean energy supply.

Hydraulic fracturing is often conducted to create artificial fractures as flow paths in Hot Dry Rock (HDR) geothermal reservoirs. However, the fractures could be closed due to the high geo-stress conditions, resulting in low efficiency of geothermal productivity. Hence, maintaining a high hydraulic permeability of the created fractures is a key point for the success of Enhanced Geothermal Systems (EGS). In this study, chemical stimulation was carried out to improve the hydraulic properties of fractured granite. The effects on the hydraulic properties of two sets of samples were examined by laboratory flowing tests. Results show that chemical stimulation solution with mixture of 12%HCl+5%HF improved efficiently permeability. The permeability was increased by four orders of magnitudes with an optimized reaction time. The mud acid was found to react fast with minerals such as feldspars and biotite, but rather inactive with quartz as revealed by the microscopic structure and ionic detection of the fluid and rocks. Thereby, the quartz particles can be used as fracturing proppant to support opening of aperture. The findings obtained from this study indicated that a hydraulic fractured granite based HDR geothermal reservoir can be effectively improved by chemical stimulation. [ABSTRACT FROM AUTHOR]

• Reconstruction of reservoir rock samples by three-dimensional carving technology. • The effect of fracture morphology on fracture evolution was analyzed. • Distilled water and simulated geothermal fluid were used in seepage experiments. • The error between the actual aperture and the inversion aperture was verified. Long-term seepage under high temperature and high pressure affects fracture morphology, ultimately impacting the development efficiency of Enhanced Geothermal Systems (EGS). In this study, rock samples with different morphologies were produced and their morphologies were described quantitatively using fractal dimension theory. Experimentally, distilled water and simulated geothermal fluid were injected into fractures at a constant flow rate for 360 h. The behaviors of different fluid components during the fracture evolution process under high temperature and high pressure were observed. Results show that apertures and permeability decreased by 7.1% and 19.5% when distilled water was used, while they decreased by 0.6% and 1.8% respectively when simulated geothermal fluid was injected. The fracture apertures first decreased and then gradually increased. The more complex the fracture shape, the faster the mineral dissolution rate. With fractal dimensions of 1.007, 1.027 and 1.056, the dissolution rates of rock-like minerals in deionized water were 9.43×10−2, 1.33×10−1 and 1.51×10−1 mg/L per 12 hours, respectively. This study lays a foundation for further research on fracture network evolution and provides a basis for the long-term efficient and stable operation of EGS. [ABSTRACT FROM AUTHOR]

Enhanced geothermal systems (EGS) is currently the most efficient method to mine heat from hot dry rock (HDR) reservoirs. The coupled thermo-hydro-mechanical (THM) effect therein is of critical importance to predict the long-term performance of EGS. A novel 3D THM coupling model integrating displacement discontinuity method (DDM), finite volume method (FVM) and finite element method (FEM) is developed in this study to aid such prediction. The present model is first validated against analytical solutions, and then applied to a penny shape EGS to simulate a 30-year prolonged injection. The simulation results reveal that EGS maintains high productivity during the early injection stage but once thermal drawdown takes place, the EGS productivity degrades rapidly. The fracture aperture model employed in the present study is capable of simulating hydraulic fractures in which the fluid pressure is large enough to overcome the normal stress exerted to the fracture faces and thus jack the fracture open. In contrast, most early studies adopted a compressive aperture model, where a substantial part of the normal stress is borne by fracture gouge/roughness. The strength of DDM + FVM + FEM scheme lies in the efficiency and versatility of simulating fractures. Our study indicates that the DDM + FVM + FEM scheme has high potential to serve as a competent alternative to performing the THM coupling simulation in EGS. [ABSTRACT FROM AUTHOR]