Search

In the context of carbon neutrality, the phase-out of coal from the energy structure has resulted in numerous old coal mines that possess abundant underground space resources suitable for underground pumped hydroelectric energy storage (UPHES). Site selection and estimation of potential are critical to the planning and implementation of UPHES in old coal mines. This paper introduces a two-step site selection concept, including a screening assessment followed by a comprehensive assessment, to determine suitable locations for UPHES. The screening indicators in the screening assessment comprise geological features, mine water disasters, and minimum installed capacity, while the analytic hierarchy process (AHP) is applied in the comprehensive assessment. Additionally, coal mines in Henan Province are preliminarily screened through the screening assessment and the potential for UPHES is thoroughly investigated. The estimated volume of the drifts and shafts in old coal mines is approximately 1.35 × 107 m3, while in producing coal mines, it is around 2.96 × 107 m3. Furthermore, the corresponding annual potential for UPHES is 1468.9 GWh and 3226.3 GWh, respectively. By consuming surplus wind and solar power, UPHES is able to reduce 4.68 × 105 tonnes of carbon dioxide (CO2) emissions. The study provides preliminary guidance for policy-makers in developing UPHES in old coal mines.

Hydraulic fracturing is the most efficient method to exploit valued geothermal energy trapped in low-permeable hard rock, e.g. granite. Most research on the hydraulic fracturing has focused on its application in shale gas and oil. However, the hydraulic fracturing performs differently in geothermal reservoir, as the rock properties are quite different. In this work, an anisotropic damage–permeability model is developed on the fundament of continuum theory to study the hydraulic fracturing of hard rock in geothermal reservoir. The plastic-hardening and damage-softening behaviours are considered in this model. A cubic law is adopted to characterize the damage enhanced permeability. Its directional information is converted from damage tensor, while the effect of compression stress on permeability is isotropic and characterized by an impact factor. The newly developed model is calibrated and validated by a series of stress–strain curve, damage and axial permeability from triaxial tests on granite. In the application to cyclic fracturing test at Aspö Hard Rock Laboratory, the capacity of newly developed model is proven by good matching of measured injection pressure, permeability, etc. The results show clearly that the fracture is mostly activated by tensile failure in this case. Moreover, the stimulated fracture will be closed during flow back and re-activated in subsequent re-fracturing. If the fracture from previous fracturing is not re-activated completely, no new fractures will be created in current re-fracturing, and the damage amasses continuously due to repeated re-activation of closed fracture during re-fracturing.

Rock-like damage and fracture are a common geological phenomenon. It not only affects the construction quality and safety of the project but also affects the economic benefits. This study uses the finite element method to calculate the numerical simulation of rock. Therefore, we must study and explore it to improve the success rate and efficiency of drilling under such problems. This study conducts systematic analysis through experimental research on the theoretical knowledge of internal failure and stability of rock-like masses and other related finite element methods. On the basis of predecessors, combined with the professional knowledge I have learned, a set of relatively complete solutions is proposed. This study mainly uses the experimental analysis method and data collection method to study cloud computing related technology and finite element method, rock tensile fracture, and numerical model. According to the experiment, it can be concluded that the degree of rock crack development gradually increases with age. In the early and late stages of stretching, the rock mass will be significantly enhanced by the stress concentration.

Although hydraulic fracturing has been massively studied and applied as a key technique to enhance the gas production from tight formations, some problems and uncertainties exist to accurately predict and analyze the fracture behavior in complex reservoirs, especially in the naturally fractured reservoirs like shale reservoirs. This paper presents a full 3D numerical model (FLAC3D) to study hydraulic fracturing behavior under the impact of preexisting orthogonal natural fractures. In this numerical model, the hydraulic fracture propagation direction is assumed perpendicular to the minimum principal stress and activated only by tensile failure, whereas the preexisting natural fractures can be activated by tensile or shear failure or a combination of them, and only tensile failure can open the natural fracture as well. The newly developed model was used to study the impact of preexisting orthogonal natural fractures on hydraulic fracturing behavior, based on a multistage hydraulic fracturing operation in a naturally fractured reservoir from the Barnett Shale formation, northwest of Texas in USA. In this multistage operation, two more representative stages, i.e., stage 1 with a relatively large horizontal stress anisotropy of 3.3 MPa and stage 4 with a comparatively small one of 1.3 MPa, were selected to conduct the simulation. Based on the numerical results, one can observe that the interaction between hydraulic and natural fracture is driven mainly by induced stress around fracture tip. Besides, the horizontal stress anisotropy plays a key role in opening the natural fracture. Thus, no significant opened fracture is activated on natural fracture in stage 1, while in stage 4 an opened fracture invades to about 90 m into the first natural fracture. Conversely, the hydraulic fracture length in stage 1 is much longer than in stage 4, as some fluid volume is stored in the opened natural fracture in stage 4. In this work, the shear failure on natural fractures is treated as the main factor for inducing the seismic events. And the simulated seismic events, i.e., shear failure on natural fractures, are very comparable with the measured seismic events.

The global warming induced by the emission of greenhouse gases, especially the carbon dioxide, has become the global climate and environmental issues. China has been working in the CO2 emission reduction and carbon sinks with the purpose of becoming the carbon-neutral country by 2060. The CO2 capture, utilization and storage (CCUS) technologies and the reforestation technology represented by the Conversion of Cropland to Forestland Program (CCFP) have great potential for sinking CO2 emission. However, the trade-off among CCFP, CCS/CCUS and Water-Energy-Food (WEF) nexus are not well evaluated. In this paper, the remote-sensing data are collected and used to evaluate the sustainability of CCFP by analyzing the variation of land use and land cover (LULC), crop production, etc. The results show that 13.29% of the cropland in 2001 vanished and converted to grassland (8.3%), mosaic cropland (3%) and urban land (0.98%) in 2019, demonstrating that the CCFP is successful in both WEF nexus and carbon sink. The total crop production has increased around 50% between 2001 and 2019, implying that the CCFP will not lead to the food risk during the conversion of croplands into other types of land in China. A sustainable implementation of CCFP and other environmental Payments for Ecosystem Services (PES) policies in 2019–2060 could reach an estimated total growth of 7.462 billion m3 in comparison of that in 2018 and the total plantation forest stock of about 10.852 billion m3 in 2060, with a corresponding minimum CO2 sink of 2.90 billion tons in 2060. The estimated peak of net equivalent CO2 emissions before 2030 is about 11.0 billion tons and could not be reduced to zero by 2060 without the large-scale application of the CCS/CCUS technologies as geological sequestration of CO2. Besides, the application of CCS/CCUS can be beneficial for WEF, e.g., through replacing the water by CO2 during energy production, especially in the shale gas production in the regions with high water risks in China. In one word, CCS/CCUS and CCFP are two decided pathways of carbon sequestration and should be systematically applied to achieve China’s carbon neutrality by 2060.

Die Kooperationsvereinbarung zwischen dem Land Niedersachsen und der Provinz Anhui durch Ministerprasident Ernst Albrecht im Jahr 1984 gab den Anstos fur eine Vielzahl von Kooperationen zwischen chinesischen und niedersachsischen Hochschulen und Wirtschaftsunternehmen. Um diese Zusammenarbeit zu koordinieren und im Sinne der China-Strategie des BMBF und der Deutschland-Strategie des chinesischen MoST weiterzuentwickeln, schuf der Niedersachsische Minister fur Wissenschaft und Kultur, Bjorn Thumler, im Oktober 2018 exklusiv die Funktion des Beauftragten des Niedersachsischen Ministeriums fur Wissenschaft und Kultur fur die Hochschulzusammenarbeit zwischen Niedersachsen und China, der diese Aufgabe gemeinsam mit dem China-Netzwerk Niedersachsen wahrnehmen soll.

In geothermal reservoirs, the typically low natural permeability needs to be stimulated for an efficient heat extraction. In this work, the stimulation of heat extraction as well as the associated CO2 sequestration in geothermal reservoirs is studied, based upon supercritical CO2-EGS (enhanced geothermal system) concept in a 3D continuum anisotropic damage-permeability model under thermal–hydraulic–mechanical framework. The developed approach has been applied into the planned Dikili EGS project in Turkey. The simulation results show that a fracture network with a stimulated reservoir volume of 8.71 × 107 m3 is created by injecting 90,000 kg CO2 in 250 h. A priority channel with high gas saturation in the fracture network is formed during heat extraction because of the viscosity difference between CO2 and water. In general, for both water and CO2 injection, the driven pressure and average thermal capacity shows a positive correlation with injection rate, while the eventual produced temperature shows a negative correlation with injection rate. In comparison with water as working fluid for heat extraction, CO2 owns some benefits like low driven pressure and high eventual produced temperature. Besides, a positive correlation between sequestrated CO2 mass and injection rate is observed. Approximately 950, 000 tons of CO2 are geologically sequestrated under injection rate of 100 kg/s over 30 years of production period.

Eogenetic karst has been regarded as a dominant origin of hydrocarbon reservoirs, whereas quantifying the fluid-rock interactions and their impacts on porosity evolution during the palaeokarst process remain persistent challenges that limit the accuracy of reservoir quality prediction. This study investigates the dissolution of carbonate rocks with potential controlling factors by reactive transport modeling that couples fluid flow, mineral reactions and porosity changes in a one-dimensional vadose meteoric water-rock system. Simulation results of base case scenario and sensitivity analyses show that the duration of subaerial exposure and recharge capacity of rainwater significantly determine the karst-affected depth and porosity increment. The amount of calcite dissolution is also affected by a downward decrease in calcite solubility (temperature-dependent) and the enrichment of Ca2+ and HCO3− in the lower part. The atmospheric carbon dioxide concentration has a minor impact on the vertical extent of karst, while it facilitates the dissolution rate under high pCO2 conditions. The influence of atmospheric pCO2 variation over geological time on the porosity increment was reconstructed under hydrogeological conditions of the base case scenario (exposure time = 130 ka; rainfall = 628 mm/a). The differences in the dissolution rates and extent of karst between limestone and dolostone can be interpreted as the results of different rate-determining reaction mechanisms, i.e., thermodynamically controlled calcite and kinetically controlled dolomite.

As one of the most important ways to reduce the greenhouse gas emission, carbon dioxide (CO 2 ) enhanced gas recovery (CO 2 -EGR) is attractive since the gas recovery can be enhanced simultaneously with CO 2 sequestration. Based on the existing equation of state (EOS) module of TOUGH2MP, extEOS7C is developed to calculate the phase partition of H 2 O–CO 2 –CH 4 –NaCl mixtures accurately with consideration of dissolved NaCl and brine properties at high pressure and temperature conditions. Verifications show that it can be applied up to the pressure of 100 MPa and temperature of 150 °C. The module was implemented in the linked simulator TOUGH2MP-FLAC3D for the coupled hydro–mechanical simulations. A simplified three-dimensional (3D) 1/4 model (2.2 km × 1 km × 1 km) which consists of the whole reservoir, caprock and baserock was generated based on the geological conditions of a gas field in the North German Basin. The simulation results show that, under an injection rate of 200,000 t/yr and production rate of 200,000 sm 3 /d, CO 2 breakthrough occurred in the case with the initial reservoir pressure of 5 MPa but did not occur in the case of 42 MPa. Under low pressure conditions, the pressure driven horizontal transport is the dominant process; while under high pressure conditions, the density driven vertical flow is dominant. Under the considered conditions, the CO 2 -EGR caused only small pressure changes. The largest pore pressure increase (2 MPa) and uplift (7 mm) occurred at the caprock bottom induced by only CO 2 injection. The caprock had still the primary stress state and its integrity was not affected. The formation water salinity and temperature variations of ±20 °C had small influences on the CO 2 -EGR process. In order to slow down the breakthrough, it is suggested that CO 2 -EGR should be carried out before the reservoir pressure drops below the critical pressure of CO 2 .

Theoretical and numerical studies of coupled hydro-mechanical processes during geothermal and hydrocarbon production, nuclear waste disposal, and CO 2 sequestration in geological media have become a research focus worldwide. However, their application in China, especially CO 2 sequestration, is still in infancy and may need several years to develop. In this paper coupled hydro-mechanical processes in CO 2 migration, formation uplift and the state of caprock integrity as a result of CO 2 sequestration have been investigated by simulation at field scale for an integrated CCS (CO 2 Capture and Sequestration) project launched in the Ordos Basin, China. The paper aims at attaining long-term safety and integrity of caprock to ensure success for subsequent project operations at field scale. The linked TOUGH2MP-FLAC3D simulator has been used to study the coupled hydraulic-mechanical responses to primary stresses, injection rate and reservoir permeability during CO 2 injection and post-injection periods by means of 4 case studies. In the short-term operation of a CO 2 sequestration project launched in the Ordos Basin 0.1 Mt CO 2 has been injected in four saline aquifers within a period of 1 year and 9 years of relaxation time allotted for simulation runs. Contrary to using hydraulic field conditions alone, a combination of hydraulic-mechanical effects accounts for lateral migration of CO 2 in aquifer layers that leads to partial volumetric expansion of the reservoir. Amongst the main factors affecting hydro-mechanical processes, injection rate has the most significant impact. This is followed by reservoir permeability that is responsible for gauging the uplift trend rate in the post-injection period through hindering the dissipation of pore pressure. Results indicate that the scope of pore pressure attained will be larger in the isotropic stress state than in the compressional stress condition, pointing to the significance of primary stress. Calculation of the maximum storage pressure within safety limits using the integrity and shear criteria reveals that failure of caprock integrity increases with the amount of CO 2 injected. During injection however, the risk of caprock failure, especially in thin sandstone-caprock interlayered systems, increases with time. This paper estimates the pressure buildup in the four injection aquifers in the Ordos Basin, as a basis to ensure safety in the long-term operation of the CO 2 sequestration project, hence providing important reference for future commercial operations at field scale.

This paper introduces and explicates a new thermal module that has been developed to study numerically the heat transport in a hydraulic fracture and heat exchange between the fracture and the surrounding reservoir rocks, as well as the THM coupled processes that take place during hydraulic fracturing in a tight gas reservoir. The new module was embedded in the simulator FLAC3Dplus and validated by comparison with several analytical examples. In this paper the new module was subsequently used to model the stimulation job that was executed in a tight gas reservoir (for anonymity reasons only known as X6) in the Northern German Basin. Its basis is the history-matching of the in-situ measured wellhead pressure. The paper presents the exceptional ability of the new thermal module to simulate the entire thermal evolution process during hydraulic fracturing, including a period of temperature recovery towards the original geothermal reservoir temperature (also termed temperature warm back) after shut-in. Further observations by this paper include the significant temperature increase at the fracture front during treatment. This is caused by a delay in the arrival of the fracturing fluid at the fracture tip. In addition, the paper draws a comparison between two simulations: one executed with a thermal module to investigate the THM coupled effects on fracture development and the other without taking the thermal effects into consideration. The basis for the comparison is that the models use (in terms of geometry and stratigraphy) identical mechanical, hydraulic and thermal parameters in both simulations. Results show that the cooling effect of the injection fluid leads to contraction of the rock formations thereby causing them to experience an extra load of tensile stress. In this regard, the hydraulic fracture will easily propagate in the direction of minimum horizontal stress (where rock stress also seems to have decreased remarkably). Finally, the paper observed that simulations carried out using the new thermal module achieved a wider but shorter fracture than that obtained in simulations without consideration of heat effects. Nevertheless, the fracture heights in both cases were almost identical due to the existence of caprocks with similar integrity. Trends indicate that the simulation without thermal module caused a greater leak off volume at the end of the operation, since the fracture surface attained in this case was larger.

The uncertainty of reservoir and operating parameters challenges the accuracy of risk assessment, as well as the efficiency of optimization in carbon capture and storage (CCS) operation. To quantitatively analyze the role of uncertainty parameters on the response of CO2 injection, the effects of geomechanical and hydrogeological parameters on CCS are investigated using the approaches of distance correlation and machine learning support vector regression (SVR). In addition, a risk factor is introduced as a combination of brittleness and stress increment to assess the potential risk of caprock integrity. Using quantitative analysis, the order of importance of the parameters on the fluid pressure in the reservoir and the caprock, and the formation deformation at the ground surface and the bottom and top of the caprock are obtained. Compared to formation deformation, the pressure change can provide more valuable information regarding the assessment of the integrity of the caprock. The trained SVR surrogate model based on SVR can predict both the pressure change as well as formation deformation with reliable accuracy.

In the aerospace manufacturing industry, it is impossible to achieve precise and efficient automatic drilling and riveting for largescale composite board parts. The bottleneck is that the depth detection of rivet holes still relies on manual operation, which seriously affects the assembly efficiency and stability of composite board parts. In order to realize accurate and efficient on-machine automatic measurement for through holes in the automatic drilling and riveting process of largescale composite board parts, this paper presents a novel hole depth measuring device. Its mechanical structure is developed based on our newly designed measurement scheme and optical path, the purpose of which is to convert the hole depth data into displacement data of the probe motion. Its electrical hardware consists of three units: a laser transceiver unit to pick up laser spots, a displacement measuring unit to capture the probe movement in real time, and a driving unit to achieve motion control of the probe. Finally, the experimental results indicated that the proposed method and device are capable of performing automatic measurements for through-hole depth. In addition, factors affecting the measuring accuracy and stability of the device are initially analyzed and discussed, which lay a foundation for subsequent research on error compensation and probe calibration.

The Thematic Issue is dedicated to different aspects of using the potential of the geologic subsurface as a resource for energy production (e.g., hydrothermal and petrothermal resources), energy storage as well as for safe deposition of energy waste, and energy conversion (e.g., as biochemical reactors to convert hydrogen and carbon dioxide into methane). Figure 1 shows an overview map of hydrothermal systems in China including a classification to high-, midand low-temperature reservoirs and basins (Kong et al. 2014). Current research efforts concerning hydrothermal resources focus on the sustainable development of large-scale geothermal fields. Pang et al. (2012) designed a roadmap of geothermal energy development in China and reported the recent progress in geothermal research in China (Pang et al. 2014). Recently, Pang et al. (2015) presented a new classification of geothermal resources based on the type of heat source and followed by the mechanisms of heat transfer. The present Thematic Issue is focusing on petrothermal resources and particularly enhanced geothermal systems. Some of the basins indicated in Fig. 1 are also of interest for unconventional gas resources which is discussed in a separate Thematic Issue in Environmental Earth Sciences (Hou et al. 2015a). Geothermal energy resources are considered to provide a significant contribution to renewable energy supply from both shallow and deep systems (Arola et al. 2014; Huenges et al. 2013; Kolditz et al. 2013; Scheck-Wenderoth et al. 2013). Understanding the thermally induced coupled processes is important for reservoir operations (McDermott et al. 2006; Kolditz and Diersch 1993). Breede et al. (2014) provide a systematic review of enhanced or engineered geothermal systems (EGS). Ramos et al. (2015) provide a systematic review of projects implemented worldwide and a methodology for screening geothermal projects. Geothermal shallow surface systems based on ground source heat pumps have already proved to be a feasible option for the heating and cooling of buildings. The present work studies the concept of using abandoned mines for geothermal heat recovery in various countries such as Germany, the Netherlands, Norway, Russia, Spain, UK and United States. De Filippis et al. (2015) are discussing the geothermal potential of the Salento peninsula in southern Italy. They propose a simple methodology for the assessment of low-enthalpy geothermal energy systems based on stochastic approaches. They found that coastal areas might lower potential due to saltwater intrusion problems. Hou et al. (2015b) introduce the first & Hongwei Zhou zhw@cumtb.edu.cn

Geothermal energy is attracting more and more attention due to its large capacity and lack of dependency on the weather. Currently, many countries have planned enhanced geothermal system (EGS) projects. In this paper the first EGS project in Turkey, which is being implemented at the license area of SDS Energy Inc., in Dikili of the Izmir province, is introduced. Extensive geological, paleostress (279 fault-slip data from 33 locations), geophysical (magnetotelluric and vertical electrical sounding at 80 and 129 locations, respectively) and geochemical studies as well as paleostress measurements have been conducted in this area within the scope of this project. In the light of all these studies, it has been determined that the Dikili region is remarkable in terms of its high thermal gradient of about 7 °C/100 m. The geothermal reservoir formation “the Kozak granodiorite” is a homogeneous, crystalline volcanic rock mass with high radiogenic heat production, and suitable for an EGS application. The analysis shows that the dominating fault system is normal, and the corresponding primary stress regime is extensional. Based on the geological, geophysical surveys and the estimated in situ stresses, numerical studies were carried out to assess the results of the hydraulic fracturing and geothermal energy production using the numerical codes FLAC3Dplus and TOUGH2MP, respectively, in the area A of the Dikili site. The simulation results show that the stimulated reservoir volume and area could reach 44.5 million m3 and 1 km2, respectively, with an injection volume of 122,931 m3. Assuming the fractured zone has a height of 1000 m and a half-length of 1200 m (the distance between injection and production wells being 1000 m), an average overall geothermal capacity of 83.7 MWth in 20 years could be reached with an injection rate of 250 l/s. The injection strategy and design parameters of the reservoir stimulation and geothermal production will be further optimized with the project running.

Natural gas storage caverns are usually built either in a rock salt dome or in a bedded rock salt formation. In China, the rock salt stratum has the following characteristics: thin, inter-layered, bedded and highly impure. The height of a cavern in bedded rock salt deposit in China depends largely on the thickness of the salt formation (80–300 m) and is much smaller than in thick domes and thick layers of rock salt deposits in Europe and the USA, where a storage cavern is typically, cylindrically shaped. This renders the diameter of a bedded salt cavern in China to be greater than its height to obtain sufficient storage volume. The stability of horizontal salt caverns with different diameters and varied minimal internal pressures during its long-term operation was numerically analyzed in this paper regarding convergence, damage, stress to peak or dilatancy strength ratio and effective strain. The Hou/Lux constitutive model including some relevant parameters from previous laboratory investigations were used for the numerical simulations. The simulation results show that a horizontal cavern (height ≪ length) for underground gas storage is more efficient, profitable and suitable because of its few times larger storage capacity than the most used vertical cavern (height ≫ diameter) or a prolate cavern (height ≈ or < diameter) under the Chinese special geological condition of thin-bedded rock salt deposits, provided with appropriate construction and operation parameters.