Underground Energy Research Articles

Evaluation Methods of Salt Pillar Stability of Salt Cavern Energy Storage

Underground energy storage is essential for the country’s development, and underground salt cavern groups are a productive way to store energy. Safety pillar design is the key to ensuring the safe operation of large salt cavern gas storage groups. Therefore, this paper is based on the salt pillar stability design problem and analyzes three aspects: (1) Three kinds of pillar stability design theories—reliability theory, strain energy theory, and catastrophe theory; (2) Two methods for designing stable pillars—determining pillars through formulas and numerical simulations; (3) The form and influencing factors of salt pillar instability—macro and microform and the main influential factors. From the current research, the major problem is that the design of safety pillars has not been systematic, due to the differences in salt rock in the region, and the engineering of salt pillar design still relies on experience. Finally, the impact of salt pillar width and gas injection and withdrawal on the pillars during the design of the salt cavern is analyzed, and the existing salt pillar theoretical characteristics and development trend are summarized.

Design and analysis of a floating photovoltaic based energy system with underground energy storage options for remote communities

Remote communities are highly dependent on transported food and fuel and require resilient energy systems. This study proposes a solar energy-based resilient system with energy storage options which is uniquely designed to generate electricity, heat, cooling, hydrogen and hence ammonia. Floating photovoltaic plant integrated with an anion exchange membrane electrolyser, pressure swing adsorption air separator with ammonia reactor and a heat pump. The underground energy storage options are pumped-hydro storage, high-grade heat storage, medium-grade heat storage and cold storage. The proposed system intends to exploit the infrastructure of abandoned mines with underground storage, as well as unutilized water surfaces with floating photovoltaic plant. Among the direct useful outputs of the system, agricultural activities and food production can be supported with a greenhouse heater, a food drying system and an ammonia fertilizer. The proposed system is then analyzed through the first and second laws of thermodynamics by using mass, energy, entropy and exergy balance equations. A time-dependent analysis with simulation is carried out to analyze each hour in terms of energy and exergy balances by considering commercially available component data as well as actual meteorological data. The proposed system with a 120 MWp floating PV plant and energy storage options, is determined to be sufficient to meet all of 51 GWhe/year non-thermal electrical, 23.8 GWhth/year heating, and 7.7 GWhth/year cooling loads for a community with 5320 people by exploiting unutilized natural bodies and abandoned mines. The pumped-hydro storage system has 67.24 % round trip energy efficiency, while the hydrogen-based energy storage option possesses 46.50 % round trip energy efficiency in a typical meteorological year under average conditions. The overall energy and exergy efficiencies of the overall system are found to be 15.9 % and 17.1 % on average.

Optimal Homotopy Asymptotic Analysis of the Dynamics of Eyring-Powell Fluid due to Convection Subject to Thermal Stratification and Heat Generation Effect

In the present study, the effect of thermal stratification and heat generation in the boundary layer flow of the Eyring-Powell fluid over the stratified extending surface due to convection has been investigated. The governing equations of the flow are transformed from partial differential equations into a couple of nonlinear ordinary differential equations via similarity variables. The optimal homotopy asymptotic method (OHAM) is used to acquire the approximate analytical solution to the problems. Impacts of flow regulatory parameters on temperature, velocity, skin friction coefficient, and Nusselt number are examined. It is discovered that the fluid velocity augments with a greater value of material parameter E , mixed convection parameter λ , and material fluid parameter σ . The result also revealed that with a higher value of the Prandtl number Pr and the stratified parameter ε , the temperature and the velocity profile decreases, but the opposite behavior is observed when the heat generation/absorption parameter γ increases. The results are compared with available literature and are in good harmony. The present study has substantial ramifications in industrial, engineering, and technological applications, for instance, in designing various chemical processing equipment, distribution of temperature and moisture over agricultural fields, groves of fruit trees, environmental pollution, geothermal reservoirs, thermal insulation, enhanced oil recovery, and underground energy transport.

Creep-fatigue characteristics of rock salt under different loading paths

The salt behavior subjected to creep-fatigue load is meaningful to ensure the stability of the salt cavities used for underground energy storage. The salt specimens are tested under three paths of creep-fatigue load, including creep followed by fatigue (C + F), fatigue followed by creep (F + C) and creep-fatigue (CF). The stress-strain curves, deformation, loading modulus and acoustic emission (AE) count were systematically analyzed. The axial, lateral and volumetric deformations present a four-phase and two-phase change trend at C + F (F + C) and CF loading, respectively. The deformation curve for an individual creep or fatigue stage at C + F and F + C loading resembles that in pure creep or fatigue tests. The prior creep or fatigue load decreases the strain rate in the subsequent fatigue or creep phase. The prior creep load decreases the loading modulus of rock salt in the subsequent fatigue stage, and the loading modulus presents a downward trend with increasing the cycle numbers for the salt samples at C + F loading. The hold time during the loading-unloading stage at CF loading exerts little effect on the variation trend of loading modulus. The hold time increases the irreversible deformation for each cycle and induces the salt samples to be prone to dilate at CF loading. Continuous AE activities were monitored in the creep stage at C + F loading, while minor AE responses develop in the creep phase at F + C loading and the hold time stage at CF loading. These results suggest that the combined creep and fatigue load in sequence and simultaneously result in a negative and positive interaction between them, respectively. The deformation behavior is explained from the viewpoint of micro-mechanism.

Experimental Study on the Compressive and Shear Mechanical Properties of Cement–Formation Interface Considering Surface Roughness and Drilling Mud Contamination

In a casing-cement sheath-formation system, the cement–formation interface is usually weakly cemented for the residual of drilling mud, in which a leakage path would easily form, threatening the safe operation of underground energy exploitation and storage. To evaluate the compressive and shear mechanical behavior of the cement–formation interface, cement–rock composite cylindrical specimens were prepared. Uniaxial and triaxial compression and direct shear tests were implemented. The flushing efficiency of the rock surface, compressive strength, interface incompatible deformation, parameters of shear strength, and morphology of shear failure surface were acquired and analyzed. Results show that the flushing efficiency of shale surface decreases from 76.7% to 64.2% with the surface roughness increasing from 0 to 2 mm. The flushing efficiency of sandstone is only 44.7%, remarkably lower than that of shale. With the stress condition transforming from uniaxial to triaxial compression, the feature of the stress–strain curves changes from elastic-brittle to elastoplastic, and the compressive strength increases from 20.6~60.1 MPa to 110~120 MPa. The cement part presents noteworthy plastic deformation and several micro shear fractures develop. There is incompatible deformation between cement and rock, which induces interface debonding for almost all the composite specimens. The internal friction angle and cohesive strength both decrease with the increase in pollution degree of drilling mud, and increase with the rise in surface roughness. The shear facture surface is not exactly the rock–cement interface, but usually manifests as a shear zone, in which the rock, cement, and interface all contribute to the final shear failure. The above findings would be valuable for the revealing of cement–formation interface failure mechanism.

A Finite Element Model for Investigating Unsteady-State Temperature Distribution and Thermomechanical Behavior of Underground Energy Piles

The underground energy geostructure represented by the energy pile is one of the key paths for the cooperative development of underground space and geothermal energy. Because of its advantages of low cost, high efficiency and no extra occupation of underground space, it has become a feasible alternative to the borehole heat exchanger. The change in the temperature field of the energy pile and its surrounding ground not only affects the geological environment but also influences the thermomechanical performance and the durability of the structure. However, the temporal and spatial unsteady-state temperature distribution of piles and surrounding rock under typical intermittent and unbalanced thermal load conditions is still unclear. In this paper, a finite element model was applied to analyze the unsteady-state temperature distribution, and the thermomechanical behavior of the energy pile group was developed and verified. The temperature field distribution of pile and surrounding rock under typical intermittent working and unbalanced thermal load conditions were determined. Moreover, the thermomechanical behavior characteristics of the energy pile group were investigated. Finally, the influences of pile layout on the thermomechanical behavior of the energy pile group were identified by designing six different scenarios. The results indicate that under typical intermittent operation conditions, the temperature of the energy pile and surrounding ground near the heat exchange pipe varies periodically. For areas with unbalanced cooling and heating loads, long-term operation of energy piles leads to thermal accumulation, and the maximum temperature of energy piles occurs in the first daily cycle. In summer/winter working conditions, the increase/decrease in pile temperature induces axial compression/tensile stress. When the pile group is partially used as the energy pile, the non-energy pile acts as the “anchor pile”, and it generates the added tensile stress.

Investigation of heat exchange efficiency and thermal migration of Deeply Buried Pipe Energy Pile group under the groundwater seepage

The seepage has a significantly positive effect on the heat transfer of underground energy structures. However, the effect of thermal migration caused by seepage is ignored. Previous studies mainly focused on the Ground Source Heat Pump (GSHP) and the Inside Buried Pipe Energy Pile (IBP-EP). In this study, a novel underground energy structure named Deeply Buried Pipe Energy Pile (DBP-EP) was proposed and thermal-seepage coupled models of DBP-EP groups with three different layouts was developed. Meanwhile, the variation of heat exchange efficiency and the effect of thermal migration caused by seepage were analyzed. The results showed that seepage could eliminate heat accumulation around the DBP-EP group and enhance the heat exchange efficiency. However, the thermal migration transferred the heat from the upstream of DBP-EPs to the downstream, and resulted in the thermal disturbance for the downstream. When the layout of DBP-EP group was horizontal parallel, the heat exchange efficiency of the downstream DBP-EPs was 4 %-6% lower than the upstream. The staggered layout could decease the influence of thermal migration on the downstream DBP-EPs and further reduce unnecessary heat exchange loss. Besides, the appropriate spacing of the DBP-EP group, which achieved the optimum utilization of land space, was related to the seepage velocity. When the seepage velocity increased from 60 m/a to 120 m/a, the minimum spacing of the DBP-EP group with the staggered layout could be reduced by about 1 m. The heat exchange efficiency of the DBP-EP could be improved by 9 %-10 % than GSHP. This study will have certain engineering guiding significance for the design method of DBP-EP project site selection, operation and DBP-EP group layout.

Research on Application of the Bolt-Truss Coupling Support Technology in Roadway With Water-Rich and Soft Rock

In the process of underground energy mining, the stability of roadway support is an important guarantee. In order to study the application of the anchor cable-truss support technology in water-rich soft rock roadways, the mechanical analysis of the anchor cable-truss structure is carried out, and the surrounding rock deformation of different supporting methods is numerically simulated under the consideration of the fluid–solid coupled interaction. We observed that the anchor cable (rod)-double arch truss coupling support can control the deformation of the surrounding rock and the expansion of the plastic zone well. The maximum vault subsidence of the roadway is 0.017 m, the horizontal convergence is 0.054 m, and the deformation of floor heave is 0.02 m, which are 3.8, 16.3, and 4% of the deformation under unsupported conditions, respectively; the roadway deformation is effectively controlled. The research results have certain guiding significance for the support design of the water-rich broken soft rock roadway.

Analysis of Stress State and Damage Characteristics of the Cement Sheath

The sealing problem of the cement sheath often appears in gas wells for underground energy exploitation, especially when horizontal multistage fracturing technology is used in the shale gas industry. In this article, according to the elastic–plastic mechanics and Mohr–Coulomb yield criteria, an analytical solution of the equations is obtained by considering the effect of pressure on the fluid column, volume shrinkage, or expansion and geological characteristics on the initial stress of the cement sheath. The analysis of the example indicates that the smaller the initial stress of the cement sheath, the lower its radial stress and circumferential stress, which is under the maximum inner casing pressure of 150 MPa. With the increase of initial stress of the cement sheath, it is easier for the first and second interfaces to enter the plastic and damage state for the cement sheath. The smaller the initial stress of the cement sheath, the earlier the damage appears, and it develops with the increase of inner casing pressure more quickly. When the initial stress of the cement sheath is less than 7.9 MPa, the damage factor finally reaches 1 with the increase of internal casing pressure; however, when the initial stress of the cement sheath is greater than 34.2 MPa, the damage factor always remains 0 with the increase of inner casing pressure. The results preliminarily revealed that the initial stress of the cement sheath plays a decisive role in promoting its integrity and may provide guidance for the choice of the formula of cement and construction methods in the oil field.

Permeability Modeling and Estimation of Hydrogen Loss through Polymer Sealing Liners in Underground Hydrogen Storage

Fluctuations in renewable energy production, especially from solar and wind plants, can be solved by large-scale energy storage. One of the possibilities is storing energy in the form of hydrogen or methane–hydrogen blends. A viable alternative for storing hydrogen in salt caverns is Lined Rock Cavern (LRC) underground energy storage. One of the most significant challenges in LRC for hydrogen storage is sealing liners, which need to have satisfactory sealing and mechanical properties. An experimental study of hydrogen permeability of different kinds of polymers was conducted, followed by modeling of hydrogen permeability of these materials with different additives (graphite, halloysite and fly ash). Fillers in polymers can have an impact on the hydrogen permeability ratio and reduce the amount of polymer required to make a sealing liner in the reservoir. Results of this study show that hydrogen permeability coefficients of polymers and estimated hydrogen leakage through these materials are similar to the results of salt rock after the salt creep process. During 60 days of hydrogen storage in a tank of 1000 m2 inner surface, 1 cm thick sealing liner and gas pressure of 1.0 MPa, only approx. 1 m3 STP of hydrogen will diffuse from the reservoir. The study also carries out the modeling of the hydrogen permeability of materials, using the Maxwell model. The difference between experimental and model results is up to 17%, compared to the differences exceeding 30% in some other studies.

In-depth analysis of ultrasonic-induced geological pore re-structures

Understanding and manipulating geological pore structures is of paramount importance for geo-energy productions and underground energy storages in porous media. Nevertheless, research emphases for long time have been focused on understanding the pore configurations, while few work conducted to modify and restructure the porous media. This study deploys ultrasonic treatments on typical geological in-situ core samples, with follow-up processes of high-pressure mercury injections and nitrogen adsorptions and interpretations from nuclear magnetic resonance and x-ray diffraction. The core permeability and porosity are found to increase by 8.3 mD, from 4.1 to 12.4 mD, and by 0.95%, from 14.03% to 14.98%, respectively. Meanwhile, the number and size of the micro- and mesopore are increased with progressing of ultrasonic treatment, while those of the macropore decrease, which finally increase the permeability and porosity. The increase of micro- and mesopore number, from x-ray diffraction results, is attributed to the migration and precipitation of clay minerals caused through ultrasonic wave. The relocation of clay minerals also helps to improve the pore-throat connectivity and modify the micro-scale heterogeneity. Basically, this study reveals the characterizations of geological pore reconfigurations post-ultrasonic treatments and interprets the associated mechanisms, which provides guidance to manipulate the geological pores and be of benefit for further porous media use in science and engineering.

Compaction and restraining effects of insoluble sediments in underground energy storage salt caverns

Compared with the salt domes formed by marine deposits abroad, the salt formations in China have bedded strata of lacustrine deposition, which contain rock salts and nonsalt interlayers. During the leaching phase, rock salts will dissolve to form a cavern that is used for storing oil or natural gas, whereas the nonsalt interlayers will soften and detach from the cavern walls, accumulating to the cavern bottom. These sediments will restrain the cavern walls and increase the working capacity because of the pore space in the sediments. Therefore, the stress and porosity of the sediments are key parameters for the assessment of the compaction and restraining effects of insoluble sediments. In this work, a mechanical element model of the sediments is proposed to predict the stress and porosity of the sediments in a cylindrical salt cavern. The depth of the sediments is introduced to analyze the compaction effect. The influencing factors of the equations of stress and porosity are then discussed using different friction coefficients, lateral stress coefficients, and hydraulic radii. To investigate the restraining effect of the sediments on the stability of the salt cavern, coupled numerical simulations are carried out using the discrete-continuous coupled method. Comparing the numerical simulation results of the salt cavern with and without sediments, the porosity of the sediments decreases, and the effective stress increases with creep time. The increasing rates of deformation and shrinkage gradually decrease because of the presence of sediments, which is favorable to improving the stability of the salt cavern. The numerical simulation results of the salt cavern with different variables indicate that the shrinkage of the cavern and porosity of the sediments are not sensitive to the sediment density, ball friction, or wall friction. This study can provide a reference for predicting the stress and porosity of the sediments and for investigating the stability of salt caverns with sediments.

Multiscale and multiphysics influences on fluids in unconventional reservoirs: Modeling and simulation

Unconventional reservoir resources are important to supplement energy consumption and maintain the balance of supply and demand in the oil and gas market. However, due to the complex geological conditions, it is a significant challenge to develop unconventional reservoirs efficiently and economically. At present, unconventional reservoirs are exten-sively studied, covering a wide range of areas, with special attention to the multiscale characterization of pore structures and fracture networks, description of complex fluid transport mechanisms, mathematical modeling of flow properties, and coupled analysis with multiphysics fields. This work briefly describes the multiscale and multiphysics influences on fluids in unconventional reservoirs, and the modeling and simulation work conducted to analyze them, with the aim to provide some theoretical basis for enhanced recovery from these geo-energy resources. The present article also aims to enhance the community’s knowledge of other potential utilizations associated with some unconventional reservoirs, specially related to environmentally-driven projects, including permanent greenhouse gas storage and cyclic underground energy storage. Cited as: Cai, J., Wood, D. A., Hajibeygi, H., Iglauer, S. Multiscale and multiphysics influences on fluids in unconventional reservoirs: Modeling and simulation. Advances in Geo-Energy Research, 2022, 6(2): 91-94. https://doi.org/10.46690/ager.2022.02.01

Experimental Investigations on the State-Dependent Thermal Conductivity of Sand-Rubber Mixtures

Soil-rubber mixtures are potential construction materials for use in civil engineering applications such as the thermal insulation layer for buried pipelines in the permafrost zone and underground energy storage systems. Currently, however, the thermal conductivity of soil-rubber mixtures in various stress, saturation, and void ratio conditions is not fully understood. In this study, a stress-controlled apparatus based on the thermal needle probe technique was developed. It was employed to study the influence of the aforementioned factors on the thermal conductivity of sand-rubber mixtures. Thirty-four tests were completed with a wide range of net stresses (0–600 kPa), initial degrees of saturation (0%–100%), void ratios (0.45–0.95), and rubber contents (0%, 10%, and 20%). Three replicates were used in each condition. The results showed that the thermal conductivity was strongly dependent not only on void ratio, rubber content, and degree of saturation but also on net stress. For instance, the thermal conductivity of a dry sand-rubber mixture increased by 30% when the stress increased from 0 to 600 kPa. This increase in thermal conductivity can mainly be attributed to an increase in interparticle contact area, accounting for about 25% of the 30% increase; the reduction in void ratio played a minor role. A semiempirical equation was proposed to calculate the thermal conductivity of sand-rubber mixtures at different stress levels.

Gas fields and large shallow seismogenic reverse faults are anticorrelated

We investigated the spatial relationships among 18 known seismogenic faults and 1651 wells drilled for gas exploitation in the main hydrocarbon province of northern-central Italy, a unique dataset worldwide. We adopted a GIS approach and a robust statistical technique, and found a significant anticorrelation between the location of productive wells and of the considered seismogenic faults, which are often overlain or encircled by unproductive wells. Our observations suggest that (a) earthquake ruptures encompassing much of the upper crust may cause gas to be lost to the atmosphere over geological time, and that (b) reservoirs underlain by smaller or aseismic faults are more likely to be intact. These findings, which are of inherently global relevance, have crucial implications for future hydrocarbon exploitation, for assessing the seismic–aseismic behaviour of large reverse faults, and for the public acceptance of underground energy and CO2 storage facilities—a pillar of future low carbon energy systems—in tectonically active areas.

Experimental Study on the Mechanical and Permeability Properties of Lining Concrete Under Different Complex Stress Paths

A promising large-scale energy storage is underground compressed air energy storage (CAES) in lined rock caverns. To ensure the safety and stability of storage caverns because of the influence of periodic injection during production, it is crucial to understand the mechanical behavior of lining concrete under different complex stress paths. In this study, three types of uniaxial compressive fatigue test and uniaxial creep test were conducted on concrete. The following conclusions were obtained from the results. 1) The irreversible deformation after the interval was larger than that before the interval in the discontinuous multi-step cyclic loading (DMCL) test. 2) Loading velocity significantly influenced concrete fatigue, and the irreversible deformation in the cycle of low loading velocity was greater than that in the cycle of high loading velocity. 3) The residual strain increased with an increase in stress level. 4) The creep strain increased with an increase in stress level during the multi-step creep loading test; the fractional derivative results were more consistent with the experimental results. 5) The permeability of concrete increased rapidly under the influence of an external force when the stress level exceeded 0.73.

Hydro-mechanical coupling of granite with micro-defects: Insights into underground energy storage

In order to revel the hydro-mechanical coupling characteristics of granite at various pore pressures and confining pressures. The spatial micro-pore structure before and after the failure of granite is extracted by high-precision CT scanning system. The results show that the UCS of the damaged granite decreases with the increase of pore pressure under certain confining pressure. The sensitivity of UCS of granite to the change of pore pressure is lower than that of confining pressure. In the rock compaction stage, the permeability decreases slightly, while it increases slowly in the elastic and plastic stages. From the perspective of microscopic fracture structure, the number of roar channels increased by 134 times, the total length increased by nearly 100 times, the average equivalent radius was expanded to 1.64 times, and the permeability increased by 443 times before and after rock failure. The permeability characteristics of granite are most affected by pore-throat structure.

Dynamic Service Restoration for Integrated Energy Systems Under Seismic Stress

Seismic events and following aftershocks can cause devastating impacts on overground and underground energy system infrastructures. This paper designs a novel approach to restore the system functionality in response to seismic stress by using coordinated dynamic system reconfiguration and operation. First, seismic impact on system infrastructures is evluated, considering substations, pipes and distribution lines. Load curtailment is assumed to be caused by the overloading of system branches, buckling towers, reversed gas flows, and isolated sub-circuits. Smart system operation and reconfiguration are coordinatedly applied by using combined soft open points (SOP) and energy storage units, aiming to restore system service by minimizing load curtailment and system recovering time. Service index is quantified considering both load curtailment and recovering time. A representative integrated electricity and gas system is employed to demonstrate the effectiveness of the proposed method. Results illustrate that the adverse impact of seismic on systems can be effectively reduced by the proposed dynamic service restoration method. The model provides system operators with a powerful tool to restore the functionality of distribution systems under seismic events.

Development of strength of materials for underground energy systems

In order to utilize underground space as a site for energy extraction and material circulation, it is a prerequisite to ensure the mechanical reliability of subsurface systems as well as aboveground systems. For this purpose, it is essential to address the issues related to strength of materials in rocks under the unique underground environment. In this presentation, research activities concerning the underground development, such as design and formation of engineered geothermal reservoirs by using hydraulic stimulation technology, are reviewed and future issues are discussed. The topics to be covered include modeling of the crack propagation behavior in underground systems under mechanical loads and characterization of subsurface complex crack systems using the fluid flow behavior in rocks.

Damage evaluation of rock salt under multilevel cyclic loading with constant stress intervals using AE monitoring and CT scanning

When salt caverns are used for the medium of underground energy storage, the surrounding rock experiences cyclic loading due to periodical injection-production. The cyclic load contains constant stress intervals and varied stress amplitudes, which threatens the stability of the salt caverns. In this study, multilevel cyclic loading with constant stress intervals on rock salt were conducted under uniaxial compression conditions. Acoustic emission (AE) monitoring and post-test computed tomography (CT) scanning were used to evaluate the damage evolution during the loading process. Results show that the density of the hysteresis loops in overall stress-strain curves present two-stage alternating sparse and dense loops during the loading. The cumulative AE count/energy increases with increasing the number of cycles and loading stage, and the growth rate for a single stage gradually becomes constant. The AE signals were divided into six kinds according to frequency-amplitude distribution. The presence of stress intervals increases the number of cracks and the median scale cracks account for about 90% of the total number of cracks. By analyzing the CT images, the crack morphology of the salt sample without hold time shows several inclined main cracks with numerous branch microcracks while that with the hold time of 20 s shows multiple branch cracks and numerous microcracks. To quantitatively evaluate damage evolution during the loading process, a damage variable is defined based on the relative variation of AE parameters. The damage accumulation for a single loading stage presents a two-phase pattern, and the damage degree during the accelerated deformation accounts for approximately half of the total damage. This study can provide guidance for designing the operation parameters when salt caverns are used for the storage of natural gas, air and CO 2 . • Multilevel cyclic loading tests with constant stress intervals are conducted on rock salt. • The AE parameters characteristics are analyzed. • The crack distribution and fracture behavior are visualized by CT images. • Damage evolution behavior is quantitatively evaluated.