Efficient Exploitation Research Articles

Control of strike-slip faults on Sinian carbonate reservoirs in Anyue gas field, Sichuan Basin, SW China

The largest Precambrian gas field (Anyue gas field) in China has been discovered in the central Sichuan Basin. However, the deep ancient Ediacaran (Sinian) dolomite presents a substantial challenge due to their tightness and heterogeneity, rather than assumed large-area stratified reservoirs controlled by mound-shoal microfacies. This complicates the characterization of “sweet spot” reservoirs crucial for efficient gas exploitation. By analyzing compiled geological, geophysical and production data, this study investigates the impact of strike-slip fault on the development and distribution of high-quality “sweet spot” (fractured-vuggy) reservoirs in the Ediacaran dolomite of the Anyue gas field. The dolomite matrix reservoir exhibits low porosity (less than 4%) and low permeability (less than 0.5×10−3 μm2). Contrarily, fractures and their dissolution processes along strike-slip fault zone significantly enhance matrix permeability by more than one order of magnitude and matrix porosity by more than one time. Widespread “sweet spot” fracture-vuggy reservoirs are found along the strike-slip fault zone, formed at the end of the Ediacaran. These fractured reservoirs are controlled by the coupling mechanisms of sedimentary microfacies, fracturing and karstification. Karstification prevails at the platform margin, while both fracturing and karstification control high-quality reservoirs in the intraplatform, resulting in reservoir diversity in terms of scale, assemblage and type. The architecture of the strike-slip fault zone governed the differential distribution of fracture zones and the fault-controlled “sweet spot” reservoirs, leading to wide fractured-vuggy reservoirs across the strike-slip fault zone. In conclusion, the intracratonic weak strike-slip fault can play a crucial role in improving tight carbonate reservoir, and the strike-slip fault-related “sweet spot” reservoir emerges as a unique and promising target for the efficient development of deep hydrocarbon resources. Tailored development strategies need to be implemented for these reservoirs, considering the diverse and differential impacts exerted by strike-slip faults on the reservoirs.

No-Man's-Land: Texas, Mexico, and International Deal-Making

In 2012, the United States and Mexico negotiated an agreement to determine how to address the transboundary hydrocarbon reservoirs in the Gulf of Mexico. This agreement generally comported with international law’s commitment to safe, efficient, and effective exploitation. Notably, however, the agreement did not cover any possible transboundary hydrocarbon reservoirs discovered within the first nine nautical miles of Texas’s coastline. This Comment discusses the constitutional and international law implications of this gap in the 2012 agreement between the United States and Mexico and how Texas and Mexico may cooperate to resolve any potential issues. As Texas cannot enter into a treaty with Mexico, this Comment proposes that Texas’s best option is to negotiate a memorandum of understanding (“MOU”) with the Mexican federal government that directs how exploration, development, and production should be handled in the event of a transboundary reservoir discovery. Although not binding in the same sense as a treaty, such an instrument would indicate the parties’ consent to act in a manner that comports with international custom. Any breach of such an agreement could have serious reputational repercussions, making each party more likely to abide by the MOU’s terms.

Battery–integrated combined cooling, heating and power plant (CCHP) through NH3 – H2O absorption system in a hospital facility

Combined heat and power (CHP) systems have been employed in various applications for years. They are gaining increasing attention in the residential and small industrial sectors for their primary energy saving potential and CO 2 emissions reduction. Additionally, in specific applications such as the hospital sector, CHP plants can play a critical role in replacing fossil-fueled electric generators to supply electricity during local grid outages, thus enhancing hospital facilities’ energy efficiency, while also securing reliability and efficiency of operation. Hospitals have unique energy demand profiles, with high and constant demand (particularly heating and cooling demands), making them an ideal use-case for trigeneration systems. Combined, cooling, heating and power plants (CCHP) are capable to provide not only heating and electricity, but also cooling through the efficient exploitation of a single energy source, reducing Hospitals’ reliance on the local grid. In this context, this work aims at evaluating the potential of a battery-integrated CCHP plant through an innovative ammonia-water absorption system, whose energy analysis is based on data acquired from an Italian Hospital facility. The potential of integrating an experimental combined cooling and power production ammonia-water absorption system has been investigated starting from an optimized battery energy storage system (BESS) CHP plant configuration, studied for the same facility. A proper control strategy has been developed to maximize the cooling production when required from the end-user through CHP plant’s exhaust gases waste heat recovery. The energy analysis demonstrated the advantages of the CCHP-BESS plant over the optimal CHP-BESS configuration, with an 11.22% increase in primary energy saving, a 9.85% reduction in CO 2 emissions, and a 5.03% decrease in electric peak power demand.

Impacts of Surface Deformation Induced by Underground Mining of Metal Mines on Above-Ground Structures: A Case Study

The surface deformation caused by underground mining seriously affects the normal life and personal safety of local residents and also causes unfavorable factors for the safe and efficient exploitation of underground resources. While the study of surface deformation caused by underground mining of metal mines requires a large amount of measured data as support, the measured data is particularly scarce, which severely hinders the study of surface deformation caused by underground mining. In this paper, in order to study the impact of underground mining on surface structures in metal mines, we take the Fu Lao Zhuang Iron Mine in Anhui Province, China, as the research object and put forward a comprehensive measurement method based on the flat plate beam theory. Using empirical formulas combined with the methods of thickness-to-span ratio and the relaxation coefficient, etc., we carry out numerical simulation calculations for the displacement of the surface triggered by the mining of the ore body by using FLAC3D software. We calculate the maximum inclination deformation, curvature, and horizontal deformation values of the ground surface by referring to the displacement and deformation with reference to the displacement and deformation formula; the maximum tilt deformation, curvature, and horizontal deformation values of the ground surface are calculated, and finally, the permissible values of the design specifications are combined to make a judgment. The research results of this paper put forward the prerequisite for improving the surface deformation induced by underground mining.

A model-based study of the evolution of gravel layer permeability under the synergistic blockage effect of sand particle transport and secondary hydrate formation

When using gravel-packed completions to exploit hydrates, the synergistic blockage of gravel filling layer by sand and hydrate often arises. In this paper, we completed two kinds of permeability measurement experiments: experiments with different volume proportions of sand particles and experiments with different hydrate saturations in the pores of gravel filling layer. The variation in the permeability of a gravel filling layer under the simultaneous blockage due to small sand particles invasion and hydrate formation was studied experimentally. Based on the different assembly forms of spherical particles and classic permeability theory models, this paper first established an experimental model of the permeability of gravel filling layer formed by the secondary formation of hydrates under different sand volume proportions. Second, this paper analyzed the permeability measurement data. A modified permeability model based on the synergistic blockage mechanism of sand and hydrate in the gravel filling layer was obtained. Finally, this study revealed the permeability evolution of the gravel filling layer under sand intrusion and hydrate formation. The experimental data and permeability theory model results supported the following conclusions: (1) The assumption of sand particles gathering in clusters in the pores has a similar impact on permeability as the formation of hydrates at the center of the pores, resulting in an exponential decrease in permeability as the proportion of sand particles increases. (2) Based on the changes in the surface areas of pores in a gravel filling layer during the processes of sand intrusion and hydrate growth, a theoretical model of the relative permeability of gravel filling layer was established. (3) Analyzing the permeability measurement experimental data revealed that the presence of small sand particles in the experiment was more conducive to the formation of hydrates, and a high saturation of hydrates may have formed in some areas, leading to more serious blockages in the gravel filling layer. This study validated the effectiveness of the theoretical model of relative permeability through permeability measurement experiments and explored the mechanism of cooperative sand and hydrate plugging, providing a theoretical basis for the safe and efficient exploitation of hydrate reservoirs.

Mechanism of increasing or inhibiting laser-weakened rocks by saturated fluids and mechanical behavior of rocks after laser damage

Laser technology has a wide range of application prospects as a novel means of assisting rock breaking. The realization of efficient laser-assisted breaking of deep hard rocks is based on revealing the mechanism of the saturated fluid increase or inhibition of laser-weakened rock. The laser irradiation of basalt with different water contents was used to analyze the mechanical response characteristics and investigate the destruction law of basalt under different laser power irradiations to explore the mechanism of the influence of water on the effect of laser rock breaking. The results show that the presence of water can significantly increase the drilling depth, and a laser irradiation of 1250 W can penetrate 56.5 mm deep into the rock in 30 s. The uniaxial compressive strength of basalt was reduced by a maximum of 38.23 % and the modulus of elasticity by a maximum of 74.99 % after the laser irradiation. Under low- power conditions, water has a positive effect on the laser rock-breaking efficiency. After 30 s of a laser irradiation in the range of 250–750 W, the uniaxial compressive strength of the water-saturated rock (WSR) samples was 12.64–13.43 % less than that of the original rock (OR) samples, and that of the natural state rock (NSR) samples was 1.09–3.93 % less than that of the OR samples. Both acoustic emission events and basalt energy significantly increased after a laser irradiation, and water promoted the expansion of rock microcracks. The degree of rock fragmentation increased with an increasing laser power. Laser irradiation can significantly reduce the strength of the rock, and water can effectively promote a low-power laser to break the rock, which is effective in reducing the difficulty and improving the efficiency of rock breaking. The obtained results provide theoretical and technical support for the application of the laser technology in the effective crushing of deep hard rock, and efficient and safe exploitation of deep resources.

Experimental study on the rheological characteristics and viscosity-enhanced factors of super-viscous heavy oil

To reveal the viscosity-enhanced mechanism of super-viscous heavy oil and improve the recovery rate of super-viscous heavy oil, the four components, elemental composition, rheological properties, and effects of asphaltenes and resin on the viscosity of super-viscous heavy oil from well TH12434 in Tahe Oilfield, China have been analyzed from macro and microscopic perspectives by Anton Paar rotational rheometer, gas chromatography-mass spectrometry and scanning cryo-EM to solve the problems of poor fluidity and high asphaltene content. The experimental results showed that in the temperature range of T= 40-100°C, the viscosity of super-viscous heavy oil decreases sharply from 352000 mPa∙s to 1620 mPa∙s, and the super-viscous heavy oil exhibits clear thermo-sensitivity. With T= 100°C and shear rate ranging from γ= 0-800 s-1, the viscosity of super-viscous heavy oil decreases sharply from 45000 mPa∙s to 956 mPa∙s, and the oil sample shows typical pseudoplasticity. The baseline of super-viscous heavy oil analysis by gas chromatography shows too high, and more than 80 % of super-viscous heavy oil compounds have a matching degree of less than 70 % with standard compounds, indicating that the super-viscous heavy oil had poor heterogeneity and many impurities. It is observed by scanning cryo-EM that the micromorphology of super-viscous heavy oil is large granular, strong continuity, asphaltene micromorphology presents an obvious layered structure, the layer spacing is 637.7 nm, and its asphaltene molecules form an order-like or crystal-like association structure through several unit sheet layers, resulting in high viscosity of super-viscous heavy oil. Based on the analysis results of the influencing factors of the viscosity of super-viscous heavy oil, a theoretical basis for the selection of viscosity reduction technology for super-viscous heavy oil the efficient exploitation in Tahe Oilfield, China could be provided.

NTNU-SINTEF SolarNet: A solar irradiation monitoring network at high latitudes

This study presents a monitoring network for solar irradiation at high latitudes, called NTNU-SINTEF SolarNet. The network collects, with a time resolution ranging from seconds to hours, solar irradiance data, e.g. global horizontal irradiation, diffuse horizontal irradiation, direct normal irradiation, global tilted irradiation, solar energy generation, which are required in solar irradiation modelling in built environments. The network will be used for specific applications, such as (i) anomalies detection, (ii) influences of ground albedo, and (iii) ageing/degradation of solar modules, that are described in this paper. Some characteristics that make the NTNU-SINTEF SolarNet relevant for solar energy research at high latitudes are identified: short distances among the sensors, the ease of data accessibility, the use of the same sensor typologies, and different solar module technologies. The research holds the potential to boost the solar energy digitalization, impacting on several aspects such as predictive and adaptive control strategies for energy management, design of renewable energy system, multi-scale optimization and efficient exploitation of solar energy.

Design Considerations of Time-Interleaved Discrete-Time Beamformers Toward Wideband Communications

Efficient exploitation of wide bandwidth data communication requires antenna array providing high gain across all frequency components for both transmit and receive equipment. In contrast to the frequency-dependent phased array, true-time-delay (TTD) arrays are appealing yet insufficiently investigated alternative for both fast initial access (IA) process and wideband directional data communications. In this brief, the mathematical relationship of delay before and after the frequency conversion is first discussed, which lays a foundation of the TTD beamformed system followed by a step-by-step design procedure and its tradeoff. Finally, system-level analysis is presented to estimate the minimum interleaving factor and silicon area for both IA and data communications associating the circuit design tradeoffs. The three-part analysis aims to provide a quick starter guide to design a TTD array.

Study on the performance and mechanism of high thermal conductivity and low-density cementing composite for deep geothermal wells

The efficient exploitation and utilization of deep geothermal energy can alleviate the energy crisis in the world today. The high thermal conductivity and low-density cementing composite (HTLDC) is helpful to improve the extraction efficiency of deep geothermal energy. In this paper, the optimal formulation of HTLDC is obtained by theoretical analysis, orthogonal test, matrix analysis, SEM, XRD, MIP and other methods. Then the basic properties, microstructure and performance changes at different temperatures are studied. The results show that the best formula of HTLDC is 0.8 W/S ratio, 60 wt% cement, 9 wt% graphite, 15 wt% quartz powder, 6 wt% silica fume, 3 wt% fly ash and 7 wt% admixture. The density of HTLDC is 1.53 g/cm3. The thermal conductivity is 1.6702 W/(m·K), which is 94.71 % higher than that of conventional low density cementing material (CLDC). The 48 h compressive strength is 7.63 MPa, which is 40.77 % higher than CLDC. The content of Ca(OH)2 is as high as 68.9 %, which is 20 times higher than CLDC. The harmful porosity is only 15.6 %, which is 46.65 % lower than CLDC. The thermal conductivity of HTLDC increases rapidly and then tends to be stable with the increase of temperature. The maximum growth rate of thermal conductivity is 0.0026 W/(m·K2), which is 136 % higher than that of CLDC. The thermal conduction mechanism of HTLDC is consistent with the heat conduction pathway theory. HTLDC has a good high-temperature resistance due to the SiO2 participates in the hydration reaction and generates C5S6H5 and C6S6H with the increase of temperature. The research results can provide strong technical support for the efficient exploitation of deep geothermal energy.

Model of shear strength of ultra-deep fractured sandstone considering fracture morphology

Ultra-Deep fractured tight sandstone gas reservoir, the hotpot unconventional petroleum resources, has been the subject of investigation recently. Due to the presence of natural fractures, drilling in ultra-deep fractured tight sandstone may cause complicated wellbore instabilities, which resulted in a substantial challenge to the efficient exploitation and production of tight gas resources. In order to understand the rock mechanical properties of the ultra-deep fractured tight sandstone under shear stress, tight sandstone core samples were artificially shear-fractured, the 3D characteristics of the fractures were reconstructed by CT scanning, and the fracture aperture and roughness characteristics were systematically evaluated. Simultaneously, triaxial shear strength tests were conducted to evaluate the mechanical properties of fractured rock subjected to varying confining pressures. Considering the contact relationship of the fracture surface during shearing, two indexes of average effective shear angle and contact area ratio were proposed to correct the fracture roughness, and a new fracture shear strength model was constructed, which served as the failure criterion for the fractured formation. The results show that the intact rock has a high compressive strength and can only collapse under conditions of substantial differential stress. Consequently, shear failure of fracture ought to be the primary source of wellbore instability. The fracture aperture's spatial distribution is irregular and conforms to a Gaussian distribution. A clear anisotropy in the fracture roughness is present, and there is less roughness along the initial shear failure path and more roughness perpendicularly. With an increase in normal stress, the fracture's peak shear strength rises, while the friction angle steadily falls. The innovative model's accuracy in predicting shear strength is higher than that of the weak-surface model and the JRC-JCS model.

Exploring high thermal conductivity polymers via interpretable machine learning with physical descriptors

The efficient and economical exploitation of polymers with high thermal conductivity (TC) is essential to solve the issue of heat dissipation in organic devices. Currently, the experimental preparation of functional polymers with high TC remains a trial-and-error process due to the multi-degrees of freedom during the synthesis and characterization process. Polymer informatics equips machine learning (ML) as a powerful engine for the efficient design of polymers with desired properties. However, available polymer TC databases are rare, and establishing appropriate polymer representation is still challenging. In this work, we propose a high-throughput screening framework for polymer chains with high TC via interpretable ML and physical feature engineering. The hierarchical down-selection process stepwise optimizes the 320 initial physical descriptors to the final 20 dimensions and then assists the ML models to achieve a prediction accuracy R2 over 0.80, which is superior to traditional graph descriptors. Further, we analyze the contribution of the individual descriptors to TC and derive the explicit equation for TC prediction using symbolic regression. The high TC polymer structures are mostly π-conjugated, whose overlapping p-orbitals enable easy maintenance of strong chain stiffness and large group velocities. Ultimately, we establish the connections between the individual chains and the amorphous state of polymers. Polymer chains with high TC have strong intra-chain interactions, and their corresponding amorphous systems are favorable for obtaining a large radius of gyration and causing enhanced thermal transport. The proposed data-driven framework should facilitate the theoretical and experimental design of polymers with desirable properties.

Study on acoustic properties of hydrate-bearing sediments with reconstructed CO2 hydrate in different layers during CH4 hydrate mining

Natural gas hydrate (NGH), a clean energy source with huge reserves in nature, and its safe and efficient exploitation fits perfectly with the UN Sustainable Development Goals (SDG-7). However, large-scale NGH decomposition frequently results in subsea landslides, reservoir subsidence, and collapse. In this work, in order to achieve safe and efficient exploitation of NGHs, the stability variation of different reservoir layers by depressurization/intermittent CO2/N2 injection (80:20 mol%, 50:50 mol%) was investigated using acoustic properties (P-wave velocity, elastic modulus), as well as reservoir subsidence under an overburden stress of 10 MPa. The P-wave velocity increased from 1282 m/s to 2778 m/s in the above-reservoir and from 1266 m/s to 2564 m/s in the below-reservoir, significantly increasing reservoir strength after CO2 hydrate formation. The P-wave velocity and elastic modulus in the top reconstructed reservoir were continually decreased by the shear damage of the overlying stress, while they remained stable in the bottom reconstructed reservoir during hydrate mining. However, due to superior pressure-bearing ability of the top CO2 hydrate reservoir, which was lacking in the bottom CO2 hydrate reservoir, the reservoir subsidence was relieved greatly. Despite the stiffness strength of reconstructed reservoir was ensured with CO2/N2 sweeping, the skeletal structure of CH4 hydrate reservoir was destroyed, and only the formation of CO2 hydrate could guarantee the stability of P-wave velocity and elastic modulus which was most beneficial to relieve reservoir subsidence. A large amount of CO2 was used in reservoir reconstruction and CH4 hydrate mining, which achieved the geological storage of CO2 (SDG-13). This work provided a new idea for safe and efficient NGHs mining in the future, and the application of acoustic properties served as a guide for the efficient construction of reconstructed reservoirs and offers credible technical assistance for safe exploitation of NGHs.

QTL mapping for kernel-related traits in a durum wheat x T. dicoccum segregating population.

Durum wheat breeding relies on grain yield improvement to meet its upcoming demand while coping with climate change. Kernel size and shape are the determinants of thousand kernel weight (TKW), which is a key component of grain yield, and the understanding of the genetic control behind these traits supports the progress in yield potential. The present study aimed to dissect the genetic network responsible for kernel size components (length, width, perimeter, and area) and kernel shape traits (width-to-length ratio and formcoefficient) as well as their relationships with kernel weight, plant height, and heading date in durum wheat. Quantitative Trait Locus (QTL) mapping was performed on a segregating population of 110 recombinant inbred lines, derived from a cross between the domesticated emmer wheat accession MG5323 and the durum wheat cv. Latino, evaluated in four different environments. A total of 24 QTLs stable across environments were found and further grouped in nine clusters on chromosomes 2A, 2B, 3A, 3B, 4B, 6B, and 7A. Among them, a QTL cluster on chromosome 4B was associated with kernel size traits and TKW, where the parental MG5323 contributed the favorable alleles, highlighting its potential to improve durum wheat germplasm. The physical positions of the clusters, defined by the projection on the T. durum reference genome, overlapped with already known genes (i.e., BIG GRAIN PROTEIN 1 on chromosome 4B). These results might provide genome-based guidance for the efficient exploitation of emmer wheat diversity in wheat breeding, possibly through yield-related molecular markers.

Numerical modeling of flashing in TFC expanders for the efficient exploitation of low-grade heat

This paper presents a novel semi-empirical thermodynamic model for the simulation of two-phase expansion in twin-screw expanders. The model utilizes a limited number of unknown parameters, the values of which are calculated through a calibration-optimization process that exploits numerical experiments available in the literature. Two-phase expansion simulations are performed for a wide variety of operating conditions, revealing that the vapor quality at the suction port of the expander is the main factor affecting its efficiency. Increased vapor qualities at the onset of expansion lead to an increase in the expander’s efficiency because a better matching between the WF’s volume ratio and the expander’s built-in volume ratio can be achieved. The thermodynamic model is applied to assess the performance of a TFC engine operating under varying heat duties and temperatures of the heat source, indicating that a detailed analysis is necessary to conclude whether the TFC or the ORC is the optimal solution for a given application.

Integrated experimental system and method for gas hydrate-bearing sediments considering stress-seepage coupling.

It is of great significance to study the mechanical behavior and permeability properties of hydrate-bearing sediments for a safe, efficient, and sustainable exploitation of hydrate. However, most of the studies conducted so far have focused only on a single stress field or seepage field, which is detached from practical engineering. In this paper, a new integrated experimental system (IES) was proposed, which realizes the coupling study of stress and seepage. The main body of IES is a triaxial subsystem and a seepage subsystem. The triaxial subsystem can realize in situ synthesis and triaxial shear of hydrate-bearing sediments (HBS). Stable seepage can be effectively formed using a constant pressure infusion pump and a back pressure valve. A series of shear-seepage coupling tests were carried out to verify the effectiveness of the IES and explore the stress-seepage coupling characteristics of HBS. The results show that stress has a significant influence on permeability, and its essence is the stress compression on the seepage channel. The stress-strain relationship, volume response, and permeability are related to each other. The permeability will be affected by the coupling of hydrate saturation (pore plugging), effective confining pressure (pore compression), and shear (fracture generation).

Dynamic characteristics of net anthropogenic phosphorus input to the upper Yangtze River Basin from 1989 to 2019: Focus on the phosphate ore rich area in China

Phosphorus (P), a non-renewable essential resource, faces heavy exploitation and contributes to eutrophication in aquatic environments. Assessing P input is vital for a healthier P cycle in the Upper Yangtze River (UYR), a phosphate ore rich basin, where P mining and P chemical enterprises have prominent pollution problems. This study modified the net anthropogenic phosphorus input (NAPI) model to include ore mining P input (Pore). We analyzed the evolutionary characteristics of P input in five sub-basins of UYR from 1989 to 2019 using prefecture-level data, and assessed the uncertainty of the data. NAPI in all sub-basins exhibited an upward and then downward trend during 1989–2019, with the inflection point occurring in 2015 or 2016, showing a net increase of about 1.1 times (568–1162 kg P km−2 yr−1) in the whole UYR basin. Among the components of NAPI, P fertilizer inputs (Pfer) and food/non-food and feed P inputs (Pf/nf&feed) contributed comparably, though the growth rate of Pfer was most notable basin-wide. Pore proportion increased significantly (about 3-fold), with a peak of 20%, especially in Wujiang sub-basin. The multi-year (1989–2019) average NAPI in UYR rose sequentially from west to east, with hotspot areas mainly concentrated in the Sichuan-Chongqing urban agglomeration and cities of Hubei province. The regional P input closely related to the population density and the level of agricultural development, certainly the phosphate mining was also unignorable. This study emphasizes that based on current status of NAPI development in UYR, targeted management for different regions should focus on improving agricultural P use efficiency and rational exploitation of P mineral resources.

Permeability properties of natural gas hydrate-bearing sediments considering dynamic stress coupling: A comprehensive experimental investigation

As a key exploitative parameter, permeability affects the extraction efficiency and program for natural gas hydrate (NGH). However, few studies have considered the effect of dynamic stress (continuously changing stress) on permeability, let alone permeability evolution during reservoir instability. Given the dynamic stress-seepage coupling process of actual NGH exploitation, this paper upgrades the triaxial apparatus to new equipment that can realize the unified tests of dynamic stress and water permeability. Subsequently, the seabed clay recovered from the South China Sea and quartz sand were used to remold fine-grained hydrate-bearing clayey-silty sediments (HBCSS). Finally, a series of stress-seepage coupling tests were carried out to reveal the permeability characteristics during sediment destabilization and to explain the coupling mechanism. The results show that seepage can reduce the strength and stiffness, and enhance the softening or weaken the hardening of the stress-strain curve. The dynamic stress-influenced permeability exhibits a non-monotonic evolution, which is essentially an expansion or contraction of seepage channels. Stress-strain relationship, volume deformation, and permeability are coupled and have good correspondence. The shear-seepage coupling process can be divided into four stages: initial compression, plastic strain development, plastic expansion and fracture development, plastic failure, and rapid expansion. This paper enriches the existing research from three aspects: permeability properties affected by dynamic stress, mechanical behavior affected by seepage, and stress-seepage coupling mechanism. It has played a significant reference for the safe and efficient exploitation of fine-grained NGH.

Operational characteristics of the super-long gravity heat pipe for geothermal energy exploitation

As a novel geothermal heat transfer technology, the super-long gravity heat pipe (SLGHP) demonstrates great potential for the efficient exploitation of deep geothermal energy. Despite the rapid advances made to the SLGHP technology, its operational characteristics remain to be fully investigated. In this study, the underlying phenomenological mechanisms of the SLGHP are compared with those of normal-size gravity heat pipes (NSGHPs), and the operational characteristics of the SLGHP are analyzed in view of its distinctive geometric features. It is noted that the thermal resistance caused by the vapor flow is negligible in the NSGHPs, while it dominates the overall thermal resistance of the SLGHP. The dominance of the vapor flow thermal resistance allows making the one-dimensional simplification (along the pipe length direction) in the mathematical model of the SLGHP geothermal energy extraction system. Under these circumstances, the temperature gradient along the SLGHP length can be theoretically deduced based on the pressure drop of vapor flow. Close observation of the temperature gradient expression indicates that the desirable SLGHP working fluid should have the following properties: i) large latent heat of vaporization, and ii) a strong dependence of the saturated vapor pressure on the temperature. In cases where the saturated vapor pressure of the working fluid is highly dependent on the temperature, as in the case of ammonia, the working temperature profile of the SLGHP can be formulated as a linear function of the depth. This unique working temperature characteristic offers valuable guidance to determine the length of the SLGHP adiabatic section in practical geothermal applications. The on-site observed operational characteristics provide the necessary theoretical foundation for modeling the multiphase heat transfer process in SLGHP, which will lead to a valuable engineering analysis and design tool supporting the application and commercialization of the SLGHP technology.

Study on mechanical properties and microscopic damage mechanism of tight sandstone reservoir under uniaxial compression

Due to the characteristics of low porosity, low permeability and serious anisotropy in tight reservoirs, it is difficult for conventional hydraulic fracturing theory to accurately guide the efficient exploitation of tight reservoirs. It has been shown that the reservoir rock mechanical properties are the key factor impacting the fracturing effect, but the current research on the damage properties of tight reservoir rocks is not comprehensive enough. Therefore, in order to improve the fracturing theory of tight reservoirs, this paper first explores the evolution mechanism of rock fractures through uniaxial compression experiments. Secondly, based on the particle discrete element method, the damage and failure process of tight sandstone under uniaxial compression is simulated from the microscopic scale. The test results show that the rock failure mainly includes tensile failure, shear failure, and tensile-shear failure; Internal micro-fractures will interconnect during rock destruction to form primary fractures through the rock mass, while secondary micro-fractures will also be generated. The numerical simulation results show that when the rock is subjected to tensile-shear failure, with the increase of load, tensile micro-fractures are mainly produced in the specimen, accompanied by a few shear fractures. Under the joint action of shear failure and tensile failure, V-shaped cracks are easily formed in rock. The tensile strength of rock is mainly affected by the microscopic tensile strength, and the cohesive force, modulus, stiffness ratio, friction coefficient and friction angle have significant effects on the compressive strength of rock. Therefore, a reasonable choice of microscopic parameters can realistically simulate the compression-tensile strength ratio of the rock. The research results of this paper can provide the theoretical basis of rock mechanics for the efficient exploitation of tight reservoirs.