Porous Rocks Research Articles

Experimental study into the propagation and attenuation of blasting vibration waves in porous rock-like materials

Rock blasting vibration can cause harm to the surrounding environment. This article aims to investigate the propagation and attenuation of vibration waves in the blasting excavation of porous rock. Similar materials were used to simulate porous rock media and indoor blasting experiments were conducted on 12 porous rock-like models poured to estimate influences of the media material, porosity, moisture conditions, and decoupling coefficient of blast holes on the propagation of blasting stress waves. The results show that: 1) the crack propagation path of vibration waves in foam ceramics similar materials (FC) is not a completely straight line: cracks tend to produce a large deflection during the development process; 2) damage modes of low-porosity similar materials are mainly dominated by crack development, while damage and failure of high-porosity similar materials involve crack expansion and crushed fragments; 3) the peak vibration acceleration presents exponential decay with the distance, which will not vary with changes in the media material, porosity, moisture conditions, and the decoupling coefficient of blast holes; 4) the peak vibration acceleration of cement-based similar materials (SM) demonstrates the exponential decay coefficient of −1.4 ∼ −1.0, the exponential decay coefficient of the peak vibration acceleration for FC is −0.8 ∼ −0.4. The peak vibration acceleration of high-porosity similar material shows a faster decay rate, which is generally 0.3 less than that of the low-porosity similar material; 5) the type of material exerts the most significant controlling effect on the decay coefficient of peak vibration acceleration, followed by the effects of porosity and degree of water saturation; the decoupling coefficient of blast holes does not exert any significant influence on the decay of peak vibration acceleration.

Microstructural Controls on the Uniaxial Compressive Strength of Porous Rocks Through the Granular to Non‐Granular Transition

AbstractUnder uniaxial compression, a porous rock fails by coalescence of stress‐induced microcracks. The micromechanical models developed to analyze uniaxial compressive strength data consider a single mechanism for the initiation and propagation of microcracks and a fixed starting microstructure. Because the microstructure of clastic porous rock transitions from granular to non‐granular as porosity decreases during diagenesis, their strength cannot be captured by a single model. Using synthetic samples with independently controlled porosity and initial grain radius we show that high‐porosity granular samples, where microcracks grow at grain‐to‐grain contacts, are best described by a grain‐based model. Low‐porosity non‐granular samples, where microcracks grow from pores, are best described by a pore‐based model. The switch from one model to the other depends on porosity and grain radius. We propose a regime plot that indicates which micromechanical model may be more suitable to predict strength for a given porosity and grain radius.

Can effective porosity be used to estimate near-well protection zones in fractured chalk?

Protection of fractured carbonate aquifers is often based on a single-porosity description of a dual-porosity system. However, it is difficult to assess a trustworthy value of the effective porosity based on scientific principles; thus, a range of estimates is often suggested. The complexity of the problem is compounded by the fact that the effective porosity may be scale-dependent. This paper investigates whether it is possible to describe solute transport in fractured carbonate rocks with an equivalent porous medium model using a constant value of effective porosity. It is assumed that the dual-porosity model provides an acceptable description of transport mechanisms in fractured porous rock and that it is possible to estimate the parameters needed in the single-porosity models from results generated by the dual-porosity model. The effective porosity is estimated from the dual-porosity results that are used as targets. For Danish chalk, an effective porosity of 13% (11–17%) is estimated. However, it is demonstrated that the estimated effective porosity is only valid at the specific transport time (1 year) from which simulation results of the dual-porosity model were extracted. The effective porosity is shown to increase with travel time until equilibrium conditions are realised between the fractures and matrix, following which, the effective porosity equals the matrix porosity and will maintain this value at larger transport times. Assuming that the dual-porosity model provides a trustworthy description of solute transport in fractured chalk and limestone, a method to estimate the effective porosity of an equivalent porous medium model is presented.

Effect of the loading condition on the statistics of crackling noise accompanying the failure of porous rocks.

We test the hypothesis that loading conditions affect the statistical features of crackling noise accompanying the failure of porous rocks by performing discrete element simulations of the tensile failure of model rocks and comparing the results to those of compressive simulations of the same samples. Cylindrical samples are constructed by sedimenting randomly sized spherical particles connected by beam elements representing the cementation of granules. Under a slowly increasing external tensile load, the cohesive contacts between particles break in bursts whose size fluctuates over a broad range. Close to failure breaking avalanches are found to localize on a highly stressed region where the catastrophic avalanche is triggered and the specimen breaks apart along a spanning crack. The fracture plane has a random position and orientation falling most likely close to the centre of the specimen perpendicular to the load direction. In spite of the strongly different strengths, degrees of 'brittleness' and spatial structure of damage of tensile and compressive failure of model rocks, our calculations revealed that the size, energy and duration of avalanches, and the waiting time between consecutive events all obey scale-free statistics with power law exponents which agree within their error bars in the two loading cases.

Study of nuclear magnetic resonance response mechanism in shale oil and correction of petrophysical parameters

Shale oil reservoirs, characterized by a low porosity, complex mineral composition, and heterogeneity of organic matter distribution, are common unconventional oil and gas reservoirs. Nuclear magnetic resonance (NMR) methods have great potential for the evaluation of petrophysical parameters in porous rocks. However, it is challenging to characterize the petrophysical properties of shale oil due to the complex mineral composition and heterogeneity of organic matter distribution. In this paper, a new NMR relaxation theory for shale was proposed for the first time, and the frequency dependencies of transverse surface relaxivity and internal magnetic field gradient for pore fluids were presented. The accuracy was verified by the extraction experiments, gas chromatography experiments, and multi-frequency NMR experiments for the first time. These experiments were also utilized to determine the transverse surface relaxivity and the transverse bulk relaxation time. Subsequently, the effects of the pyrite and clay minerals on shale oil T2 distributions at different instrument frequencies were analyzed with random walk simulations based on digital core technology for the first time. The results show the echo spacing, instrument frequency, pyrite content, and clay minerals are important parameters affecting the NMR response. Compared with the oil in organic pore, these factors have a greater impact on the NMR response characteristics for water in inorganic pore. When the content of pyrite is 5.3 % and the echo spacing is 0.2 ms, the numerical simulation results show that the NMR porosity of water in inorganic pore is 65.3 %, 12.3 %, and 1.6 % of the real porosity at the instrument frequencies (IF) of 2 MHz, 21.36 MHz, and 200 MHz, respectively. When chlorite with high magnetic susceptibility exists near the formation pores, it can also affect the NMR response of shale oil. The results show the effects of these factors on porosity and T2ML are non-negligible. The cross-plots of petrophysical parameters correction from shale oil T2 distributions were established for the first time. This work provides a theoretical guide for the NMR-based shale oil evaluation, which may be helpful for petrophysical parameter conversion between downhole and laboratory NMR instruments.

Effects of Mixing at Pore Intersections on Large‐Scale Dissolution Patterns and Solute Transport

AbstractThe flow‐induced dissolution of porous rocks governs many important subsurface processes and applications. Solute mixing, which determines pore‐scale concentration fields, is a key process that affects dissolution. Despite its importance, the effects of pore‐scale mixing on large‐scale dissolution patterns have not been investigated. Here, we use a pore network model to elucidate the mixing effects on macroscopic dissolution patterns and solute transport. We consider two mixing rules at pore intersections that represent two end members in terms of the mixing intensity. We observe that the mixing effect on dissolution is the strongest at moderate Damköhler number, when the reactive and advective time scales are comparable. This is the regime where wormholes spontaneously appear. Incomplete mixing is shown to enhance flow focusing at the tips of the dissolution channels, which results in thinner wormholes and shorter breakthrough times. These effects on passive solute transport are evident independent of initial network heterogeneity.

Investigating the Impact of Pores on Rock Damage during Thermal Spalling Drilling

Numerous microcracks and pores in geological rock formations cause early flaws. High temperatures increase these fractures and pores, thermally damaging reservoir rocks and changing the rock failure mechanism. However, research on pores' high-temperature thermal spalling and fragmentation effects on heterogeneous rocks is sparse. This study built a finite element numerical model of heterogeneous granite rock thermal damage with pores based on rock thermal fracture theory and the Voronoi method and explored the mechanism under varied pore settings. The research's findings indicate that the application of high temperatures to local heterogeneous porous rocks results in a higher proportion of tensile damage. The proportion of shear damage and tensile damage constantly varies due to the changing position and shape of the pores. The rock's porosity has the effect of decreasing temperature in the direction of heat transfer while increasing the extent of temperature transfer along the pore parallel to the heating surface. The potential degree of damage increases as the density of pores increases, the distances between them decrease, and the pore lengths increase. The thermal damage resulting from heating in the vicinity of the pore is primarily localized in the area between the pore and the heated surface. This effect becomes more significant as the distance between them decreases. The findings of this study can serve as a theoretical framework for understanding the impact of rock pores on rock thermal fracturing and fragmentation in the thermal spalling-assisted development of deep oil and gas resources.

Acoustic signatures of porous rocks permeated by partially saturated, aligned planar fractures

AbstractThe presence of sets of vertical, parallel fractures is very common in the Earth's upper crust. Notably, open fractures exert critical control on the mechanical and hydraulic properties of the host formation. There is great interest in understanding how fractures interact with seismic waves, as this knowledge could be used to detect and characterize fractures from seismic data. When a seismic wave travels through a fractured formation, it induces oscillatory fluid pressure diffusion between the fractures and the embedding porous background, a physical process that produces attenuation and dispersion of the seismic wave. Although there are numerous studies on this topic, the case of parallel fractures saturated with different immiscible fluids, such as brine and CO2, remains rather unexplored. With these motivations, in this work, we propose an analytical approach to compute the phase velocity and attenuation of P‐waves travelling perpendicularly to a set of planar, parallel fractures. While we consider that the background is saturated with brine, the fractures can be saturated with brine or gas. Our numerical analysis shows for the first time that two manifestations of fluid pressure diffusion arise in these cases. One of them, associated with relatively high levels of attenuation, is due to fluid pressure diffusion occurring between consecutive fractures saturated with different fluids. In this case, the fluid pressure diffusion process is initiated at a fracture saturated with brine and reaches a consecutive fracture saturated with gas. The other fluid pressure diffusion manifestation, on the other hand, arises at higher frequencies, is characterized by lower levels of attenuation and results from the interaction in the background of fluid pressure diffusion processes initiated at consecutive fractures, irrespective of the fluid content. Finally, by considering a stochastic distribution of the two fluids and a classical frequency of interest in seismic experiments, we obtain P‐wave attenuation and phase velocity as functions of the fracture gas saturation. This approach could be of interest for the remote detection and quantification of pore fluids using seismic waves.

Cyclic confining pressure and rock permeability: Mechanical compaction or fines migration

The apparent rocks permeability, measured under cyclic loading, tends to constantly decrease with each subsequent loading/unloading cycle, the main reason for this, the researchers consider irreversible deformations. However, this understanding is not complete, as attention is not paid to the method of determining the permeability. Under cyclic loading, permeability is measured by fluid flow, which is able to carry colloids and they can clog pore throats and reduce permeability, but this is not taken into account in the study of porous rocks. Colloidal migration is also not taken into account in routine core tests, although colloids can significantly reduce the apparent permeability of rocks during migration. Also it is impossible to determine exactly which of the factors (mechanical compaction or colloid migration) contributed to the change in apparent permeability. The purpose of this research is to provide experimental evidence that under cyclic loading, permeability decreases not only due to mechanical compaction, but also due to colloid migration within the porous medium. For this task, a methodology has been developed, the feature of which is that between loading/unloading cycles, the core is blowning with a large pressure gradient. It has been established that under cyclic loading, the apparent permeability of rock samples changes as a result of colloid migration up to 20 %. The dynamics of permeability becomes unpredictable and weakly depends on the confining pressure when changing the direction and regime of the gas flow. At the end of the multi-cycle confining pressure test, the apparent permeability of 3 out of four core samples, despite the decrease during injection, recovered to the initial value which also confirms that the apparent permeability has affected by colloid migration. A mechanism for changing the apparent permeability during gas injection and cyclic confining pressure is proposed. The outcomes indicate that colloid migration should be taken into account during core tests not only in cyclic loading but also in routine coreflooding tests.

U–Pb Dating of Fibrous Dolomite in the Hydrothermal Dolostone of the Dengying Formation, Central Sichuan Basin, and Its Response to Supercontinent Breakup

Super-deep drilling in the central Sichuan Basin encountered volcanic rocks of the Suxiong Formation, which are overlain by multiple hydrothermal alterations within the upper section of the Ediacaran Dengying Formation. This provides an excellent research opportunity to understand the pre-Cambrian hydrothermal activity and geological evolution of the western margin of the Yangtze Craton. Observations revealed the development of a series of hydrothermal dolomite aggregates characterized by the presence of brown sphalerite within fractures and pores of the carbonate rock. Microscopically, the dolomite exhibited fibrous columnar crystallization, forming radial bands with a gradual decrease in crystallization intensity from the center to the periphery. Cathodoluminescence analysis revealed the presence of approximately eight dolomite bands within the aggregate. U–Pb dating from the inner to outer bands yielded isotopic ages of 781 ± 12 Ma for the second band, 683 ± 12 Ma for the fifth, 562 ± 12 Ma for the sixth, and 545.4 ± 6.9 Ma for the seventh. The in situ rare earth element (REE) distribution patterns of the 781 and 683 Ma dolomite bands exhibited similarities. They both showed low total REE content (∑REE), with significant fractionation between light and heavy REEs. Additionally, they exhibited negative anomalies in cerium (Ce) and europium (Eu), while heavy REEs were relatively enriched. The dolomite bands at 562 and 545 Ma also exhibited similar REE characteristics, with low ∑REE and weak fractionation between light and heavy REEs. They also displayed distinct negative anomalies in Ce and Eu, indicating similar distribution patterns. These findings suggested that formation of the banded hydrothermal dolostone occurred during different tectonic events, and the presence of heavy REE-enriched hydrothermal fluids suggested a deep-seated origin. This study has provided preliminary evidence that the Dengying Formation, previously considered to be of the Ediacaran age, has undergone multiple episodes of deep-seated fluid infiltration and alteration since the Mesoproterozoic Era. Importantly, these events coincide with the rifting of the Rodinia and Pannotia supercontinents, aligning with their respective timeframes. This finding raises questions regarding the stratigraphic division and correlation of the formations in the deeply buried core area of the basin.

Gravity-driven flow in a cross-bedded porous rock

Many geological layers include cross bedding, which leads to different values for permeability along and across the bedding planes. We explore how such cross bedding impacts buoyancy-driven flow through an inclined aquifer. For each bedding angle and ratio of the permeability along and across the bedding, a free buoyancy-driven plume rises at a particular angle to the horizontal. If the angle of inclination of the aquifer to the horizontal is smaller than this angle, then the plume rises along the upper boundary, otherwise, somewhat surprisingly, the buoyant plume rises along the lower boundary of the aquifer. We present new laboratory experiments to support these predictions. We also test a model for the effective permeabilities which control the speed and the rate of spread of the plume along one or other boundary of the aquifer. We consider the impact of our results for modelling geological storage of ${\rm CO}_2$ or aquifer thermal energy storage.

Fractal analysis of rock pore structure: A comparative study of the Matlab-based image method and the mercury intrusion method

As a naturally porous medium, rocks have a very strong heterogeneity in their internal pore structure, which has an important impact on the mechanical and chemical properties of rocks, etc. Therefore, it is very important to quantitatively characterise the heterogeneity of the pore structure of rocks. Fractal dimension has long been recognised as an effective means of characterising the heterogeneity of the pore structure of porous media and is widely used in oil and gas exploitation, construction materials, mining, and water engineering. There are different methods to calculate the fractal dimension. To verify the consistency between different methods, this paper compares the results of fractal analysis using the mercury intrusion method and MATLAB image fractal analysis using three rock samples with large differences in porosity and calculates the fractal dimension in three different ways on the basis of the mercury intrusion method. The results demonstrate that the fractal dimension of the mercury intrusion method and the box counting dimension of the image analysis obtained by the three methods, although slightly different in numerical value, are consistent in their numerical relationship, i.e., they all conform to the rule that the stronger the non-homogeneity of the pore throat, the larger the fractal dimension. The results of this paper show that fractal dimension is indeed an effective means of characterising rock homogeneity.

Research on flow field characteristics in the karst tunnel face drilling hole (conduit) under the coupling between turbulence and seepage

Tunnel water inrush is one of the most common engineering problems during tunnel construction and operation, especially in karst aquifers. Among them, tunnel face drilling holes and karst conduits are strong water conductivity channels (called the “conduit” here), while the porous rock matrix is a weak water conductivity area. A second-order accuracy tunnel face water inrush model is established to investigate the coupling between the turbulent flow in the conduit and the seepage in the porous rock matrix. In the model, the incompressible Reynolds average Navier–Stokes (RANS) equation and the Brinkman-extended Darcy equation are used to govern the turbulence in the conduit and seepage in the porous rock matrix, respectively. The expressions of velocity distribution and water inflow discharge are derived requiring the velocities and their gradients of the turbulent and seepage flow to be equaled at the flow-solid interface. The validity of the present model is verified by the existing analytical solutions and COMSOL numerical solutions. Furthermore, parameter sensitivity results show that the velocity has a positive relation with the Laplace parameter, Reynolds number, Darcy number, and conduit radius. Finally, the tunnel face water inrush in the new Zhongliangshan expansion tunnel is selected as a real field engineering case. The error of the water inflow discharge in the tunnel face drilling hole between the present predicted results and the field measured results is within 10%. After introducing cosine fluctuation with amplitude 0.15 and period 500 s to optimize the model, the error for the stable water gushing stage can be reduced to within 5%. This model can provide a reference for predicting the water inflow discharge in the drilling holes or karst conduits on tunnel faces.

Using digital cores and nuclear magnetic resonance to study pore-fracture structure and fluid mobility in tight volcanic rock reservoirs

Pore-fracture structure characterization and fluid mobility analysis are key to the effective development of tight reservoirs. However, it is difficult to accurately characterize the pore-fracture structure of tight volcanic rocks using a single method, and the mechanisms of fluid mobility are not fully understood. A combination of X-ray diffraction, casting thin section observations, scanning electron microscopy, high-pressure mercury injection, constant-speed mercury injection, nuclear magnetic resonance (NMR), X-ray computed tomography, and the advanced mathematical algorithms in the AVIZO visualization software was used to analyze the Permian tight volcanic rock in the ZhongGuai area of the Junggar Basin to investigate its overall pore-fracture structure characteristics. On this basis, NMR centrifugation experiments were conducted to monitor the fluid migration dynamics in the tight volcanic rocks, and the mobile fluid migration characteristics were studied based on the NMR T2 spectra. The results show that the porosity and permeability of the samples are 0.2–18.7 % (avg. of 8.3 %) and 0.001–6.680 × 10−3μm2 (avg. of 1.602 × 10−3μm2), respectively, and the various pore types are due to the differences in cementation and compaction. The distribution of the pore throats are mainly contiguous and isolated, and the connected pores are mainly distributed in enriched bands, which is due to the interconnection of gas pores, intergranular pores, and dissolution fractures; while the disconnected pores are mainly isolated, which is related to the development of inter-gravel dissolved pores and matrix dissolution pores. In addition, the contribution of the pore connectivity to seepage is greater than that at the pore scale. The pores in the tight volcanic rock have a wide range of sizes, from 3 nm to 120 μm, and are dominated by Gaussian and bimodal distribution patterns. The micron-scale pore radius in this area is mainly 4.47–31.56 μm. Also, the fractures can be divided into three types according to their occurrence and openings. They are mainly high-angle structural fractures and vertical fractures; the pore-fracture structures have strong heterogeneity, and the fractures play a larger role on the seepage of oil and gas. The connectivity of the pore throats in the tight matrix is poor, and the pore throat structure has a great influence on fluid mobility. Subsequently, the movable fluid saturation increases with increasing permeability, and the fractures and micropores have less flow resistance and are more conducive to water flow than small pores. This study provides new insights for the exploitation of similar tight reservoirs.

Ultrasonic P-wave to determine pore parameters of Spergen limestone

Abstract Ultrasonic waves are capable of helping characterize pores of rocks. A model of viscous squirt is used to simulate phase velocity and the quality factor of ultrasonic P-wave measured in water-saturated Spergen limestone, thus determining pore parameters of the limestone. The measured P-wave had a centroid frequency of ∼0.75 MHz, and two simulations are conducted in this paper. The first simulation yields a dispersion curve with a dipping spike followed by a rising spike. However, it cannot yield the measured quality factor. Another drawback is that one of the pore parameters violates rock physics. The second simulation yields a dispersion curve with a small velocity depression followed by an upward velocity concave; the measured phase velocity and quality factor are simultaneously predicted. The resulting dimensions of the rock unit are 0.3 by 0.333 mm, which is consistent with the mean grain diameter of 0.3 mm. The relative first and second porosities are ascertained to be 0.97 and 0.03, respectively. The aperture distance at contact of grains is inverted as 1.8 µm. Remarkably, the minimum phase velocity of the water-saturated limestone is lower than the Gassmann velocity.

Frost-heaving characteristics and formation mechanism of saturated sandstone under unidirectional freezing conditions

In this study, a series of frost-heaving experiments were conducted on saturated sandstone at varying cooling rates under unidirectional freezing conditions. The frost-heaving strain in the freezing process was examined using a novel laboratory testing methodology. The results showed that the frost-heaving behavior of the saturated sandstone was significantly affected by the freezing direction and rate. Specifically, the rapid freezing of sandstone parallel to the freezing direction resulted in a more pronounced frost-heaving strain than that perpendicular to the freezing direction. The rock freezing rate had a positive correlation with the peak frost-heaving strain parallel to the freezing direction, and a negative correlation with that perpendicular to the freezing direction. A microstructure-based conceptual model is proposed to determine the frost-heaving pressure. The conceptual model showed that the ice pressure, determined by the pore-water pressure and capillary force, resulted from the crystallization of the pore water within the pore structure. A nonlinear relationship was observed between the ice pressure and crystallization rate. Capillary forces were more significant for slow crystallization processes, resulting in a larger difference between ice and water pressures. At high crystallization rates, the hydraulic pressure acted on the pore structure before the capillary forces become significant, leading to a smaller difference between the water and ice pressures. Rapid crystallization processes resulted in the uneven distribution of ice pressure within the pore structure, with subsequent re-equilibration occurring with the migration of unfrozen water. The proposed conceptual model presents a new perspective for obtaining an improved understanding of the microscopic frost-heaving mechanism involved in the freeze-thaw damage of porous rocks. It provides an evaluation method for the behavior of rocks during weathering by frost heave in cold regions.

Modeling of formation damage during smart water flooding in sandstone reservoirs

Impairment of permeability has been observed as an effective factor in production decline during secondary and tertiary recovery processes such as water flooding. Among different permeability damage mechanisms, fines migration and deposition is known as the main mechanism which occurs due to pore throat clogging and blocking. Because injected water and formation water are usually incompatible, permeability damage evaluation and scale formation prediction must be done before the water flooding process in the oil field is implemented. For this purpose, compatibility tests and core flood experiments are common, but experimental approaches with time and facility limitations are expensive. Thus, by decreasing the time required for conducting experiments, modeling approaches can replace the routine laboratory experiments. Based on thermodynamic balance and the solubility of ions in water, scale development due to seawater injection in an Iranian oil field was predicted in this work using the OLI ScaleChem software. After that, it was suggested that special water be introduced to help reduce the amount of scales that had accumulated in the rock pore space. The extent of permeability damage in various seawater injection scenarios was then assessed via dynamic core flood experiments. Finally, scales-seawater injection into the core was simulated using digital core analysis (DCA) results and the pore scale modeling approach. The core flood experiment data are consistent with the scale formation prediction made by the OLI ScaleChem software, which indicates that smart water can be determined by optimizing the salinity and ion content of injected water. Also, results of permeability damage prediction by our modeling approach have good agreement with the core flood experiment data. Therefore, our modeling approach can replace the conventional core flood experiments as a low-cost method with high computational efficiency and high enough accuracy to evaluate formation damage in the water flooding process.

Hydrogen-Driven Microbial Redox Reactions in Deep Geosystems

In the subsurface, biotic and abiotic processes can generate and consume hydrogen. Hydrogen has a low reduction potential and is thus a highly energetic electron donor when involved in sulfate, carbon dioxide or ferric iron reduction. Although known as important drivers for the deep biosphere, the contributions of different processes to hydrogen turnover in different geosystems still are not well understood. In context with the ongoing transformation to renewable energy resources, underground H2 storage (UHS) in deep porous or salt cavern systems came into focus. In situ microbial and geochemical reactions that consume H2 are highly relevant topics in deep biosphere research, and also are still a major uncertainty during UHS. Consequently, we studied the potential microbial hydrogen oxidation rates – combined with the possible production of metabolic products like H2S, acetic acid or CH4 - in formation fluids from natural gas fields and salt caverns, thereby considering the importance of in situ pressure and temperature conditions, fluid chemistry and mineral composition. In addition, more defined experiments were conducted with selected pure cultures representing important metabolic groups of deep biosphere microorganisms. Several original formation fluids showed immediate H2 consumption. Microorganisms oxidized hydrogen at relevant in situ pressure conditions (up to 100 bar) and tolerated dynamically changing pressure and temperature conditions. The microbial hydrogen oxidation rate was strongly dependent on H2 partial pressures and the availability of e.g., sulfate as a terminal electron acceptor. High-throughput sequencing of 16S rRNA gene amplicons indicated hydrogen oxidation by sulfate reducing bacteria to be the presumed process in the studied porous rock reservoir fluids. In addition, hydrogen turnover by methanogenic and acetogenic as well as iron-reducing microorganisms was investigated. Also, the importance of biotic reactions in relation to abiotic hydrogen turnover processes at mineral surfaces will be discussed.

Underground hydrogen storage: A recovery prediction using pore network modeling and machine learning

Understanding the hydrogen-brine transport properties and the hydrogen trapping rate (i.e., ratio of residual hydrogen saturation after recovery to initial hydrogen saturation) is critical to the site selection for underground hydrogen storage (UHS). In this study, a three-dimensional pore network model (PNM) was used to simulate hydrogen-brine two phase flow in various porous media, including sandstone, carbonate, and sand packs. Surface contact angles measured from previous experiments were used to study the influence of the wettability on hydrogen transport in porous media. Many studies have investigated the impact of these factors on carbon dioxide sequestration. However, because of the difference in the thermal dynamic properties of the fluids and the purpose of UHS and carbon dioxide sequestration, it is still essential to analyze the UHS performance under the different rock and fluid properties. PNM simulations showed that a relatively larger contact angle with low water affinity was more suitable for UHS due to its low hydrogen trapping rate. Two machine learning methods, the least square fitting and the support vector machine (SVM), were developed to classify the ability of a rock to trap hydrogen and to predict hydrogen trapping rates. Hydrogen trapping rates simulated using the PNM were used as the training data in the machine learning models. The SVM classified rock samples into two groups which had high hydrogen trapping rates (>50 %) and low hydrogen trapping rates (<50 %). The machine learning results showed that rock samples with a low ratio of pore size to throat size and high pore connectivity (i.e., average number of throats connected to a given pore) were favorable for a low hydrogen trapping rate. This study illustrated that the impact of both rock surface wettability and pore structural geometry should be accounted for when evaluating a hydrogen-brine two-fluid system in porous media. This work is the first study that combined PNM simulation with machine learning to investigate the influence of rock surface wettability and pore structure features on the hydrogen trapping rate, which will provide insights into site selections and decision making in large-scale UHS projects.

Analysis of Economic Benefits of Using Deep Geological Storage Technology to Treat High-Salinity Brine in Coal Mines

Some provinces in China require zero discharge of coal mine wastewater, with a focus on the disposal of high-salt water, because evaporation ponds have been completely banned. Deep geological storage (DGS) technology is a novel geological environment solution that uses rock pores and microfissures within deep strata for safely storing liquid or gas to avoid its environmental impact on the biosphere. The author and his research team were the first to put forward the research idea of using DGS technology to dispose of high-salinity brine in coal mines in China and performed related research. Taking a coal mine in the south of Ordos Basin as an example, this study designed a conventional, mine-specific, zero-discharge water treatment process route based on a evaporation–crystallization process. This strategy was tailored to the unique water inflow conditions of the mine. Furthermore, the technical and economic efficiencies were assessed for the generation and treatment scenarios of a four-stage highly concentrated brine solution. In addition, the comparative analysis of the economic prospects of using DGS technology to treat high-salinity brine revealed that combining DGS with post-conventional treatment in secondary reverse osmosis, whose flow quality is 481 m3/h and TDS is 24,532.66 mg/L, can maximize the economic benefits. This integration heightened water resource utilization while maintaining a cost-effective, comprehensive water treatment approach. These results provide a valuable reference value for the future zero-discharge treatment of coal mine water.