Heterogeneous Mineral Research Articles

Fracability evaluation model for unconventional reservoirs: From the perspective of hydraulic fracturing performance

Fracability evaluation for unconventional reservoir is critical to the selection of candidate zones for post-frac productivity and plays a key role in fracturing design. Historically, the prevailing models for assessing fracability have been largely relied on brittleness indices. Brittleness indices focus mainly on rock fracture characteristics and offers limited assessment of fracture surface area and the complexity of fracture network, which are more relevant to the practical production. We explored a new fracability evaluation model for unconventional reservoirs from the perspective of fracturing performance, which comprehensively characterizes the rock's ability to generate larger fracture surface areas, more shear fractures and complex fracture networks. The new fracability index considers both the physical processes of rock failure and fracture propagation, and is directly associated with the dynamic production capacities of reservoir. According to the analysis of energy conservation during hydraulic fracturing, we quantify the rock fracture surface area using the KGD and the PKN models. The ability of rock formation to generate shear fractures is mainly influenced by Poisson's ratio and mode II fracture toughness. Brittle mineral content and mineral heterogeneity are two vital criteria that significantly affect the complexity of fracture networks. Based on the logging and production data, this fracability model was applied to two types of unconventional reservoirs. Preliminary results show that this fracability model has an improved correlation with the pay zones and actual production, which is beneficial for optimizing fracturing strategies and identifying production sweet spots.

Characterization of regolith-hosted rare earth element deposits using reflectance spectroscopy: Framework towards an efficient and reliable field exploration tool

With a growing demand for the rare earth elements (REE), exploration of regolith-hosted REE resources worldwide has been thriving in recent years and development of a rapid and reliable field-based tool will greatly facilitate the survey and exploration. In this study, we use visible and short-wave infrared (VNIR-SWIR) reflectance spectroscopy to comprehensively evaluate the applicability of the technique to explore regolith-hosted REE resources, exemplified by three representative regolith-hosted REE deposits in China. Neodymium among the REE shows reliably detectable spectral features in the VNIR-SWIR spectroscopy down to concentrations of 10–50 ppm in field samples with heterogeneous mineral grain sizes. The Nd spectral intensity of electronic transition at the band of ∼800 nm is correlated with bulk Nd concentrations and can be used as semi-quantitative indicators for the Nd concentrations, thereby the total REE in regolith. Moreover, VNIR-SWIR spectroscopy is demonstrated to be capable of delineating favorable ore-bearing mineralogy by characterizing the abundance and type of clay minerals and Fe (oxyhydr)oxides, and the crystallinity of kaolinite-group minerals. However, the Nd spectral features of samples with high bulk Fe2O3 contents (>3 wt%) are significantly masked due to overlapping by the strong absorption features of ferric (oxyhydr)oxides. VNIR-SWIR spectroscopy is deemed to be applicable to the exploration of regolith-hosted REE resources developed from Fe-poor felsic rocks.

Altered extracellular matrix and mechanotransduction gene expression in rat bone tissue following long-term estrogen deficiency.

Osteoporosis is primarily associated with bone loss, but changes in bone tissue matrix composition and osteocyte mechanotransduction have also been identified. However, the molecular mechanisms underlying these changes and their relation to bone loss are not fully understood. The objectives of this study were to (1) conduct comprehensive temporal gene expression analyses on cortical bone tissue from ovariectomized rats, with a specific focus on genes known to govern matrix degradation, matrix production, and mechanotransduction, and (2) correlate these findings with bone mass, trabecular and cortical microarchitecture, and mineral and matrix composition. Microarray data revealed 35 differentially expressed genes in the cortical bone tissue of the ovariectomized cohort. We report that catabolic gene expression abates after the initial accelerated bone loss period, which occurs within the first 4wk of estrogen deficiency. However, in long-term estrogen deficiency, we report increased expression of genes associated with extracellular matrix deposition (Spp1, COL1A1, COL1A2, OCN) and mechanotransduction (Cx43) compared with age-matched controls and short-term estrogen deficiency. These changes coincided with increased heterogeneity of mineral-to-matrix ratio and collagen maturity, to which extracellular matrix markers COL1A1 and COL1A2 were positively correlated. Interestingly, mineral heterogeneity and collagen maturity, exhibited a negative correlation with PHEX and IFT88, associated with mechanosensory cilia formation and Hedgehog (Hh) signaling. This study provides the first insight into the underlying mechanisms governing secondary mineralization and heterogeneity of matrix composition of bone tissue in long-term estrogen deficiency. We propose that altered mechanobiological responses in long-term estrogen deficiency may play a role in these changes.

Hydration of mafic crustal rocks at high temperature during brittle‐viscous deformation along a strike‐slip plate boundary

AbstractThe Poroshiri ophiolite (Hidaka metamorphic belt, Japan) occurs within a crustal‐scale network of high‐temperature, dextral shear zones that accommodated hundreds of kilometres of displacement due to the opening of the Japan Sea in the Neogene. The opholitic rocks comprise ultramafic, mafic and sedimentary protoliths that have been variably hydrated and metamorphosed. This work investigates the mechanisms and timing of fluid influx relative to viscous deformation in metagabbros and amphibolites deformed during exhumation from granulite‐ to amphibolite‐facies conditions. We consider a range of microstructures, from low strain domains and 1–2 mm thick shear bands to mylonites with a thickness of a few meters. Low strain domains of metagabbros exhibit corona textures with symplectites consisting of pargasitic amphibole (Ed0.7) ‐ anorthitic plagioclase (An80–92) ± orthopyroxene ± clinopyroxene forming around olivine and igneous pyroxene and with similar plagioclase–amphibole‐orthopyroxene ± clinopyroxene granoblastic aggregates in micrometric‐thick fractures. These textures result from hydration under low fluid‐rock ratio, with elevated H2O content only occurring locally (H2O > 1–1.2 wt%). Igneous mineral replacement leads to grain size reduction from 1 mm to ~10 μm. Amphibole exhibits a strong core‐rim zonation primarily controlled by the high diffusivity of Fe, Mg and OH and the low diffusivity of Al. Mineral compositional equilibrium is achieved at the scale of 100–200 μm. In mm‐thick localized shear bands and in metric‐scale mylonitic amphibolites, the heterogeneous mineral composition of amphibole (Ed0.2–0.5) and plagioclase (An40−An80) indicates only partial re‐equilibration (at the scale of 200–500 μm) despite higher fluid‐rock ratios and more pervasive fluid percolation than in metagabbros. Plagioclase–amphibole thermometry and equilibrium phase diagrams indicate that the initial fluid infiltration and corona formation occurred at 800–850°C by fracturing and percolation along grain boundaries. This was followed by the main fluid percolation and mylonitization event, which occurred during exhumation and cooling at conditions of 720–580°C, ~4 kbar. Continuous hydration during retrogression was achieved by the influx of dominantly seawater‐derived fluid, as attested by the high chlorine (Cl) contents (>300–400 ppm) of amphiboles in fractures. The heterogeneous distribution of fractures controls the distribution of fluid from the earliest stages of hydration, creating positive feedback where the growth of hydrous minerals as amphibole (up to +300 vol% of amphibole in high strain areas relative to the low‐strain ones) and the formation of fine‐grained mixed domains that led to further localization of viscous strain and mass transfer (variations of ± 30–40% in major elements). The Poroshiri ophiolite therefore represents a good fossil example of a former transpressional plate boundary where coupled metamorphic and deformation processes were triggered by seawater‐derived fluid that percolated to depths of ~15 km.

Accelerating CO2 Storage Site Characterization through a New Understanding of Favorable Formation Properties and the Impact of Core-Scale Heterogeneities

CO2 sequestration in deep geologic formations can permanently reduce atmospheric CO2 emissions and help to abate climate change. Target formations must undergo a time- and resource-intensive site evaluation process, assessing storage capacity, environmental safety, and suitability for CO2 trapping via reactive transport models based on data from a limited number of core samples. As such, simulations are often simplified and omit heterogeneities in formation properties that may be significant but are not well understood. To facilitate more rapid site assessment, this work first defines the aquifer properties of favorable storage formations through the analysis of promising and active storage sites. Data show quartz is the most prevalent formation mineral with carbonate minerals, highly reactive with injected CO2, present in over 75% of formations. Porosity and permeability data are highly clustered at 10–30% and 10–1000 mD. Field-scale reactive transport simulations are then constructed and used to analyze CO2 trapping efficiency. The models consider porosity and carbonate mineral heterogeneity as well as the impacts of typical temperature gradients. Simulated sequestration efficiencies are compared to results from a comparable homogenous model to understand the implications of aquifer non-uniformities. The results show a lower sequestration efficiency in the homogeneous model during the injection phase. During the post-injection phase, the homogenization of porosity and carbonate mineralogy results in a higher sequestration efficiency. Incorporating the temperature gradient also increases the sequestration efficiency. Importantly, the maximum deviation between the homogeneous and heterogeneous simulations at the end of the 50-year study period is only ~10%. Larger impacts may be incurred for properties outside the defined, promising ranges suggested here.

Thermal spalling failure mechanism of heterogeneous granite under moving heat source

The conventional mechanical methods employed for rock breaking encounter substantial challenges in deep, hard formation drilling, characterized by low penetration rates and exorbitant drilling costs. Consequently, there exists an urgent necessity to explore novel drilling methods and technologies. Thermal spallation drilling, as a non-contact rock breaking technique, offers high efficiency in rock fragmentation and substantially reduces drilling costs. However, the mechanism underlying rock fragmentation induced by thermal spallation remains unclear. Therefore, this study establishes a thermo-mechanical coupling model based on heterogeneous granite subjected to a high-temperature moving heat source. The model comprehensively considers the geometric shape, contact, and strength heterogeneity of rock minerals, and analyzes the effects of heating source speed, radius, mineral composition, heterogeneity index, and grain size on the thermal spallation mechanism of rock. The research findings indicate that lower heating source speeds and larger radii result in higher surface temperatures and greater damage volumes, whereas increasing speed and reducing radius enhance fragmentation efficiency, with speed exerting a more significant influence. Thermal spallation primarily results in tensile damage to rocks, with the surface damage and depth of damage being respectively influenced by the heating source speed and radius. Particle size exhibits minimal impact on thermal spallation, while increased quartz content enhances the rock's thermal properties, resulting in significant thermal spallation effects. Additionally, the location of rock damage is closely correlated with thermal parameters. These findings significantly contribute to a better understanding of the thermal spallation fracture mechanism in granite and facilitate the development of technical solutions for thermal spallation drilling methods.

Research on the influence of mineral heterogeneity under different CO2 injection schemes in low permeability reservoirs

In the pursuit of sustainable oil and gas resource extraction, the innovative integration of carbon capture, utilization, and storage (CCUS) technology has emerged as the most promising approach. During the CCUS process, intricate physicochemical interactions between the injected CO2, facilitated through various injection strategies (Water Alternative Gas: WAG/Continue Gas Injection: CGI) and the formation fluids and heterogeneous mineral assemblages within the reservoir trigger alterations in mineral structures, consequently impacting permeability and recovery factors, constituting a pivotal aspect. Precisely delineating and quantifying these interactions is paramount for optimizing process design and evaluating reservoir dynamics in the successful implementation of CCUS operations. This study has carried out qualitative and quantitative characterization of mineral heterogeneity, different pore types, and mineral combination characteristics from a low-permeability sandstone reservoir. Additionally, the effect on the physical properties of minerals from different development methods (WAG/CGI) was investigated using numerical simulation for CCUS applications. The results indicate that the saturated CO2 fluid selectively dissolves the potassium feldspar (orthoclase) in intergranular pores, while the intergranular pores are filled with illite and secondary precipitated clay minerals. It initially dissolves the sensitive mineral (ankerite) in the intergranular pores. The decrease of ankerite and increase of illite result from the prolonged contact period between saturated CO2 and minerals, which changes the mineral cementation to argillaceous type, thus affecting permeability in the context of CCUS. The spatial impact on reservoir physical properties depends on the spatial heterogeneity of the original sensitive minerals (ankerite, anorthite, illite, etc.) distributed in the study area. In the WAG scheme, the physicochemical interaction between saturated CO2 and reservoir minerals is more intense than in the CGI scheme for CCUS operations, significantly impacting cumulative production.

Study on the characteristics of granite rock impact crack based on grain-based model

The type and heterogeneity of minerals control the initiation, aggregation, and expansion of rock blasting cracks and significantly affect the rock failure process. Granite was used to investigate the law of rock cracking under impact loading at the scale of mineral particles, and a two-dimensional grain-based model (GBM) was developed using PFC based on CT) scanning and laboratory mechanical tests. The GBM’s reliability was validated using laboratory uniaxial compression tests and numerical simulations. The specimen failure modes and evolution of microcracks during rock impact were analyzed, as well as the impact cracks’ characteristics under various impact velocities. The findings show that the GBM can simulate the microfracture behavior of several types of mineral fractures during rock impact. The evolution of granite cracks under impact loading can be divided into three stages: rapid growth, decreased growth, and gradual stabilization. The impact failure cracks of the samples were primarily tensile and intragranular. Granite’s tensile and shear fractures and intergranular and intergranular cracks of granite exhibit various trends with varying impact velocities. The GBM is viable for researching the dynamics of crystalline rocks and is a powerful tool for exploring the dynamic characteristics of rocks at the mineral particle level.

Nanomechanical behavior of coal with heterogeneous minerals and pores using nanoindentation.

The coal's mechanical properties have a significant influence on mining safety and the mine environment. Preparing a standard sample and conducting repeat mechanical testing are challenging because the coal is primarily soft, fragmented, and rich in developed fractures. This study used nanoindentation technology, combined with X-ray diffraction, small-angle X-ray, a high magnification microscope, and mechanical parameter scale-up analysis, to study the micromechanical of three coals being dominated by heterogeneous components and pores. The results show that load-displacement curves with different maximum loads (50 mN, 100 mN, and 200 mN) all appear the pop-in events, and coal heterogeneity affects the frequency of their occurrence. As the maximum load is increased, pop-in event of DSC appears once each, YW increases from zero to three times and HM decreases from four to two times. The heterogeneity of pore structure has little effect on residual displacement, which is mainly affected by hard minerals, and hard minerals reduce the law that residual displacement increases with the increase in maximum load. The micromechanical parameters of soft coals are mainly affected by large pores, while hard coals are mainly affected by hard minerals. The coal's heterogeneity does not affect the linear relationship between hardness and elastic modulus, but stronger heterogeneity will weaken the linear relationship between fracture toughness and elastic modulus. Compared to the mechanical parameters after scale-up, the values obtained based on nanoindentation are less than 15.588% larger, and the increase in the heterogeneity and hard minerals can make the predicted parameters more accurate. The nanoindentation technique can not only provide an efficient and accurate method for studying the mechanical properties of heterogeneous coal at the nanoscale, an important guide for large-scale coal.

Lower microhardness along with less heterogeneous mineralization in the femoral neck of individuals with type 2 diabetes mellitus indicates higher fracture risk

Abstract There is still limited understanding of the microstructural reasons for the higher susceptibility to fractures in individuals with type 2 diabetes mellitus (T2DM). In this study, we examined bone mineralization, osteocyte lacunar parameters, and microhardness of the femoral neck trabeculae in 18 individuals with T2DM who sustained low-energy fracture (T2DMFx: 78 ± 7 years, 15 women and 3 men) and 20 controls (74 ± 7 years, 16 women and 4 men). Femoral necks of the T2DMFx subjects were obtained at a tertiary orthopedic hospital, while those of the controls were collected at autopsy. T2DMFx individuals had lower trabecular microhardness (P = .023) and mineralization heterogeneity (P = .001), and a tendency to a lower bone area with mineralization above 95th percentile (P = .058) than the controls. There were no significant intergroup differences in the numbers of osteocyte lacunae per bone area, mineralized lacunae per bone area, and total lacunae per bone area (each P > .05). After dividing the T2DMFx group based on the presence of vascular complications (VD) to T2DMFxVD (VD present) and T2DMFxNVD (VD absent), we observed that microhardness was particularly reduced in the T2DMFxVD group (vs. control group, P = .02), while mineralization heterogeneity was significantly reduced in both T2DMFx subgroups (T2DMFxNVD vs. control, P = .002; T2DMFxVD vs. control, P = .038). The observed changes in mineralization and microhardness may contribute to the increased hip fracture susceptibility in individuals with T2DM.

Alterations in compositional and cellular properties of the subchondral bone are linked to cartilage degeneration in hip osteoarthritis

ObjectiveThe subchondral bone is an emerging regulator of osteoarthritis (OA). However, knowledge of how specific subchondral alterations relate to cartilage degeneration remains incomplete. MethodFemoral heads were obtained from 44 patients with primary OA during total hip arthroplasty and from 30 non-OA controls during autopsy. A multiscale assessment of the central subchondral bone region comprising histomorphometry, quantitative backscattered electron imaging, nanoindentation, and osteocyte lacunocanalicular network characterization was employed. ResultsIn hip OA, thickening of the subchondral bone coincided with a higher number of osteoblasts (controls: 3.7±4.5 mm-1, OA: 16.4±10.2 mm-1, age-adjusted mean difference 10.5 mm-1 [95% CI 4.7 to 16.4], p<0.001) but a similar number of osteoclasts compared to controls (p=0.150). Furthermore, higher matrix mineralization heterogeneity (CaWidth, controls: 2.8±0.2 wt%, OA: 3.1±0.3 wt%, age-adjusted mean difference 0.2 wt% [95% CI 0.1 to 0.4], p=0.011) and lower tissue hardness (controls: 0.69±0.06 GPa, OA: 0.67±0.06 GPa, age-adjusted mean difference -0.05 GPa [95% CI -0.09 to -0.01], p=0.032) were detected. While no evidence of altered osteocytic perilacunar/canalicular remodeling in terms of fewer osteocyte canaliculi was found in OA, specimens with advanced cartilage degeneration showed a higher number of osteocyte canaliculi and larger lacunocanalicular network area compared to those with low-grade cartilage degeneration. Multiple linear regression models indicated that several subchondral bone properties, especially osteoblast and osteocyte parameters, were closely related to cartilage degeneration (R2 adjusted=0.561, p<0.001). ConclusionSubchondral bone properties in OA are affected at the compositional, mechanical, and cellular levels. Based on their strong interaction with cartilage degeneration, targeting osteoblasts/osteocytes may be a promising therapeutic OA approach. Data and materials availabilityAll data are available in the main text or the supplementary materials.

Five-dimensional facies-driven seismic inversion for igneous reservoirs based on rock-physics modelling

Abstract The igneous reservoir has become an important exploration target for increasing reserves and production of oil and gas in the Junggar Basin. However, igneous reservoir exploration is restricted because the seismic exploration of high-quality igneous reservoirs is difficult and the anisotropy induced by high-angle fractures cannot be neglected. To implement the characterization of igneous reservoirs, we first study the correlation between anisotropy parameters and physical properties of igneous rock, and we propose a 5D facies-driven inversion method based on rock physics, which means we employ 3D seismic data at different incidence angles and azimuths to implement the estimation of a hydrocarbon reservoir constrained by the igneous rock facies. We also present an anisotropic igneous rock-physics model, in which micro-petrophysical characteristics, strong heterogeneity of skeleton minerals, and pore structures are considered. As a reasonable initial model is important for seismic inversion, we propose a facies-driven modelling seismic inversion method in which we use facies obtained on the basis of the difference between rock composition, reservoir physical parameters, and elastic parameters of different lithofacies igneous rocks to constrain the seismic inversion. Finally, we present a step seismic inversion method of using seismic data to estimate multi-parameters of horizontal transversely isotropic media. Therefore, the comprehensive processes of rock-physics modelling, inversion model establishment, and reservoir prediction of high-quality igneous rocks are proposed in this study, which demonstrates effective application for igneous reservoirs in China.

Clay composition patterns and their influence on the adhesive strength of earthen plasters

The clay fraction of earthen plasters is the part responsible for the acquisition of cohesion and adherence that they possess against deterioration factors. Adherence is the property responsible for keeping the plaster together with the wall and is influenced by the percentage content of clay as well as by its mineralogy and the heterogeneity of minerals that may be present. However, it is still unknown in depth how clay minerals perform in the adherent properties of earthen plasters when the composition is heterogeneous in the material. The objective of this study is to evaluate the incidence of the mineralogical complexity of clay mineralogy in the variability of the adherence of earth plasters. To evaluate the variability, considering that the mineralogy of the soils depends directly on the place and the formation processes, eight soils from Tucumán (Argentina) corresponding to different physiographic units were analyzed. A methodology was designed for sample preparation that allows soils to be compared through adhesion tests. They were characterized by XRD to determine their mineralogical composition and by the hydrometric method to determine their granulometry. To evaluate the adherence of mixtures made with the respective soils, it was proposed in the first instance to compensate the granulometry of the soils to equate them and, once the plasters were made, this property was evaluated through shear and pull-off tests. The results showed that they allowed us to identify that the soils presented a pattern of mineralogical composition common to all the physiographic units, made up of the Ill and K pair, the former being predominant. For this pattern, it was observed in particular that there is a positive correlation between the increase in Ill content with the increase in the adhesive strength of the plasters. Clay minerals from the Sm group also contribute to the increase in adherence when the percentage is greater than or equal to 11%. On the contrary, K and Cl do not influence the increase in adhesive strength.The clay fraction of earthen plasters is the part responsible for the acquisition of cohesion and adherence that they possess against deterioration factors. Adherence is the property responsible for keeping the plaster together with the wall and is influenced by the percentage content of clay as well as by its mineralogy and the heterogeneity of minerals that may be present. However, it is still unknown in depth how clay minerals perform in the adherent properties of earthen plasters when the composition is heterogeneous in the material. The objective of this study is to evaluate the incidence of the mineralogical complexity of clay mineralogy in the variability of the adherence of earth plasters. To evaluate the variability, considering that the mineralogy of the soils depends directly on the place and the formation processes, eight soils from Tucumán (Argentina) corresponding to different physiographic units were analyzed. A methodology was designed for sample preparation that allows soils to be compared through adhesion tests. They were characterized by XRD to determine their mineralogical composition and by the hydrometric method to determine their granulometry. To evaluate the adherence of mixtures made with the respective soils, it was proposed in the first instance to compensate the granulometry of the soils to equate them and, once the plasters were made, this property was evaluated through shear and pull-off tests. The results showed that they allowed us to identify that the soils presented a pattern of mineralogical composition common to all the physiographic units, made up of the Ill and K pair, the former being predominant. For this pattern, it was observed in particular that there is a positive correlation between the increase in Ill content with the increase in the adhesive strength of the plasters. Clay minerals from the Sm group also contribute to the increase in adherence when the percentage is greater than or equal to 11%. On the contrary, K and Cl do not influence the increase in adhesive strength.

Mechanistic insights into the co-recovery of nickel and iron via integrated carbon mineralization of serpentinized peridotite by harnessing organic ligands.

The rising need to produce a decarbonized supply chain of energy critical metals with inherent carbon mineralization motivates advances in accelerating novel chemical pathways in a mechanistically-informed manner. In this study, the mechanisms underlying co-recovery of energy critical metals and carbon mineralization by harnessing organic ligands are uncovered by investigating the influence of chemical and mineral heterogeneity, along with the morphological transformations of minerals during carbon mineralization. Serpentinized peridotite is selected as the feedstock, and disodium EDTA dihydrate (Na2H2EDTA·2H2O) is used as the organic ligand for metal recovery. Nickel extraction efficiency of ∼80% and carbon mineralization efficiency of ∼73% is achieved at a partial pressure of CO2 of 50 bars, reaction temperature of 185 °C, and 10 hours of reaction time in 2 M NaHCO3 and 0.1 M Na2H2EDTA·2H2O. Extensive magnesite formation is evidence of the carbon mineralization of serpentine and olivine. An in-depth investigation of the chemo-morphological evolution of the CO2-fluid-mineral system during carbon mineralization reveals several critical stages. These stages encompass the initial incongruent dissolution of serpentine resulting in a Si-rich amorphous layer acting as a diffusion barrier for Mg2+ ions, subsequent exfoliation of the silica layer to expose unreacted olivine, and the concurrent formation of magnesite. Organic ligands such as Na2H2EDTA·2H2O aid the dissolution and formation of magnesite crystals. The organic ligand exhibits higher stability for Ni-complex ions than the corresponding divalent metal carbonate. The buffered environment also facilitates concurrent mineral dissolution and carbonate formation. These two factors contribute to the efficient co-recovery of nickel with inherent carbon mineralization to produce magnesium carbonate. These studies provide fundamental insights into the mechanisms underlying the co-recovery of energy critical metals with inherent carbon mineralization which unlocks the value of earth abundant silicate resources for the sustainable recovery of energy critical metals and carbon management.

Mineralogical control on the dissolution induced microscopic pore alternations of high-rank coal during CO2 sequestration

The microstructure of deep coal seam plays a crucial role in determining the CO2 geological sequestrations. To investigate the effects of pore and mineral distribution on the changes and mechanisms of coal microstructure under supercritical CO2 (SC–CO2) interactions, we employed low-temperature N2 (LTN2) adsorption, low-temperature CO2 (LTCO2) adsorption, and X-ray diffraction (XRD) techniques to explore the development of microstructures and mineral compositions of coal samples with distinct pore distribution characteristics under SC-CO2 exposure. The results revealed that mineral dissolution under SC-CO2 saturation led to an increase in both specific surface area (SSA) and pore volume (PV) of mesopores, with SSA increasing by 13.6%–332.31% and PV increasing by 4.59%–148.61%. Subsequently, SC-CO2 exposure induced the formation of numerous new pores with diameters smaller than 3 nm within the coal matrix, particularly an abundance of new ultra-micropores (r > 0.5 nm) and large micropores (r > 0.85 nm). This significantly altered the irregularity of pore structure and adsorption capacity, thereby playing a crucial role in CO2 storage. Micropores exhibited much larger SSA and PV than mesopores, underscoring their significant contribution to CO2 geological sequestration. The spatial mineral distribution in coal influences the alteration characteristics of pores, which explains the different microscopic pore structures formed after SC-CO2 exposure. The impact of spatial mineral distribution on SC-CO2-induced pore alteration is analysed, emphasising the intrinsic heterogeneity of minerals within coal as the fundamental factor behind the diversity observed in pore development during CO2 sequestration. Finally, we found that the changes in micropores and mesopores under SC-CO2 exposure are closely related to the initial structure of microscopic pores. The development of micropores under SC-CO2 exposure is associated with the average pore size and fractal dimension of micropores, with the increment in mesopore irregularity increasing with the increase in micropore volume fractal dimension. By studying the influence of mineral and pore distribution on the evolution of coal microstructures, this study provides new insights into pore development under SC-CO2 geological sequestration, which can be valuable for predicting reservoir development in CO2-ECBM.

Spatial Characterization of Wetting in Porous Media Using Local Lattice-Boltzmann Simulations

Wettability is one of the critical parameters affecting multiphase flow in porous media. The wettability is determined by the affinity of fluids to the rock surface, which varies due to factors such as mineral heterogeneity, roughness, ageing, and pore-space geometry. It is well known that wettability varies spatially in natural rocks, and it is still generally considered a constant parameter in pore-scale simulation studies. The accuracy of pore-scale simulation of multiphase flow in porous media is undermined by such inadequate wettability models. The advent of in situ visualization techniques, e.g. X-ray imaging and microtomography, enables us to characterize the spatial distribution of wetting more accurately. There are several approaches for such characterization. Most include the construction of a meshed surface of the interface surfaces in a segmented X-ray image and are known to have significant errors arising from insufficient resolution and surface-smoothing algorithms. This work presents a novel approach for spatial determination of wetting properties using local lattice-Boltzmann simulations. The scheme is computationally efficient as the segmented X-ray image is divided into subdomains before conducting the lattice-Boltzmann simulations, enabling fast simulations. To test the proposed method, it was applied to two synthetic cases with known wettability and three datasets of imaged fluid distributions. The wettability map was obtained for all samples using local lattice-Boltzmann calculations on trapped ganglia and optimization on surface affinity parameters. The results were quantitatively compared with a previously developed geometrical contact angle determination method. The two synthetic cases were used to validate the results of the developed workflow, as well as to compare the wettability results with the geometrical analysis method. It is shown that the developed workflow accurately characterizes the wetting state in the synthetic porous media with an acceptable uncertainty and is better to capture extreme wetting conditions. For the three datasets of imaged fluid distributions, our results show that the obtained contact angle distributions are consistent with the geometrical method. However, the obtained contact angle distributions tend to have a narrower span and are considered more realistic compared to the geometrical method. Finally, our results show the potential of the proposed scheme to efficiently obtain wettability maps of porous media using X-ray images of multiphase fluid distributions. The developed workflow can help for more accurate characterization of the wettability map in the porous media using limited experimental data, and hence more accurate digital rock analysis of multiphase flow in porous media.

Identification of quartz cement in sandstone through deep learning segmentation of electron microscopy images

Quartz cementation is a crucial factor in controlling the petrophysical properties of sandstone reservoirs. However, reliable identification of sandstone cementation often requires petrographic analysis of rock thin sections using a combination of optical light microscopy, backscattered electron (BSE), and cathodoluminescence (CL) images. This process is currently carried out manually and is both costly and time consuming. In this study, we present the first attempt to automate this process by identifying sandstone cement through convolutional neural networks (CNNs). We used a combination of BSE and CL images acquired from sandstone thin sections sourced from formations in the US, Israel, and the Netherlands. For each image pair we created a labeled mask with 4 classes: (i) quartz grains; (ii) quartz cement; (iii) porosity; and (iv) Other phases. We developed a U-Net with a total of 10 layers: 5 layers for the contracting path and 5 layers for the expansive path. The model is trained to predict the segmentation mask for a given input of paired BSE and CL images. A high level of accuracy is achieved for quartz grains (91%), quartz cement (78%), and porosity (95%). By contrast, the Other phase class was poorly predicted due to a combination of mineral heterogeneity and low representation. Our results indicate that Deep Learning algorithms provide a promising avenue for the automation of quartz cement detection in sandstone images.

An application for nonlinear heterogeneity-based isotherm models in characterization of niobium sorption on clay rocks and granite

Abstract The nonlinear heterogeneous adsorption behaviors of niobium (Nb) on clay rocks (bentonite and argillite) and granite in synthetic groundwater and seawater systems were evaluated by adsorption experiments, applying two heterogeneity-based isotherm models: the Langmuir–Freundlich (LF) and generalized-Freundlich (GF) models. According to the root mean square error (RMSE) between the experimental results and numerical simulation, the two heterogeneous sorption models (LF and GF), which correspond to a different heterogenous constant (β), were more adequate than Langmuir models for characterizing the Nb adsorption mechanism. The fitting results demonstrated that the sorption of Nb on granite, bentonite, and argillite exhibited a different adsorption affinity spectrum as a result of the heterogeneous mineral surface. Consequently, the Nb sorption capacity of bentonite and argillite was higher than that of granite and was estimated at 9.24E-01 mmol/g for bentonite, 8.44E-01 mmol/g for argillite, and 2.33E-02 mol/kg for granite.

Numerical Simulation of Multi-cluster Fracturing Using the Triaxiality Dependent Cohesive Zone Model in a Shale Reservoir with Mineral Heterogeneity

Numerical Simulation of Multi-cluster Fracturing Using the Triaxiality Dependent Cohesive Zone Model in a Shale Reservoir with Mineral Heterogeneity

The Influence of Surface Heterogeneity of Fluorite on the Adsorption of Alkyl Sulfonates

Surface heterogeneity of minerals can significantly affect the adsorption of collectors. Petroleum sulfonate is widely used as a fluorite collector, but how the surface heterogeneity of fluorite influences the adsorption of alkyl sulfonates remains unknown. Herein, two kinds of surface heterogeneity situations, i.e., edge and (1 1 1) _vacancy, were modeled, and the adsorption of sodium dodecyl sulfonate on them was simulated. The results show that the stable adsorption configuration of sodium dodecyl sulfonate on the edge was in a bridged mode, and the stable interaction configuration with vacancy was in a tridentate mode. The 2p orbit of fluorine on the surface of the edge and the vacancy could hinder collector adsorption. After adsorption, the 3d orbit of calcium interacted with the collector orbit above Fermi level, and moved towards the lower energy level, benefiting the adsorption process. It was also found that the adsorption intensity/strength of alkyl sulfonate on fluorite was directly proportional to the interaction intensity of the collector with the 3d orbits of calcium ions on the surface and vacancy. Therefore, the rough fluorite surface had a stronger adsorption effect on the collector, and the existence of vacancy could improve the surface adsorption energy, and thus enhance the adsorption of the collector on the fluorite surface. The rough fluorite surface requires high collector concentration to achieve saturated monolayer adsorption, so increasing vacancy was the better choice to improve the adsorption capacity of alkyl sulfonate on the fluorite surface. This study provides novel insights into the flotation mechanism, in the context of surface heterogeneity, and could guide the design of high-performance collectors for fluorite ore flotation.