Mercury Injection Porosimetry Research Articles

Comparative study on preparation and hydration mechanism of composite cementitious materials containing copper slag

Copper slag (CS) is a kind of non-ferrous solid waste with low content of aluminosilicate and relatively high content of iron oxide. In this paper, the feasibility and mechanism of CS as the mineral admixture and precursor of alkali activated cementitious material were studied. The effects of CS on mechanical properties were studied and the hydration products were determined by several characterization methods. Scanning Electron Microscopy (SEM) and Mercury Injection Porosimetry (MIP) were performed to characterize the microstructure. The results show that CS composite fly ash (FA) has more advantages than CS alone, but its application as a mineral admixture is limited by the low activity. The alkali activated cementitious material prepared by CS and FA as precursor has excellent mechanical properties. The compressive strength of specimen at 28 days age can reach the maximum of 81.3 MPa when the dosage of CS is 30 %. The 3d and 28d compressive strength of the specimen contained 60 % CS obtained 57.7 MPa and 70.7 MPa, respectively. The research exhibits great potential for the future utilization of CS.

The Significance of Lignocellulosic Raw Materials on the Pore Structure of Activated Carbons Prepared by Steam Activation.

Five different lignocellulosic raw materials (coconut shells, Moso bamboo, sawtooth oak, Chinese fir, and Masson pine) were used to prepare activated carbons by steam activation at 850 °C to evaluate the effects of their structures on physical activation. The chemical compositions, botanic forms, and pore structures of the lignocellulose-based charcoal samples were systematically characterized by proximate and ultimate analyses, scanning electron microscopy, and mercury injection porosimetry. It was found that the rate of the activation reaction between charcoal and steam is determined by the porosity of the precursor. Pore structure results show that the steam activation of coconut shell and bamboo charcoals primarily produced micropores, thus yielding microporous activated carbon materials with just a few mesopores, even following a high burn-off of >66%. The steam activation of sawtooth oak charcoals produced mainly micropores at a low burn-off of <50% and both micropores and mesopores at a high burn-off of >50%. The steam activation of Chinese fir and Masson pine charcoals produced mainly mesopores at a burn-off of 0-80%. These mesopores were remarkably broadened to >20 nm on extending the activation time, resulting in a high vitamin B12 (VB12) adsorption capacity of ~530 mg/g. In conclusion, the raw lignocellulosic materials used as precursors have a decisive effect on the development of pore structures in activated carbon materials obtained through physical activation.

Dry-Wet Cycles Durability of Solid Waste Based Cementing Materials Solidifying Different Characteristic Soils

Abstract A comparative study of the durability of multi-source solid waste-based soil solidification materials in solidifying different soil types has not yet been conducted. Therefore, the properties of multi-source solid waste-based solidification materials (SBM) solidifying clay soil (CS), sandy soil (SS) and organic soil (OS) subjected to dry-wet cycles of damage were studied in this work. The unconfined compressive strength (UCS) of the SBM solidified soil was tested to evaluate the mechanical properties of the solidified soil. Scanning electron microscopy (SEM) and mercury injection porosimetry (MIP) tests were conducted in order to study the micro-action mechanism. The results demonstrated that the SBM showed wide applicability and good long-term performance. The rate of strength increase of the SBM solidified soil during the long-term curing period was found to be dependent on soil characteristics. All the types of SBM solidified soils exhibited increased UCS during the first 10 cycles of the D-W. As the number of D-W cycles increased from 10 to 50, the UCS loss rate for CS reached 78%, with OS experiencing the least at 58%. The structure of SBM solidified soil exhibited softening and weakened resistance to deformation with each additional D-W cycle. The types of hydration products were consistent across all three soil types. The quantity of hydration products was influenced by the characteristics of the soil, which also contributed to the deterioration of damage resistance in D-W cycles. The number of pores within the SBM solidified soil increased with the number of D-W cycles (&gt;10 cycles), resulting in a deterioration of the compact structure.

Effects of steam curing on the pore structure of recycled aggregate concrete

The pores in steam-cured concrete affect its strength and durability, and optimisation of the steaming system can improve the pore structure to some extent. However, the influence mechanism of evaporation on recycled aggregate concrete (RAC) remains unclear. In this paper, the effects of curing temperature (20°C, 40°C, 60°C and 80°C), steam curing time (6 h, 9 h and 12 h) and pozzolans (slag powder and fly ash) on the compressive strength and pore structure of RAC were studied. The microstructure characteristics such as pore size and pore size distribution were analysed by mercury injection porosimetry. The results show that steam curing can significantly improve the compressive strength and reduce the most probable aperture and average pore sizes of RAC. Moreover, the effect of slag powder on RAC pore structure is better than that of fly ash. Also, there is a linear correlation between the compressive strength and the average pore size of steam-cured RAC. This could be an important parameter for constructing future steam-cured RAC strength prediction models.

Characteristics of Permeability Evolution and Pore Structure of Coal with High Gas

To study the influence of gas pressure on coal permeability evolution, we conducted experiments on coal samples from the No. 9 coal seam in Tangshan Coal Mine, Hebei Province, China. Different gas pressures (helium and nitrogen) were applied, and nitrogen-induced deformations were measured. We also analyzed the coal samples’ pore structure using mercury injection porosimetry, obtaining pore surface fractal dimensions. The increase in nitrogen pressure from 0.3 MPa to 3 MPa resulted in an elevation of adsorption strain from 0.168 × 10−3 to 1.076 × 10−3, with a gradual decrease observed in the extent of this increase. However, the permeability of coal samples initially decreased from 16.05 × 10−18 m2 to 4.91 × 10−18 m2 and subsequently rose to 5.69 × 10−18 m2. Helium showed similar trends to nitrogen, with average permeability 1.42–1.88 times higher under the same pressure. The lowest permeability occurred at 1.5 MPa for helium and 2.5 MPa for nitrogen. Gas absorptivity plays a crucial role in coal permeability evolution. Additionally, we observed coal’s compressibility to be 7.2 × 10−11 m2/N and corrected porosity to be 53.8%, considering matrix compression. Seepage pores larger than 100 nm accounted for 80.4% of the total pore volume, facilitating gas seepage. Surface fractal dimension Ds1 correlated positively with micropore volume, while Ds2 and Ds3 correlated negatively with pore volume and gas permeability.

Pore to Core Scale Characterization of Hydraulic Flow Units Using Petrophysical and Digital Rock Analyses

This article illustrates an integrated method for characterizing hydraulic flow units in subsurface reservoirs using multiscale images and petrophysical data analysis from well cores. This method is applied to 35 meters of cores drilled from a Late Jurassic Upper Jubayla Formation outcrop in Saudi Arabia, which is analogous to the lower section of the Arab‐D reservoir. Scanning electron microscope (SEM) imagery, thin‐section petrography, and X‐ray computed tomography (CT) images of whole cores and core plugs are used to visualize and characterize these carbonate rocks across multiple length scales. Core plug porosity and permeability data are used for petrophysical characterization and deriving hydraulic flow units (HFU). The multimodal pore types associated with the HFUs and their connectivity using thin section and SEM image analysis, combined with mercury injection porosimetry are used. Whole‐core CT textures and heterogeneity logs are correlated with HFU and digital image analysis of core plugs. Whole‐core CT data prove to be a highly valuable tool for bridging the scale gap between well data and reservoir models. This methodology can be applied to datasets from a large number of wells, especially when combined with machine learning tools, allowing improved characterization of complex carbonate reservoirs.

Integrated controls of tectonics, diagenesis and sedimentation on sandstone densification in the Cretaceous paleo-uplift settings, north Tarim Basin

The densification mechanism of the Lower Cretaceous sandstone in the western Tabei Uplift is still unclear, restricting successful petroleum exploration and development. Seismic interpretation, thin sections observation, Field emission scanning electron microscope (FE-SEM) and cathodoluminescence (CL), porosity and permeability measurements, mercury injection porosimetry (MIP) are compiled in this study, to investigate the paleo-uplift and facies co-evolution, tight reservoir characteristics, and densification mechanism of the Lower Cretaceous sandstones around the paleo-uplift. Results indicate the sedimentary facies around the Xinhe and the Sandaoqiao uplifts were controlled by the decreasing geomorphic slope and rising lake level, their evolution exhibited difference. The Xinhe area developed a fan delta with a reducing distribution in region, while the sandaoqiao area transitioned from fan delta to beach bar, eventually developing a braided river delta. Reservoir physical properties are primarily influenced by intense compaction, calcite and clay cementation and weak dissolution. Severe calcite cementation, compaction and weak dissolution predominantly influence the physical properties of the beach bar. Moderate compaction, weak cementation and moderate quartz and feldspar dissolution are the key factors impacting the physical properties of braided river delta. Additionally, reservoir quality varies with sedimentary facies where the sandstones of the braided-river delta have the best petrophysical properties and pore connectivity than those of the fan delta and beach bar. Tectonic movements around the uplift affected the distribution of various sedimentary facies, which later governed the diagenesis processes pathways. Collectively, this study can serve as a valuable reference for exploration and development of clastic reservoirs in the Tabei Uplift and analogous reservoirs with a paleo-uplift context.

Effect of tuff powder on the performance of low heat Portland cement-based materials

Low heat Portland cement (LHPC), a low-carbon cement variant, finds extensive application in expansive water conservancy ventures. To amplify energy efficiency and curtail CO2 emissions, supplementary cementitious materials (SCMs) are frequently harnessed as cement substitutes. This research delves into the impact of tuff powder (TP) on LHPC mortar traits via an array of evaluations encompassing rheological scrutiny, compressive strength assessment, isothermal calorimetry, X-ray diffraction (XRD), thermogravimetric analysis (TG), mercury injection porosimetry (MIP), and scanning electron microscopy (SEM). Notably, the research divulges that TP incorporation heightens yield stress and plastic viscosity within LHPC paste, concurrently expediting the hydration process and mitigating the maximum heat rate. Over a 90d span, TP efficiently increase of binding water content due to the pozzolanic reaction of TP and Ca(OH)2. Compared with FA, the addition TP can significantly enhance the 7d compressive strength of LHPC up to 39%. Augmented TP content engenders a more intricate and fine-pored structure in LHPC paste, bolstering cement paste density. Importantly, TP incorporation contributes to diminished energy consumption, lowered CO2 emissions, and amplified sustainability. In order to optimize material properties, the content of TP must be strictly controlled in the range of 20%-40%.

Comparing the Pore Networks of Coal, Shale, and Tight Sandstone Reservoirs of Shanxi Formation, Qinshui Basin: Inspirations for Multi-Superimposed Gas Systems in Coal-Bearing Strata

Transitional upper carboniferous Shanxi Formation coal-bearing strata in Qinshui Basin have been proven to be a set of mixed unconventional gas-bearing reservoirs forming a multi-superimposed gas system that consists of multiple independent fluid pressure systems vertically through the strata. An experimental protocol was designed to compare the pore networks in high-rank coal, shale, and tight sandstone reservoirs from Shanxi Formation using quantitative and qualitative experimental methods, including high-pressure mercury injection porosimetry (MIP), low-pressure nitrogen gas adsorption (LN2GA), and argon ion polishing–field emission scanning electron microscope (AIP-FESEM). The results show that genetic and structural differences in pore types, morphology, abundance, and proportion in coal, shale, and tight sandstone reservoirs are significant, reflecting strong heterogeneity characteristics. Pore networks determine the roles of different types of reservoirs in gas-bearing systems through differentiated pore structure, development degree, and spatial distribution. Due to the differences in nanopore development and connectivity, coal and tight sandstone reservoirs provide important reservoir spaces for adsorbed and free gas in the system. Thus, they become influential factors controlling the relationship between the gas-bearing subsystems with different fluid pressures. The lack of mesopores in shale and relatively weaker heterogeneity between layers lead to the phenomenon that continuously developed shales of a specific thickness are more likely to be the interlayers that divide the superimposed gas-bearing system. Systematic comparison of pore development characteristics will provide scientific support to further explain the formation mechanism of multi-superimposed gas systems in coal-bearing strata from the perspective of pore networks and provide guidance for the development of unconventional natural gas in coal-bearing strata.

Probing the Pore Structure of the Berea Sandstone by Using X-ray Micro-CT in Combination with ImageJ Software

During diagenesis, the transformation of unconsolidated sediments into a sandstone is usually accompanied by compaction, water expulsion, cementation and dissolution, which fundamentally control the extent, connectivity and complexity of the pore structure in sandstone. As the pore structure is intimately related to fluid flow in porous media, it is of great importance to characterize the pore structure of a hydrocarbon-bearing sandstone in a comprehensive way. Although conventional petrophysical methods such as mercury injection porosimetry, low-pressure nitrogen or carbon dioxide adsorption are widely used to characterize the pore structure of rocks, these evaluations are based on idealized pore geometry assumptions, and the results lack direct information on the pore geometry, connectivity and tortuosity of pore channels. In view of the problems, X-ray micro-CT was combined with ImageJ software (version 1.8.0) to quantitatively characterize the pore structure of Berea Sandstone. Based on its powerful image processing function, a series of treatments such as contrast enhancement, noise reduction and threshold segmentation, were first carried out on the micro-CT images of the sandstone via ImageJ. Pores with sizes down to 2.25 μm were accurately identified. Geometric parameters such as pore area, perimeter and circularity could thus be extracted from the segmented pores. According to our evaluations, pores identified in this study are mostly in the range of 30–180 μm and can be classified into irregular, high-circularity and slit-shaped pores. An irregular pore is the most abundant type, with an area fraction of 72.74%. The average porosity obtained in the image analysis was 19.10%, which is fairly close to the experimental result determined by a helium pycnometer on the same sample. According to the functional relationship between tortuosity and permeability, the tortuosity values of the pore network were estimated to be in the range of 4–6 to match the laboratory permeability data.

Selection Effect of Liquid Nitrogen Freeze-Thaw Cycles on Full Pore Size Distribution of Different Rank Coals.

In China, the capacity to produce coalbed methane and extract underground gas is restricted by the prevalence of low-permeability coal seams. Liquid nitrogen fracturing is a new low temperature-high-pressure anhydrous fracturing technology that uses low temperature and high frost heave forces to increase coal permeability. To better understand the liquid nitrogen fracturing effect on coal, we conduct the liquid nitrogen freeze-thaw cycle (LNCFT) experiments on different rank coals from Qinghai, Shanxi, and Shaanxi provinces. We combined the low-pressure nitrogen and carbon dioxide adsorption experiment with the non-local density functional theory model and mercury injection porosimetry with compressibility corrections to examine the full pore size distributions of untreated and water-saturated samples before and after LNCFT. The results found that LNCFT can effectively increase the pore volume (PV) and specific surface area of the water-saturated coal sample. Compared with the raw coal, the increased ratio of the full pore size PV is 70.41-100.17%. However, the scale-selective transformation effect on pores during liquid nitrogen fracturing is noticeable. Under the same conditions, LNCFT can significantly increase the pore volume of micropores (>2 nm) and macropores (>50 nm), and the increase ratios are 24.40-44.16 and 103.55-327.93%, respectively. The PV of mesopores (2-50 nm) shows a slightly increasing trend with the increase in metamorphic degree, and the increase ratio is between 8.7 and 56%. Comparing the full pore size distribution curves before and after LNCFT, it is found that the alteration of high-volatile bituminous coal (BLT coal) and anthracite (SH coal) has more significance in the range of less than 2 and 50-20,000 nm, while middle-volatile bituminous coal (YJL coal) varies between 50 and 2000 nm. Meanwhile, the ratio of micropore and mesopore PV to the total decreased gradually before and after LNCFT, while the proportion of macropores increased, indicating that small-scale pores would intersect and connect to form larger-scale pores during the fracturing. The combined effects of temperature gradient, water-ice phase transition, and heat transmission rate are the key factors that determine the impact of LNCFT on pore size distribution. Our results provide new information for enhancing the permeability of low-permeability coal seams of different ranks.

Nanoindentation of Horn River Basin Shales: The Micromechanical Contrast Between Overburden and Reservoir Formations

AbstractWe present a micromechanical characterization of shales from the Horn River Basin, NW Canada. The shales have contrasting mineralogy and microstructures and play different geomechanical roles in the field: the sample set covers an unconventional gas reservoir and the overburden unit that serves as the upper fracture barrier. Composition and texture were characterized using X‐ray diffraction, mercury injection porosimetry, and scanning electron microscopy (SEM). Grid nanoindentation testing was used to obtain the mechanical response of the dominant phases in the shale microstructure. Samples were indented parallel and perpendicular to the bedding plane to assess mechanical anisotropy. Chemical analysis of the grids with SEM‐EDS (energy dispersive X‐ray spectroscopy) was undertaken and the coupled chemo‐mechanical data was used in a statistical clustering procedure (Gaussian mixture model) to reveal the mechanical properties of each phase. The results show that the overburden consists of a soft clay matrix with highly anisotropic elastic stiffness, and stiffer but effectively isotropic inclusions of quartz and feldspar; the significant anisotropy of the overburden has been previously observed on a much larger scale using microseismic data. Creep displacement is concentrated in the clay matrix, which is the key phase for fracture barrier and seal applications. The reservoir units are harder and have more isotropic mechanical responses, primarily due to their lower clay content. Despite varied compositions and microstructures, the major phases of these shales (clay/organic matrix, quartz/feldspar, dolomite, and calcite) have unique mechanical signatures, which will aid identification in future micromechanical characterizations and facilitate their use in upscaling schemes.

Micromechanical characterisation of overburden shales in the Horn River Basin through nanoindentation

The paper presents a micromechanical characterisation of Fort Simpson shale, which overlies unconventional gas-producing lithologies in the Horn River Basin, NW Canada. The Fort Simpson formation is clay-rich and microseismic data recorded during hydraulic fracturing events in the underlying reservoir has shown the formation acts as a barrier to fracture development, with a notably anisotropic seismic response. Samples were prepared from core fragments and the composition and texture of the shale was characterised using X-ray diffraction, mercury injection porosimetry and scanning electron microscopy (SEM). Nanoindentation testing was used to obtain the mechanical response of the shale microstructure, at grain-scale. The indentation was conducted on a grid pattern and samples were oriented both parallel and perpendicular to the bedding plane to assess the inherent mechanical anisotropy. Chemical analysis of the grids was also undertaken through SEM/EDS (energy dispersive X-ray spectroscopy) and the coupled chemo-mechanical data was used to characterise the material phases of the shale through a statistical clustering procedure. The results show that Fort Simpson shale broadly consists of a soft clay phase, with strongly anisotropic elastic stiffness, and stiffer but effectively isotropic grains of quartz and feldspar. A simple upscaling scheme was also applied to link the grain-scale elastic stiffness to the field-scale microseismic data.

Rate-Controlled Mercury Injection Experiments to Characterize Pore Space Geometry of Berea Sandstone

Interpretation of the relationship between heterogeneity and the flow in porous media is very important in increasing the recovery factor for an oil or gas reservoir. Capillarity for instance, controls fluids static distribution in a reservoir prior to production and remaining hydrocarbons after production commences. Therefore, capillary pressure data are used by petroleum engineers, geologists, and petrophysicists to evaluate production characteristics of petroleum accumulations. Conventional pressure-controlled mercury porosimetry produces an overall capillary pressure curve and pore throat size distribution data that provide little information about the porous medium structure and pore geometry. The present study provides information on three capillary pressure curves obtained from rate-controlled mercury injection porosimetry; one describes the larger pore spaces or pore bodies of a rock, another describes the smaller pores or pore throats that connect the larger pores, and a final curve which corresponds to the overall capillary pressure curve obtained from the conventional pressure-controlled mercury injection. An experimental constant-rate mercury injection apparatus was constructed that consists of a piston displacement pump, a computer controlled stepper motor drive and a core sample cell designed to minimize dead volume. The apparatus was placed in a glass chamber and subjected to an air bath to maintain a constant temperature of 27o C throughout the experiments. Then constant rate mercury injection experiments were performed on three Berea Sandstone core plugs. Results show that volume-controlled or rate-controlled porosimetry provides considerably more detailed data and information on heterogeneity and the statistical nature of pore space structure than the conventional pressure-controlled porosimetry as pressure fluctuations with time reveal menisci locations in pore bodies and pore throats. Moreover, pore size distributions based on volume-accessed pores and pore radii were obtained from the pressure versus saturation relationship.

The effect of fault-induced compaction on petrophysical properties of deformation bands in poorly lithified sandstones

Although it is well documented that during faulting there is an overall fault zone porosity loss in porous sandstone, little is known about the effect of shear-induced compaction on subsequent deformation bands petrophysics. We studied the impact of fault-induced compaction on deformation band's petrophysical properties developed in poorly lithified sandstones of the Rio do Peixe Basin, Brazil. We compared undeformed sandstone and deformation bands associated with syn-rift normal faults and post-rift strike-slip faults by integrating structural, petrophysical, mineralogical and uniaxial compressive strength (UCS) measurements. The sealing behavior was interpreted from capillary pressure obtained from mercury injection porosimetry and in situ air permeability. Host rock consists of poorly lithified sandstone with 25–30% porosity, permeability of 1.3 Darcy, capillary pressure of 5 psi, and UCS of 20 MPa. The normal faults developed from soft-sediment conditions with moderate to intense cataclasis, and resulted in an overall fault zone compaction (deformation bands and zones between deformation bands) with localized porosity and permeability loss in deformation bands (10% and 12 mD) and rock volumes between deformation bands (20% and 0.7 Darcy). Compared to syn-rift deformation bands, the strike-slip deformation bands show similar porosity (12–13%), but lower permeability (1–10 mD) and greater capillary pressures (up to 114 psi). XRD analysis on clay minerals indicates that both syn-rift and post-rift faults formed under similar shallow burial depths (<1–2 km). Therefore, we propose a model that quantifies fault-induced compaction and may predict fault sealing in structurally complex, deformation bands-dominated sandstone reservoirs affected by multiple tectonic stages.

Unconfined compressive strength and pore structure evolution law of structural clay after disturbance

The clay in the Zhanjiang Formation has thixotropic properties, which has greatly influenced the foundation engineering in the Zhanjiang area. The evolution law of macroscopic strength and clay microstructure during thixotropy can be used to explain the practical engineering problems caused by thixotropy. For undisturbed and reconstituted soil curing for a different period, unconfined compressive strength test, scanning electron microscopy, and mercury injection porosimetry test were carried out to obtain the unconfined compressive strength and pore structure evolution law in the thixotropic process. The results indicate that the Zhanjiang Formation structural clay is very sensitive to disturbance and its unconfined compressive strength decreases from 180.29 to 11.73 kPa after the natural structure is completely destructed. After 300 d of curing, the unconfined compressive strength of clay increased from 11.73 to 53.43 kPa because of thixotropy, which increased by 3.55 times. The stacking flaky flocculation structure of the undisturbed soil is destructed by reconstituting, turning to flaky flocculation structure, and the large pores are homogenized, the small pores develop into medium pores, and there is a decrease in soil strength. In the process of thixotropy, the soil particles gradually coagulate and form an aggregates flocculation structure, and the strength of clay increases with the increase in the degree of cementation. Based on the results, the thixotropic pattern of clay was established and its thixotropic mechanism was explained.

Multiscale Pore Structure Characteristics and Crack Propagation Behavior of Coal Samples from High Gas Seam.

Investigation on the pore-fracture features and crack propagation behavior of coal is necessary to prevent coal mine disasters. The pore structure features of coal samples taken from high gas seam were obtained by mercury injection porosimetry (MIP) and gas adsorption methods. The process of deformation and failure for coal samples under three-point bending conditions were obtained. The results demonstrate that the adsorption pores with diameter less than 100 nm are the most developed and their surfaces are the roughest (the average surface fractal dimension Ds is 2.933). The surface of micro-cracks is smoother (Ds is 2.481), which is conducive to gas seepage. It may be the explanation for that 14-3# coal seam is a high gas seam, while there was almost no gas outburst accident so far. At the initial stage of crack propagation, the main crack on the coal sample expanded along the direction of the natural cracks. In the process of crack propagation, the surface fractal dimension of the main crack increased, suggesting that the bending degree of the main crack enhanced. The brittle characteristics of coal samples can be reflected by the ratio of the dissipated energy to the accumulated energy.

Investigation on the Fracture-Pore Evolution and Percolation Characteristics of Oil Shale under Different Temperatures

It is well known that underground in situ pyrolysis technology for oil shale production is a promising field. In the in situ modification mining process, the permeability property of a shale matrix has a great effect on the transport capacity of pyrolytic products. For oil shale undergoing pyrolysis, the changes of internal structure (fracture and pore space) have a considerable influence on the permeability network which further affects the migration of hydrocarbon products. In this study, based on an oil shale retorting experiment performed under different temperatures (20 °C, 100 °C, 200 °C, 300 °C, 325 °C, 350 °C, 375 °C, 400 °C, 425 °C, 450 °C, 475 °C, 500 °C, 525 °C, 550 °C, 575 °C, 600 °C), an investigation on the distribution characteristics of the fractures was conducted using micro-CT technology. Meanwhile, mercury injection porosimetry was used to characterize the pore structure of the oil shale samples under different temperatures. Finally, a fracture-pore dual medium model was constructed to calculate the percolation probability to quantitatively describe the permeability variation of oil shale with temperature. The test results indicated that the higher the temperature, the larger were the pore spaces. The increase in pore volume due to pyrolysis temperatures mainly affected the pores ranging from 10 nm to 100 nm and occurred in the specific temperature range (400 °C to 425 °C). Additionally, CT images show that the fracture morphology varied with increasing temperature and the number and length of fractures at different temperatures were in great accordance with the fractal law statistically. On the other hand, simulation of the percolation probabilities discovered that in a single pore media model over the whole range of tested temperatures they were too low to exceed the threshold. In contrast, in the dual medium model, the theoretical threshold of 31.16% was exceeded when the temperature reached 350 °C. Moreover, the results demonstrated that fractures dominated the seepage channel and had more significant effects on the permeability of oil shale. What has been done in this study will provide some guidance for the in situ fluidization mining of oil shale.

A Multi-Scale Fractal Approach for Coal Permeability Estimation via MIP and NMR Methods

Permeability in porous media has an important role in many engineering applications, which depends mainly on the pore size, distribution, and connectivity of porous media. As the pore structure distribution of coal has a multi-scale fractal dimension characteristic, this study aimed to propose a multi-scale fractal dimension characteristics units model (MFU) to describe the pore structure distribution by analyzing the multi-scale fractal dimension characteristics of coal pore media. Then, a multi-scale fractal permeability model was established based on MFU. The pore structure distribution was obtained by mercury injection porosimetry (MIP) and nuclear magnetic resonance (NMR) experiments. Based on MIP and NMR experimental data, the permeability contribution of different pore diameters were calculated. The results show that the permeability contribution of the micropore was minimal and can be ignored. The permeability contribution of mesopores was about 1–5%, and the permeability contribution of macropores was about 95–99%, which plays a decisive role in the seepage process. The calculated results, based on multi-scale fractal permeability model and the experimental permeability data, are in the same order of magnitude. The permeability prediction based on proposed model is better than classical single fractal permeability model.

Hydrothermal solidification of alkali-activated clay-slaked lime mixtures

Waste clay was converted into non-sintered building materials by hydrothermal solidification technology to improve resource utilization. The effects of curing time, curing temperature, initial dry density and alkaline environment on the strength development of the hydrothermally solidified clay-slaked lime mixtures were evaluated by compressive strength tests, and the microstructural evolution and solidification mechanism were analyzed by X-ray diffraction (XRD), thermogravimetry and differential scanning calorimetry (TG-DSC), scanning electron microscopy (SEM) and low-field nuclear magnetic resonance (NMR). The generated calcium silicate hydrate (C-S-H), katoite, and Al-substituted tobermorite filling pores and cementing mineral particles were the main factors responsible for the improved strength and compactness of the material. The C-S-H gel was transformed into Al-substituted tobermorite crystals in the presence of 4 mol/L NaOH solution, which greatly increased the compressive strength to 45.3 MPa. The transverse relaxation time (T2) distribution was transformed into a pore size distribution curve by combined mercury injection porosimetry (MIP) and NMR, which can effectively characterize the evolution of the pore structure. The synthesis conditions determined the type and quantity of hydrothermal products that affect the pore size distribution and microstructure, thereby exhibiting changes in structural and mechanical properties on the macroscale.