Shale Pore Network Characterization Across Thermal Maturity: A Comparison of He Pycnometry, Gas Adsorption, and Mercury Intrusion Porosimetry

Full-scale nanopore system and fractal characteristics of clay-rich lacustrine shale combining FE-SEM, nano-CT, gas adsorption and mercury intrusion porosimetry

The effect of analytical particle size on gas adsorption porosimetry of shale

Pore structure determines the gas occurrence and storage properties of gas shale and is a vital element for reservoir evaluation and shale gas resources assessment. Field emission scanning electron microscopy (FE-SEM), high-pressure mercury intrusion porosimetry (HMIP), and low-pressure N2/CO2 adsorption were used to qualitatively and quantitatively characterize full-scale pore structure of Longmaxi (LM) shale from the southern Sichuan Basin. Fractal dimension and its controlling factors were also discussed in our study. Longmaxi shale mainly developed organic matter (OM) pores, interparticle pores, intraparticle pores, and microfracture, of which the OM pores dominated the pore system. The pore diameters are mainly distributed in the ranges of 0.4–0.7 nm, 2–20 nm and 40–200 μm. Micro-, meso- and macropores contribute 24%, 57% and 19% of the total pore volume (PV), respectively, and 64.5%, 34.6%, and 0.9% of the total specific surface area (SSA). Organic matter and clay minerals have a positive contribution to pore development. While high brittle mineral content can inhibit shale pore development. The fractal dimensions D1 and D2 which represents the roughness of the shale surface and irregularity of the space structure, respectively, are calculated based on N2 desorption data. The value of D1 is in the range of 2.6480–2.7334 (average of 2.6857), D2 is in the range of 2.8924–2.9439 (average of 2.9229), which indicates that Longmaxi shales have a rather irregular pore morphology as well as complex pore structure. Both PV and SSA positively correlated with fractal dimensions D1 and D2. The fractal dimension D1 decreases with increasing average pore diameter, while D2 is on the contrary. These results suggest that the small pores have a higher roughness surface, while the larger pores have a more complex spatial structure. The fractal dimensions of shale are jointly controlled by OM, clays and brittle minerals. The TOC content is the key factor which has a positive correlation with the fractal dimension. Clay minerals have a negative influence on fractal dimension D1, and positive influence D2, while brittle minerals show an opposite effect compared with clay minerals.

Volcanic soils are important resources because of their unique mineralogical and physical characteristics, and allophanic Andosols represent some of the world’s most fertile soils. However, their unique properties can be lost when cultivated. Most soils in the Central Valley, Mexico, are derived from volcanic materials. This valley encompasses one of the largest water supply systems in the world by volume, but is affected by soil degradation and deforestation. Sustainably managing volcanic soils requires understanding how land use affects their hydrodynamic properties. Gas adsorption and mercury intrusion porosimetry, water retention curves, tension infiltrometry and X-ray tomography were used to describe pore structure characteristics. Two volcanic soils (one Andosol and one derived from indurated tuff – Tepetates), three land uses (maize monoculture, maize–wheat rotation and fallow) and two horizons (Ap and A2 for maize monoculture and maize–wheat rotation) were studied. Tillage affected topsoil by increasing the sand fraction by 38% and decreasing total porosity and macroporosity by 23% and 40% respectively. Macropore size was reduced and the number of isolated macropores was higher in the tilled layer under maize, compared with untilled subsoil. The plot under maize–wheat rotation had lower allophane content, and saturated hydraulic conductivity was reduced by nearly an order of magnitude and water retention by half, compared with maize and fallow plots. Compared with Andosols, Tepetates showed differences in mineralogical composition with lower contents of amorphous compounds and in its porous network characteristics with twice the total and percolating macroporosity compared with the maize plot. Its high content of organic carbon (3.5%) seemed beneficial for its hydrodynamic properties. Sustainable agricultural management of these volcanic soils requires reducing mechanised tillage, avoiding periods when soil is bare, not applying maize–wheat rotation and applying maize–fallow rotation allowing natural vegetation growth.

Characteristics of matrix-related pores associated with various lithofacies of marine shales inside of Guizhong Basin, South China

This study investigates unconventional resources such as coal and shale belonging to the Barakar Formation. Various analytical methods such as fluid invasive low-pressure gas adsorption (LPGA) and mercury intrusion porosimetry (MIP), small angle x-ray scattering (SAXS) and imaging methods were employed to determine the pore attributes and pore characteristics of coal and shale. The results show that coal has an abundance of nanopores that occurs in clusters, having evidence of microfractures in its structure, as observed through scanning electron microscopy (SEM). It was found that the accessible micropore surface area (SA) of coal samples is approximately 2.5 – 3 times that of shale samples, while the accessible and inaccessible mesopore SA in coal is about half of that in shale. Nevertheless, the average pore width of the coal samples is around 0.8 – 0.9 times that of the shale samples. These results suggest that the coal has a higher percentage of organic carbon that contributes to the abundance of organic pores, that leads to higher porosity in coal samples compared to shale samples. The total SA, incorporating the entire spectrum of pore sizes, is about 2 times as large in coal as in shale. Interestingly, despite disparity in pore SA and pore volume, the pore surface roughness in coal is nearly equal to or slightly higher than that of shale. The study provides a detailed analysis of the pore structures of unconventional resources, such as coal and shale from the same reservoir, considering various parameters such as depth, mineralogical content and surface roughness. During CO2 gas injection, the coal and shale formations may experience change in geomechanical responses, potentially compromising their mechanical stability. Furthermore, any loss to the caprock integrity could result in leakage and reservoir failure. Thus, this study is critical for estimating the secure CO2 storage capacity of coal and shale reservoirs. The findings aim to optimize gas adsorption while maintaining structural stability, ensuring the long-term feasibility of CO2 sequestration in other basins.

Influence of the relative volumes between catalyst and Nafion ionomer in the catalyst layer efficiency

Robust engineering of geomaterials for energy applications requires a clear understanding of the impacts of temperatures and pressures applied to the soil on their microstructures. Such understandings will facilitate better designs of new geomaterials and technologies via ensuring accurate assessments of the performance of the existing ones. In this study, we assess the changes in the microstructure—specific surface area and pore size distribution—of a saturated clay subjected to stress and temperature cycle. Clay specimens were subjected to the desired mechanical stresses and thermal cycles in a triaxial system. Then, the specimens were swiftly extracted from the triaxial, flush frozen in liquid nitrogen, then freeze-dried to preserve their microstructure. The preserved specimens were then used for specific surface area and pore size distribution assessments using nitrogen (N2)-gas adsorption and mercury intrusion porosimetry. The results established qualitative explanations of the expected microstructural changes in geomaterials under operational conditions, which facilitate the development of new geomaterials that can overcome such alternations.

The nano-scale pore systems in shale reservoirs control shale gas transportation and aggregation, which is of great significance for the resource evaluation of shale oil and gas and the selection of a “sweet spot”. Taking twelve marine shale samples from the Wufeng–Longmaxi Formation in the Zigong area, southwest Sichuan Basin, as the research target, we carried out a series of experiments, including total organic carbon (TOC) analysis, X-ray diffraction (XRD), gas adsorption (CO2 + N2), and mercury intrusion porosimetry (MIP), to study the full-scale pore structure characterization and controlling factors of pore volume and specific surface area. The results presented the following findings. (1) Marine shale samples from the target area are rich in organic matter, with an average TOC value of 3.86%; additionally, the mineral composition was dominated by quartz and clay minerals, with average contents of 44.1% and 31.4%, respectively. (2) The full-scale pore size distribution curves of pore volume developed multimodally, with the main peaks at 0.5 nm–2 nm, 3 nm–6 nm, and 700 nm–2.2 um; moreover, the full-scale pore size distribution curves of a specific surface area developed unimodally, with the main peak ranging from 0.5 nm to 1.2 nm. (3) Pore volume was mainly contributed by mesopores and macropores, with an average contribution of 46.66% and 42.42%, respectively, while the contribution of micropores was only 10.91%. The specific surface area was mainly contributed by micropores and mesopores, with an average contribution of 64.63% and 29.22%, respectively, whereas the contribution of micropores was only 6.15%. (4) The TOC content mainly controlled the pore volume and specific surface area of micropores and mesopores, while the clay and feldspar content generally controlled the pore volume and specific surface area of macropores. Additionally, the quartz content had an inhibitory effect on the development of all pore types. These results will help researchers understand the laws of gas accumulation and migration.

Hybrid approach combining multiple characterization techniques and simulations for microstructural analysis of proton exchange membrane fuel cell electrodes

Pore changes of slickwater-containing shale under supercritical CO2 treatment

The Silurian Longmaxi (S1l) marine shale and Jurassic Dongyuemiao (J1d) lacustrine shale in the Sichuan Basin, West China have attracted considerable attention from the oil-and-gas industry in China. Currently, the differences in pore types and pore structures between them are poorly understood, which has limited shale-resource exploration in the Sichuan Basin. This paper systemically compares the pore characteristics of Longmaxi shale and Dongyuemiao shale and investigates their impact on shale-gas enrichment by integrating field-emission scanning electron microscopy (FE–SEM), X-ray diffraction (XRD), low-pressure gas (CO2 and N2) adsorption and mercury-intrusion porosimetry, high-pressure sorption isotherms, gas-saturation measurement, molecular-dynamics simulation, etc. The results show that the S1l organic-rich marine shale and the J1d lacustrine shale have different pore types and pore structures. The S1l shale is dominated by organic pores, mainly micropores and mesopores with ink-bottle-like pore shapes, while the J1d shale is primarily composed of clay-mineral pores, mainly mesopores and macropores with slit- or plate-like pore shapes. Organic pores can provide considerable storage space for shale-gas enrichment in S1l marine shale, which also determines the adsorption capacity of shale reservoirs. Although organic pores are not the most prevalent in the Dongyuemiao lacustrine shale, they also play an important role in enhancing reservoir quality and absorbed-gas enrichment. Clay-mineral pores contribute weakly to the storage space of J1d-lacustrine-shale reservoirs. Mesopores are the most important form of storage space in both S1l shale and J1d shale, contributing significantly to shale-gas enrichment. Micropores are secondary in importance in S1l marine shale, while macropores are secondary contributors to pore volume in J1d lacustrine shale.

The pore structure of shale has a significant effect on hydrocarbon migration and the long-term gas supply of shale gas wells. The present study investigates the spontaneous imbibition characteristics to evaluate the pore connectivity and wettability of marine Longmaxi shale samples from the southeastern Chongqing area and continental Yanchang shale samples from the Ordos Basin. The pore-size distribution obtained from N2 adsorption and mercury intrusion porosimetry, field emission–scanning electron microscopy, and focused ion beam–scanning electron microscopy photos are used to interpret the imbibition behaviors. Our results show that the difference in dominant pore type between marine and continental samples, which is dominated by thermal maturity, controls on their imbibition behaviors as well as their wettability. Organic matter (OM) pores within Yanchang samples are poorly developed because of their low thermal maturity, and a large amount of water-wet inorganic pores are preserved in these samples because of relatively weak compaction. Oil-wet OM pores are well developed in Longmaxi samples with higher thermal maturity, and inorganic pores have been largely eliminated because of strong compaction. The low pore connectivity to water for both the Longmaxi and Yanchang samples is indicated by the low water imbibition slopes. Furthermore, the more oil-wet property of the Longmaxi samples and more water-wet characteristics of the Yanchang samples are obtained by comparing the directional water/oil imbibition slopes. In addition, the positive meaning of quartz in the protection of pore spaces is found in both the Longmaxi and the Yanchang samples used in this study.

Effect of lithofacies on pore structure and new insights into pore-preserving mechanisms of the over-mature Qiongzhusi marine shales in Lower Cambrian of the southern Sichuan Basin, China

The moisture adsorption and desorption performance of five diatomite samples with different pore structures were tested under high and low relative humidity (98% and 11% relative humidity) and constant temperatures of 20°C and 30°C. The mineral compositions and surface morphologies of diatomite samples were characterized by x-ray diffraction and scanning electron microscopy, respectively. The pore structure of diatomite samples were analyzed by nitrogen gas adsorption and mercury intrusion porosimetry. The relationship of pore structure and humidity control performance were analyzed and discussed. It is indicated that the humidity control performance of diatomite in general enhances as the environmental temperature increases. The Shengxian diatomite presents better humidity control performance among these five samples. According to the dynamics study, the moisture adsorption and desorption performance follow the pseudo-second-order model. The mesoporous in diatomite is vitally important for the humidity controlling. The specific surface area and mesoporous volume are the two key factors for the moisture adsorption and desorption ability. Diatomite with high specific surface area and large mesoporous volume has better humidity control performance. Moreover, the impurities could change the pore structure, then further affect the humidity control performance of diatomite.