Model Applicability of Characterizing Shale Pore Structure by Low-Pressure Nitrogen Adsorption Method

Adsorption thermodynamic characteristics are an important part of the methane adsorption mechanism, and are useful for understanding the energy transmission mechanism of coalbed methane (CBM) migration in coal reservoirs. To study the effect of coal pore characteristics on methane adsorption heat, five different types of rank coals were used for low-pressure nitrogen, low-pressure carbon dioxide, and methane adsorption experiments. Pore structure and adsorption parameters, including maximum adsorption capacity and adsorption heat, were obtained for five coal samples, and their relationships were investigated. The results show that the low-pressure nitrogen adsorption method can measure pores within 1.7–300 nm, while the low-pressure carbon dioxide adsorption method can measure micropores within 0.38–1.14 nm. For the five coal samples, comprehensive pore structure parameters were obtained by combining the results of the low-pressure nitrogen and carbon dioxide adsorption experiments. The comprehensive results show that micropores contribute the most to the specific surface area of anthracite, lean coal, fat coal, and lignite, while mesopores contribute the most to the specific surface area of coking coal. Mesopores contribute the most to the pore volume of the five coal samples. The maximum adsorption capacity has a significant positive correlation with the specific surface area and pore volume of micropores less than 2 nm, indicating that methane is mainly adsorbed on the surface of micropores, and can also fill the micropores. The adsorption heat has a significant positive correlation with the specific surface area and pore volume of micropores within 0.38–0.76 nm, indicating that micropores in this range play a major role in determining the methane adsorption heat.

The effect of sample particle size on the determination of pore structure parameters in shales

Granular samples are often used to characterize the pore structure of shale. To systematically analyze the influence of particle size on pore characteristics, case studies were performed on two groups of organic-rich deep shale samples. Multiple methods, including small-angle neutron scattering (SANS), low-pressure nitrogen gas adsorption (LP-N2GA), low-pressure carbon dioxide gas adsorption (LP-CO2GA), and XRD analysis, were adopted to investigate how the crushing process would affect pore structure parameters and the fractal features of deep shale samples. The research indicates that with the decrease in particle size, the measurements from nitrogen adsorption and SANS experiments significantly increase, with relative effects reaching 95.09% and 51.27%, respectively. However, the impact on carbon dioxide adsorption measurements is minor, with a maximum of only 8.97%. This suggests that the comminution process primarily alters the macropore structure, with limited influence on the micropores. Since micropores contribute the majority of the specific surface area in deep shale, the effect of particle size variation on the specific surface area is negligible, averaging only 16.52%. Shales exhibit dual-fractal characteristics. The distribution range of the mass fractal dimension of the experimental samples is 2.658–2.961, which increases as the particle size decreases. The distribution range of the surface fractal dimension is 2.777–2.834, which decreases with the decrease in particle size.

Micro-pore structure and fractal characteristics of deep shale from Wufeng Formation to Longmaxi Formation in Jingmen exploration area, Hubei Province, China

In order to explore the pore characterizations in shales during organic matter evolution, a series of simulation experiments were conducted. The artificial hydrous pyrolysis was conducted on the same seven columned oil-shale samples at 250 °C, 300 °C, 350 °C, 375 °C, 400 °C, 450 °C and 500 °C, respectively. To obtain the characteristics of pore structures in shales, the unheated and the residual solid samples were analyzed by low-pressure nitrogen adsorption method. Based on the nitrogen adsorption isotherms, fractal dimensions were calculated by the model of Frenkel–Halsey–Hill, which also contained the fractal dimension of D1 and D2 before and after the relative pressure P/Po = 0.5, respectively. And then the relationships of simulation temperatures (thermal maturity), total-, macro-, meso- and micro-pores volumes, specific surface areas and diameters to fractal dimensions were investigated. The results showed that the average value of D2 (2.6110) was higher than D1 (2.4147) and there was a positive relationship between them (R2 = 0.9237), which indicated that though D2 and D1 were more related to pore structures and surfaces, the better linear relationships suggested that both of them could be used in the representation of pore structures and surfaces in shales. With the thermal maturity increasing, the obvious fractal characteristics were, the complexity of pore structures were, which may be associated with the following cause-and-effect relationships. During the pyrolysis, the generation of hydrocarbons increased, as well as the consumption of TOC may increase the volume and surface area of total-, macro-, meso- and micro-pores but decrease the corresponding average diameter and then the quantities of smaller pores occurred and led to the strengthening of pore heterogeneity in shales. Based on the fractal characteristics, we also found the higher thermal maturity would result in the better connections among pores but worse permeability in shale, which further increased the gas adsorption quantity. Therefore, analyzing the fractal characteristics in shales could provide help for clarifying the characteristics of reservoirs as well as the comprehensive exploration and development of shale gas.

Summary The pore structure of unconventional gas reservoirs, despite having a significant impact on hydrocarbon storage and transport, has historically been difficult to characterize because of a wide pore-size distribution (PSD), with a significant pore volume (PV) in the nanopore range. A variety of methods is typically required to characterize the full pore spectrum, with each individual technique limited to a certain pore size range. In this work, we investigate the use of nondestructive, low-pressure adsorption methods, in particular low-pressure N2 adsorption analysis, to infer pore shape and to determine PSDs of a tight gas silt-stone reservoir in western Canada. Unlike previous studies, core-plug samples, not crushed samples, are used for isotherm analysis, allowing an undisturbed pore structure (i.e., uncrushed) to be analyzed. Furthermore, the core plugs used for isotherm analysis are subsamples (end pieces) of cores for which mercury-injection capillary pressure (MICP) and permeability measurements were previously performed, allowing a more direct comparison with these techniques. PSDs, determined from two isotherm interpretation methods [Barrett-Joyner-Halenda (BJH) theory and density functional theory (DFT)], are in reasonable agreement with MICP data for the portion of the PSD sampled by both. The pore geometry is interpreted as slot-shaped, as inferred from isotherm hysteresis loop shape, the agreement between adsorption- and MICP-derived dominant pore sizes, scanning-electron-microscope (SEM) imaging, and the character of measured permeability stress dependence. Although correlations between inorganic composition and total organic carbon (TOC) and between dominant pore-throat size and permeability are weak, the sample with the lowest illite clay and TOC content has the largest dominant pore-throat size and highest permeability, as estimated from MICP. The presence of stress relief-induced microfractures, however, appears to affect laboratory-derived (pressure-decay and pulse-decay) estimates of permeability for some samples, even after application of confining pressure. On the basis of the premise of slot-shaped pore geometry, fractured rock models (matchstick and cube) were used to predict absolute permeability, by use of dominant pore-throat size from MICP/adsorption analysis and porosity measured under confining pressure. The predictions are reasonable, although permeability is mostly overpredicted for samples that are unaffected by stress-release fractures. The conceptual model used to justify the application of these models is slot pores at grain boundaries or between organic matter and framework grains.

Heat-treatment of carbon blacks obtained by pyrolysis of used tires. Effect on the surface chemistry, porosity and electrical conductivity

Investigation of pore structure and fractal characteristics of organic-rich shale reservoirs: A case study of Lower Cambrian Qiongzhusi formation in Malong block of eastern Yunnan Province, South China

Influences of subcritical and supercritical CO2 treatment on the pore structure characteristics of marine and terrestrial shales

Occurrence and influence of residual gas released by crush methods on pore structure in Longmaxi shale in Yangtze Plate, Southern China

Pore structure characterization of Carboniferous shales from the eastern Qaidam Basin, China: Combining helium expansion with low-pressure adsorption and mercury intrusion

Characterization of pore structure and fractal dimension of Paleozoic shales from the northeastern Sichuan Basin, China

One of the main techniques for the exploitation of shale oil and gas is hydraulic fracturing, and the fracturing fluid (slick water) may interact with minerals during the fracturing process, which has a significant effect on the shale pore structure. In this study, the pore structure and fluid distribution of shale samples were analyzed by utilizing low-pressure liquid nitrogen adsorption (LP-N2GA) and nuclear magnetic resonance (NMR). The fractal analysis showed that the pore structure of the sample was strongly heterogeneous. It was also found that the effect of slick water on pore structure can be attributed to two phenomena: the swelling of clay minerals and the dissolution of carbonate minerals. The swelling and dissolution of minerals can exist at the same time, and the strength of them at different soaking times is different, leading to the changes in specific surface area and pore size. After the samples were soaked in the slick water for two days, the contact angle reached the minimum value (below 8°), which means the sample is strongly hydrophilic; then the contact angle increased to above 38° with longer soaking times. The connected pore space in the shale matrix is enlarged by the soaking processing. Therefore, an in-depth understanding of the interaction between the fracking fluid and shale is essential to deepen our understanding of changes in the pore structure in the reservoir and the long-term productivity of shale gas.

The impact of supercritical CO2 on the pore structure and storage capacity of shales

Characteristics and controlling factors of pore structure of shale in the 7th member of Yanchang Formation in Huachi area, Ordos Basin, China