Low-temperature Nitrogen Gas Adsorption Research Articles

Fractal Characterization of the Pore-Throat Structure in Tight Sandstone Based on Low-Temperature Nitrogen Gas Adsorption and High-Pressure Mercury Injection

The pore-throat structure is a critical factor in the study of unconventional oil and gas reservoirs, drawing particular attention from petroleum geologists, and it is of paramount significance to analyze to enhance oil and gas production. In tight sandstone, which serves as a significant hydrocarbon reservoir, the internal pore-throat structure plays a decisive role in the storage and migration of fluids such as water, gases, and hydrocarbons. This paper employs casting thin section (CTS), field emission scanning electron microscope (FE-SEM), high-pressure mercury injection (HPMI), and low-temperature nitrogen gas adsorption (LT−N2−GA) experimental tests to qualitatively and quantitatively analyze the characteristics of the pore-throat structure in tight sandstone. The results indicate that the pore types in tight sandstone include intergranular residual pores, dissolution pores, intercrystalline pores, and microfractures, while the throat types encompass sheet-shaped, curved-sheet-shaped, and tubular throats. Analysis of the physical and structural parameters from 13 HPMI and 5 LT−N2−GA samples reveals a bimodal distribution of pore-throat radii. The complexity of the pore-throat structure is identified as a primary controlling factor for reservoir permeability. The fractal dimension (D) exhibits an average value of 2.45, displaying a negative correlation with porosity (R2 = 0.22), permeability (R2 = 0.65), the pore-throat diameter (R2 = 0.58), and maximum mercury saturation (R2 = 0.86) and a positive correlation with threshold pressure (R2 = 0.56), median saturation pressure (R2 = 0.49), BET specific surface area (R2 = 0.51), and BJH total pore volume (R2 = 0.14). As D increases, reservoir pores tend to decrease in size, leading to reduced flow and deteriorated physical properties, indicative of a more complex pore-throat structure.

Experimental investigation of gas diffusion kinetics and pore-structure characteristics during coalbed methane desorption within a coal seam

The desorption-diffusion properties of coalbed methane (CBM) are not only the theoretical basis for investigating gas outburst hazards in a coal mine, but also impose a non-negligible impact on CBM recovery performance. In this paper, the pore structure of coal with five different metamorphism degrees was qualitatively and quantitatively characterized with the scanning electron microscopy (SEM) and low-temperature nitrogen gas adsorption (LTNGA) measurements. The isothermal methane ad/desorption experiments were performed with four initial gas desorption pressure scales. Subsequently, the matching degree between eight empirical correlations (ECs) and the measured methane desorption data was determined, and the methane diffusion coefficient (Dfg) was numerically calculated combining with an FGDG (free gas density gradient) diffusion model. Importantly, we evaluated and discussed the sensitivity of pore structure parameters, initial gas desorption pressure, and volatile fraction to Dfg. The results show that pore density distribution differs among coal samples and the pore structure, especially the micropores, is developed better in high-rank coals than in low- and medium-rank ones. Initial gas release velocity increases with an increase in coal rank and decreases negatively with the increasing particle size in an exponential manner. Among the eight ECs, EC-VIII can be applied to more accurately describe gas desorption process during the whole time scale, and its correlation coefficient with the measured data is larger than 0.9900. Dfg increases with an increase in pore volume and specific surface area (SSA), and it is negatively correlated with the average pore diameter. Additionally, Dfg exhibits an asymmetric U-shaped pattern with an increase in the volatile fraction, which is mainly related to the switch between the dominant roles of mesopores and micropores during coal metamorphism. At a lower volatile fraction, the diffusion coefficient is more sensitive to micropores, while, at a higher volatile fraction, mesopores contribute more to the diffusion coefficient.

Interaction of resorcinol-formaldehyde carbon aerogels with water: A comprehensive NMR study

Carbon aerogels prepared from resorcinol-formaldehyde aerogels are promising platforms for electrodes, catalysts, adsorbents in environmental chemistry and as electric conductors. For these applications the knowledge of their structure and behavior in aqueous medium is essential. In this work two resorcinol-formaldehyde (RF) carbon aerogels prepared in different ways were characterized with various NMR methods while their pore structure was stepwise saturated with water. The wetting properties were studied by vapor adsorption and low-field NMR relaxometry, while the morphology was followed by NMR cryoporometry during the hydration process. At several water saturation levels the self-diffusion of water was measured. The comprehensive evaluation of the results led to a detailed description of the wetting process of these carbon aerogels beyond the pore size distributions. At low hydration level water clusters formed on and around the hydrophilic functional groups of the surface being able to adsorb water, but no continuous water layer developed on the surface. With increasing water content, spherical water drops formed inside the pore system, and vapor phase diffusion was observed in the partially filled pores. Subsequently the interconnected pore structure was saturated. The determined wet structure was compared to low temperature nitrogen gas adsorption results and scanning electron microscopy images.

Pore Fractal Characteristics of Suancigou Long-Flame Coal after Electrochemical Treatment: An Experimental Study through the Implementation of N2 Adsorption and Mercury Intrusion Prosimetry Techniques.

The application of electrochemical treatment in coal seams for enhancing coalbed methane (CBM) recovery can also decrease the risks of outburst disasters. The long-flame coal samples were electrochemically modified with 0, 1, 2, and 4 V/cm electric potential gradients, and the pore structures were measured and analyzed by combined low-temperature nitrogen gas adsorption, mercury intrusion prosimetry, and fractal theory. The experimental test results indicated that the pore volumes of macropores (>50 nm) and mesopores (2–50 nm) increased after electrochemical modification and further increased with the increase in electric potential gradient. The fractal dimensions of pores showed a decreasing trend except for the slight fluctuation of the mesopores with a size of 2–4.5 nm after modification, which indicated that the overall roughness and irregularity index of pores decreased. The evolution mechanisms of pore size distributions and their fractal dimensions were explained by the dissolution of minerals and the falling off of alkane side chains in the coal surface, which would expand and connect the pores during the electrochemical modification process. The results obtained from this work were crucial for CBM exploration via an electrochemical method.

A new application of atomic force microscopy in the characterization of pore structure and pore contribution in shale gas reservoirs

The pore structure determined by different materials is extremely significant for shale gas accumulation. Understanding how different materials contribute to the pore remains a knowledge gap. To elucidate the surface property and pore structure qualitative and quantitative characteristics of gas-bearing shales, the Atomic Force Microscopy (AFM) and low-temperature nitrogen gas adsorption (LT-N2GA) experiments were performed on shale samples selected from the Longmaxi Formation (Fm.) in the Sichuan Basin in China in this study. A new method based on AFM technique was used to calculate shale porosity, and the double-threshold discrete integration method was proposed to estimate the pore contributions of main materials. The results show that the organic-rich shale is significantly different from the organic-poor shale in material composition. The average value of mean roughness (Ra) of the organic-rich and organic-poor shale samples are 38.74 nm and 104.58 nm, respectively, indicating that the organic-poor shale has a rougher surface. The average value of mean square roughness (Rq) of the organic-rich and organic-poor shale samples are 49.91 nm and 138.80 nm, respectively, indicating that the organic-poor shale has a severer variation degree. Moreover, organic-poor shale samples also have a more concentrated height distribution. The average surface porosity of the organic-rich shale and organic-poor shale are 17.08% and 3.31% respectively. The average form factor (ff) of the organic-rich shale and organic-poor shale are respectively 0.102 and 0.386. It indicates that organic-rich shale has more pores and more complicated pore boundaries than organic-poor shale. AFM-calculated porosity is consistent with that of LT-N2GA-converted, which indicates that the porosity calculated by AFM is real porosity and reliable. The essence of the phase change on the shale surface is the difference in material composition. Shale pores are mainly contributed by organic matter, followed by chlorite, potash feldspar, and quartz. The results show that our method provides a powerful and effective tool to characterize the nanopore structure of organic rocks, obtain shale reservoir physical properties, and analyze pore contribution of shale components etc. issues, which has a wide application prospect.

Influencing factor analysis of the coal matrix compressibility of middle-high rank coals

The pore structure and coal matrix compressibility evolution law and their influencing factors of middle-high rank coals was evaluated based on the combined application of high pressure mercury injection, low-temperature nitrogen gas adsorption and low-temperature carbon dioxide adsorption. The middle-high rank coals are dominated by macropores and micropores, along with lower developed mesopores. The porosity, total pore volume and macropores have an increasing trend with increasing coalification degree. Meanwhile, micropores had gradually become more dominant. Coal matrix compressibility of middle-high rank coals is between 0.55 × 10−10 and 1.42 × 10−10 N/m2, which had significant effects on mesopores, while its influence on macropores rise with the increases of coal rank. There was a negative correlation between coal matrix compressibility and coalification degree, resulting from the increasing peak intensity and elastic modulus during the coalification processes. The polarity of the water molecules tends to reduce the peak strength and elastic modulus of coal. As a result, the deformability becomes more enhanced resulting in increases in coal matrix compressibility. Organic compositions and inorganic minerals have opposite effects on coal matrix compressibility. Inorganic matter can effectively resist coal matrix compressibility, resulting from the higher intensities and elastic features and their roles in supporting cleats and pores while organic matter has the opposite effect. Pore structural features have important influences on coal matrix compressibility. Coal matrix compressibility has a trend of increasing with the increasing in the porosity, total pore volume and macropores or the decreasing in micropores. In the maceral composition, the vitrinite was positively correlated with the peak intensity and elastic modulus of medium rank coal, while inertinite tends to be the opposite. There is a positive correlation between coal matrix compressibility and vitrinite, but the opposite occurs for inertinite, resulting from differences in their mechanical strength and elastic properties.

Coal-bed methane geology of the No. 2 coal seam in Fengfeng Coalfield, North China

The Fengfeng Coalfield is an important coal production base in the Hebei Province of North China. It has been reported that higher coal-bed methane (CBM) contents exist in this area showing good prospects for the exploration of CBM. However, there are few studies on CBM investigation in Fengfeng Coalfield. In order to better understand the geological controls on CBM in the Fengfeng Coalfield, geological surveys and laboratory experiments were conducted that included coal petrology analysis, proximate analysis, mercury intrusion, low-temperature liquid nitrogen adsorption, coal permeability analysis, and methane adsorption/desorption. The results show that the organic compositions are dominated by vitrinites, followed by inertinites, and minor liptinites. The CBM contents are range between 0.83 and 11.52 m3/t, with an average of 4.72 m3/t. The moisture contents are range from 0.76 to 2.54%, with an average of 1.21%, and the ash yields are range from 11.46 to 24.5%, with an average of 17.07%. Low-temperature nitrogen gas adsorption indicates the micropores provide most of the pore specific surface and the pore volume is mainly determined by small pores and mesopores. The geologic structures, coal-bearing strata, and the coal depositional environments were studied through field geological research and laboratory tests. The coals were deposited in a river-dominated shallow delta environment to form the mudstones or siltstones of the coal roof and floor that were used to seal the CBM. The CBM were mainly generated during stages of the Hercynian–Indosinian and the Early-Middle Yanshanian orogeny. Underground water flowed to the Dashucun Mine and its adjacent mines in the northeast of the Gushan Anticline to form a semi-closed hydrodynamic system that created the CBM emission and preservation. The coal rank increases from the southern to the northern areas. Then, it can be speculated that the CBM formation and preservation conditions in the north are better than in the south. The best prospective targets for CBM are in the northern areas. The results of this study may be useful for CBM exploration in North China.

Characterization and controlling factors of pores in the Lower Cambrian Niutitang shale of the Micangshan Tectonic Zone, SW China

Lower Cambrian marine shale has become a new target of shale gas resource evaluations in the complex geological region of southern China, where the Niutitang Formation is among the most important shale gas reservoirs. Recently, the micropore structural characterization and main controlling factors of the ancient shale have become popular research topics. The micropore structures of certain shale samples from the Niutitang Formation in the Micangshan tectonic zone were investigated at the microscale and nanoscale using low-temperature nitrogen gas adsorption (GA) and a combination of broad ion beam (BIB) milling and field-emission scanning electron microscopy (FE-SEM). Then, we analyzed how the mineral composition and thermal evolution affect the pore development in the Niutitang shale through the results of X-ray diffraction (XRD) and vitrinite equivalent reflectance (VRoequiv) calculated by bitumen reflectance (Rb). The BSE images by the BIB/FE-SEM system show that multiple pore types are present in the ancient Niutitang shale and form a complex pore network under the influences of mineral contents and thermal maturation.

Single Plant Derived Nanotechnology for Synergistic Antibacterial Therapies.

Multiple new approaches to tackle multidrug resistant infections are urgently needed and under evaluation. One nanotechnology-based approach to delivering new relevant therapeutics involves silicon accumulator plants serving as a viable silicon source in green routes for the fabrication of the nanoscale drug delivery carrier porous silicon (pSi). If the selected plant leaf components contain medicinally-active species as well, then a single substance can provide not only the nanoscale high surface area drug delivery carrier, but the drug itself. With this idea in mind, porous silicon was fabricated from joints of the silicon accumulator plant Bambuseae (Tabasheer) and loaded with an antibacterial extract originating from leaves of the same type of plant (Bambuseae arundinacea). Preparation of porous silicon from Tabasheer includes extraction of biogenic silica from the ground plant by calcination, followed by reduction with magnesium in the presence of sodium chloride, thereby acting as a thermal moderator that helps to retain the mesoporous structure of the feedstock. The purified product was characterized by a combination of scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), and low temperature nitrogen gas adsorption measurements. Antimicrobial activity and minimum inhibitory concentration of a leaf extract of Bambuseae arundinacea was tested against the bacteria Escherichia Coli (E. Coli) and Staphylococcus aureus (S. Aureus), along with the fungus Candida albicans (C. Albicans). A S. aureus active ethanolic leaf extract was loaded into the above Tabasheer-derived porous silicon. Initial studies indicate sustained in vitro antibacterial activity of the extract-loaded plant derived pSi (25 wt %, TGA), as measured by disk diffusion inhibitory zone assays. Subsequent chromatographic separation of this extract revealed that the active antimicrobial species present include stigmasterol and 2,6-dimethoxy-p-benzoquinone.

Characterization of the micropore systems in the high-rank coal reservoirs of the southern Sichuan Basin, China

To gain a better understanding of the characteristics of micropore systems in high-rank coal reservoirs, 51 coal core samples collected from 16 wells were examined by maceral analysis, proximate analysis, scanning electron microscopy observation, low-temperature nitrogen gas adsorption, and nuclear magnetic resonance (NMR). The results show that pores in coals can be divided into plant tissue hole, blowhole, dissolved pore, and intercrystalline pore, and they have three structure shapes: open pore, semiclosed pore, and “ink bottle” pore. The total specific surface area (Brunauer-Emmet-Teller [BET]) ranges from 0.611 to (7 to ), averaged at . The total pore volume (Barrett-Joyner-Halenda model [BJH]) ranges from 0.0018 to (0.0001 to ) with an average of , and it shows a good positive relationship with . The adsorption amount shows a good positive relationship with the total and . The average pore size ranges from 5.775 to 17.842 nm. Pores that are smaller than 5 nm make up the main contribution to the pore surface area, and those larger than 10 nm contribute greatly to the pore volume. Inertinite-rich coals have higher total specific surface area, pore volume, pore size, and adsorption capacity than those in vitrinite-rich coals. Lopingian coal reservoirs are characterized by low porosity and extremely low permeability obtained from NMR tests, and the permeability has a positive correlation with the porosity. The average permeability of inertinite is almost twice that of vitrinite .

Effect of Some Admixtures on the Surface Properties of the Hydration Products of Silica Fume/Ca(OH)2 Mixtures

The effect of sodium naphthalene formaldehyde sulphonate polycondensate and stearic acid on the surface properties of calcium silicate hydrates, formed as a result of the hydration reaction between silica fume and calcium hydroxide, has been examined by the use of low-temperature nitrogen gas adsorption techniques. In addition, the interaction of such admixtures with the calcium silicate hydrates formed was investigated by infrared (IR) spectroscopic analysis. It was observed that both admixtures increased the specific surface area of the calcium silicate hydrates quite considerably relative to the behaviour of a blank sample (i.e. without addition of admixture), especially during the initial stages of hydration (after 24 h). IR examination demonstrated that the admixtures used did not interact chemically with the calcium silicate hydrates although physical interaction was possible.

The dyeing of secondary cellulose acetate. Investigation of the surfaces of dyed fibers by low-temperature nitrogen adsorption

The surface areas of dyed and undyed secondary cellulose acetate fibers have been determined using low-temperature nitrogen gas adsorption by the method of Brunauer, Emmett, and Teller. The results confirmed earlier work using p-nitrophenol, in which it was shown that there is a marked change of surface area of dyed fiber in the region corresponding to a theoretical monolayer adsorption of the dye.