Multifractal characteristics of shale and tight sandstone pore structures with nitrogen adsorption and nuclear magnetic resonance

Based on the experiments of nitrogen gas adsorption(N_2 GA) and nuclear magnetic resonance(NMR),the multifractal characteristics of pore structures in shale and tight s andstone from the Chang 7 member of Trias sic Yanchang Formation in Ordos Basin,NW China,are investigated.The multifractal spectra obtained from N2 GA and NMR are analyzed with pore throat structure parameters.The results show that the pore size distributions obtained from N2 GA and NMR are different,and the obtained multifractal characteristics vary from each other.The specific surface and total pore volume obtained by N2 GA experiment have correlations with multifractal characteristics.For the core samples with the similar specific surface,the value of the deviation of multifractal spectra Rd increases with the increase in the proportion of large pores.When the proportion of macropores is small,the Rd value will increase with the increase in specific surface.The multifractal characteristics of pore structures are influenced by specific surface area,average pore size and adsorption volume measured from N2 GA experiment.The multifractal characteristic parameters of tight sandstone measured from NMR spectra are larger than those of shale,which may be caused by the differences in pore size distribution and porosity of shale and tight sandstone.

Improved analytic methods for coal surface area and pore size distribution determination using 77 K nitrogen adsorption experiment

77 K nitrogen adsorption was the most widely used technique for determining surface area and pore size distribution of coal. Brunauer–Emmett–Teller(BET) and Barrett–Joyner–Halenda(BJH) model are commonly used analytic methods for adsorption/desorption isotherm. A Chinese anthracite coal is tested in this study using an improved experimental method and adsorption isotherm analyzed by three adsorption mechanisms at different relative pressure stages. The result shows that the micropore filling adsorption predominates at the relative pressure stage from 6.8E 7 to 9E 3. Theoretically, BET and BJH model are not appropriate for analyzing coal samples which contain micropores. Two new analytic procedures for coal surface area and pore size distribution calculation are developed in this work. The results show that BET model underestimates surface area, and micropores smaller than 1.751 nm account for 35.5% of the total pore volume and 74.2% of the total surface area. The investigation of surface area and pore size distribution by incorporating the influence of micropore is significant for understanding adsorption mechanism of methane and carbon dioxide in coal.

Pore characterization of different types of coal from coal and gas outburst disaster sites using low temperature nitrogen adsorption approach

To characterize the pore features of outburst coal samples and investigate whether outburst coal has some unique features or not, one of the authors, working as the member of the State Coal Mine Safety Committee of China, sampled nine outburst coal samples(coal powder and block) from outburst disaster sites in underground coal mines in China, and then analyzed the pore and surface features of these samples using low temperature nitrogen adsorption tests. Test data show that outburst powder and block coal samples have similar properties in both pore size distribution and surface area. With increasing coal rank, the proportion of micropores increases, which results in a higher surface area. The Jiulishan samples are rich in micropores, and other tested samples contain mainly mesopores, macropores and fewer micropores. Both the unclosed hysteresis loop and force closed desorption phenomena are observed in all tested samples. The former can be attributed to the instability of the meniscus condensation in pores,interconnected pore features of coal and the potential existence of ink-bottle pores, and the latter can be attributed to the non-rigid structure of coal and the gas affinity of coal.

The occurrence characteristics of oil in shales matrix from organic geochemical screening data and pore structure properties:An experimental study Author links open overlay panel

The occurrence characteristics of shale oil are of great significance to the movability of shale oil.In this study,the occurrence characteristics of oil in the shale matrix at Funing Formation shale in Subei Basin were quantitatively evaluated by organic geochemistry and microscopic pore structure characterization experiments.The Multiple Isothermal Stages Pyrolysis(MIS)experiment results show that the content of total oil,adsorbed oil,and free oil in the shales are 3.15-11.25 mg/g,1.41-4.95 mg/g,and 1.74-6.51 mg/g,respectively.among which the silicon-rich shale has the best oil-bearing.The relative content of free oil shows an increasing trend in pores with pore diameters greater than 3 nm.When the relative content of free oil reaches 100%,the pore size of silicon-rich shale is about 200 nm,while that of calcium-rich shale,clay-rich shale,and siliceous mixed shale is about 10 nm.The occurrence law of adsorbed oil is opposite to that of free oil,which indicates that shale oil will occur in the pores and fractures in a free state in a more extensive pore size range(>200 nm).This study also enables us to further understand the occurrence characteristics of shale oil under the interaction of occurrence state and occurrence space.

Distribution characteristics and oil mobility thresholds in lacustrine shale reservoir:Insights from N_(2)adsorption experiments on samples prior to and following hydrocarbon extraction

The pore structure and oil content of shales have an important influence on the oil mobility and enrichment.In this study,the lacustrine shale samples from the Qingshankou Formation(Q1)of Songliao Basin were selected.TOC,pyrolysis,XRD and nitrogen adsorption were performed on the original and extracted shale samples.Then the influence of mineral composition and organic matter(OM)on the development of nano-scale pore,the oil phase states and mobility were analyzed.The Q1 shale samples can be sub-divided into three types according to the isotherm characteristics.Type A samples are characterized by high kerogen content,with oil mainly existing in the free phase state.Type B samples are characterized by medium kerogen content,oil mainly exists in the absorbed phase state.Type C samples are characterized by low kerogen content,with trace oil found in the absorbed phase state.Nano-scale organic pores are well developed in the Q1 Formation.Oil is primarily found in the pore spaces with diameters less than 10 nm,this being the pore size threshold for mobile shale oil.When TOC>2.0 wt%and EOM>1.0 wt%,Q1 Formation shale oil mobility is high,resulting in prospective drilling targets.

Scanning tunneling microscopy study of surface reconstruction induced by N adsorption on Cu(100) surface

The reconstructed structure of Cu （100） surface induced by atomic N adsorption is studied by using scanning tunneling microscopy （STM）. The 2D structure of copper boundary between neighbouring N covered islands is found to be sensitive to the growth conditions, e.g. N＋ bombardment time and annealing temperature. The copper boundary experiences a transition from nano-scale stripe to nano-particle when the substrate is continuously annealed at 623~K for a longer time. A well-defined copper-stripe network can be achieved by precisely controlling the growth conditions, which highlights the possibility of producing new templates for nanofabrication.

Porosity of gas shale:Is the NMR-based measurement reliable?

Aiming at obtaining an accurate porosity of gas shale,various approaches are attempted.Therein,nuclear magnetic resonance(NMR),being treated as a kind of new-developed technique,possesses the representative significance.However,as a booming technique,the reliability of NMR-based porosity of shale is not exactly defined.Depending on NMR device,this work measured the porosity of shale experiencing different water soaking time,accordingly,judging the reliability of NMR-based porosity.Results indicate the NMR outcomes vary with the water soaking time,making a doubt about the objectivity of NMRbased porosity in reflecting the real shale porosity.Furthermore,some supplementary means were adopted to verify the water soaking-induced variation in the pore system of shale sample,which intensities the suspicion if the NMR-based porosity is reliable or not.To sum up,this work considers that the NMR-based porosity of shale is not reliable enough when water is used as the probe.Besides,this work also offers some suggestions on how to enhance the reliability of NMR-based porosity of shale sample.Basically,this work selects a fresh perspective to analyze the NMR approach in determining shale porosity,which is hopefully helpful in promoting the development of NMR technique in the shale-related field.

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

The composition of gas released under vacuum by crushing from the gas shale of Longmaxi Formation in Upper Yangtze Plate,Southern China was systematically investigated in this study.The effect of residual gas release on pore structures was checked using low-pressure nitrogen adsorption techniques.The influence of particle size on the determination of pore structure characteristics was considered.Using the Frenkel-Halsey-Hill method from low-pressure nitrogen adsorption data,the fractal dimensions were identified at relative pressures of 0‒0.5 and 0.5‒1 as D1 and D2,respectively,and the evolution of fractal features related to gas release was also discussed.The results showed that a variety component of residual gas was released from all shale samples,containing hydrocarbon gas of CH4(29.58%‒92.53%),C2H6(0.97%‒2.89%),C3H8(0.01%‒0.65%),and also some non-hydrocarbon gas such as CO2(3.54%‒67.09%)and N2(1.88%‒8.07%).The total yield of residual gas was in a range from 6.1μL/g to 17.0μL/g related to rock weight.The geochemical and mineralogical analysis suggested that the residual gas yield was positively correlated with quartz(R^2=0.5480)content.The residual gas released shale sample has a higher surface area of 17.20‒25.03 m^2/g and the nitrogen adsorption capacity in a range of 27.32‒40.86 ml/g that is relatively higher than the original samples(with 9.22‒16.30 m^2/g and 10.84‒17.55 ml/g).Clearer hysteresis loop was observed for the original shale sample in nitrogen adsorption-desorption isotherms than residual gas released sample.Pore structure analysis showed that the proportions of micro-,meso-and macropores were changed as micropores decreased while meso-and macropores increased.The fractal dimensions D1 were in range from 2.5466 to 2.6117 and D2 from 2.6998 to 2.7119 for the residual gas released shale,which is smaller than the original shale.This factor may indicate that the pore in residual gas released shale was more homogeneous than the original shale.The results indicated that both residual gas and their pore space have few contributions to shale gas production and effective reservoir evaluation.The larger fragments samples of granular rather than powdery smaller than 60 mesh fraction of shale seem to be better for performing effective pore structure analysis to the Longmaxi shale.

The pore structure of tight limestone-Jurassic Ziliujing Formation, Central Sichuan Basin, China

The pore structure of the tight limestone in the Daanzhai Member of the Ziliujing Formation, Jurassic System, in central Sichuan Basin, China, is complex but essential to the exploration and development of tight oil. The pore structure of the tight limestone is studied by using scanning electron microscopy （SEM）, nitrogen adsorption, high-pressure mercury intrusion, and nuclear magnetic resonance （NMR）. The experimental results suggest that the pores are mainly slit pores and mesopores and macropores contribute to the pore volume and specific surface. The displacement pressure, average pore size, and homogeneity coefficient correlate with porosity and permeability and can be used to evaluate the pore structure. The full pore-size distribution was obtained by combining nitrogen adsorption and high-pressure mercury intrusion. We find that the limestone mainly contains mesopores with diameter of 2-50 nm. The T2 distribution was converted into pore-size distribution, well matching the full pore-size distribution. The relation between T2 and pore size obeys a power law and the geometric mean of T2 correlates with the pore structure and can be used in the pore structure evaluation.

Microscopic pore characteristics of andesite and implication from mineral content:A case study in Huoshiling Formation of Changling fault depression,Songliao Basin

In order to study the microscopic pore characteristics of andesite reservoir and the effect of mineral content on the andesite pore,this study takes the andesite of the Huoshiling Formation in Longfengshan of Changling fault depression,Songliao Basin as a subject.The andesite reservoir space was discerned through the observation of cores and casting thin sections.Besides,the pore size distribution of andesites and their mineral contents were quantitatively characterized by high-pressure mercury injection,nitrogen adsorption and XRD,respectively.The results show that:(1)There are various types of reservoir space in andesites,including vesicles,amygdule,intergranular pores,matrix dissolution pores and dissolution pores of amygdala,and three types of fractures including dissolution,structural and explosion fractures.(2)The pore size distribution of andesite is complex.The main pore size of andesite is mid-pore(10-20 nm)with some large-pores(>50 nm).Mid-pore and large-pore provide the main specific surface area,which are the main space for gas storage.(3)The andesite reservoir space in the study area is mainly controlled by dissolution,as supported by the relationship between the change of mineral content and porosity evolution.The porosity of andesites decreases with the increase of quartz and chlorite content,but increases with the increase of soluble mineral,e.g.,feldspar content.

Multi-Scale Pore Structure Heterogeneity in Tuff Reservoirs Investigated with Multi-Experimental Method and Fractal Dimensions in Chang 7 Formation,Southern Ordos Basin

The tight tuff reservoir is an unusual type of unconventional reservoir with strong heterogeneity.However,there is a lack of research on the microscopic pore structure that causes the heterogeneity of tuff reservoirs.Using the Chang 7 Formation in Ordos Basin,China as a case study,carbon-dioxide gas adsorption,nitrogen gas adsorption and high-pressure mercury injection are integrated to investigate the multi-scale pore structure characteristics of tuff reservoirs.Meanwhile,the fractal dimension is introduced to characterize the complexity of pore structure in tuff reservoirs.By this multi-experimental method,the quantitative characterizations of the full-range pore size distribution of four tuff types were obtained and compared in the size ranges of micropores,mesopores and macropores.Fractal dimension curves derived from full-range pores are divided into six segments as D1,D2,D3,D4,D5 and D6 corresponding to fractal characteristics of micropores,smaller mesopores,larger mesopores,smaller macropores,medium macropores and larger macropores,respectively.The macropore volume,average macropore radius and fractal dimension D5 significantly control petrophysical properties.The larger macropore volume,average macropore radius and D5 correspond to favorable pore structure and good reservoir quality,which provides new indexes for the tuff reservoir evaluation.This study enriches the understanding of the heterogeneity of pore structures and contributes to unconventional oil and gas exploration and development.

Characterization of the pore system in an over-mature marine shale reservoir:A case study of a successful shale gas well in Southern Sichuan Basin,China

During the past two years the shale gas exploration in Southern Sichuan basin received some exciting achievements.Data of a new appraisal well showed that the gas producrtions of vertical well and horizontal well are^1.5×104 m3/day/well(with maximum^3.5×104 m3/day/well)and^12.5×104 m3/day/well(with maximum^40×104 m3/day/well),respectively,indicating a good gas potential in this area.Eight core samples from the reservoir were investigated by using a carbon sulfur analyzer,microphotometry,x-ray diffractometry,field-emission scanning electron microscopy(FE-SEM),mercury injection porosimetry(MIP),and low-pressure nitrogen adsorption to obtain a better understanding of the reservoir characteristics of the Upper OrdovicianeLower Silurian organic-rich shale.Results show that the total organic carbon(TOC)content ranges from 0.5%to 5.9%,whereas the equivalent vitrinite reflectance(VRr)is between 2.8%and 3.0%.Pores in the studied samples were observed in three modes of occurrence,namely,interparticle pores,intraparticle pores,and intraparticle organic pores.The total porosity(P)ranges from 1.6%to 5.3%,and MIP data sets suggest that pores with throats larger than 20 nm contribute little to the pore volume.Low-pressure N2 adsorption isotherms indicate that the total specific surface area(SBET)ranges from 9.6 m2/g to 18.9 m2/g,and the pore volume(V)ranges from 0.011 cm3/g to 0.020 cm3/g.The plot of dV/dW versus W shows that the fine mesopores(pore size(BJH)<4 nm)mainly contribute to the pore volume.The P,SBET,and V show a good positive correlation with TOC and a weak positive correlation with the total clay mineral content,thus indicating that the nanopores are mainly generated by the decomposition of organic matter.The reservoir characteristics of the Upper OrdovicianeLower Silurian organic-rich shale are comparable with commercial shale gas plays in North America.The sample gas contents with TOC>2%are more than 3.0 m3/ton.The observation can be a good reference for the future exploration and evaluation of reservoir in this area.

Micro-pore Structure and Gas Accumulation Characteristics of Shale in the Longmaxi Formation, Northwest Guizhou

For further study on characteristics of micro-pores in the marine shale reservoirs in Northwest Guizhou,shale samples in the Lower Silurian Longmaxi Formation were selected to investigate effect of micro-pores on the characteristics of gas accumulation in shales,through methods of the argon broad ion beam-field emission scanning electron microscope and the nitrogen adsorption/desorption,analysis on the characteristics of micro-pores and related geochemical analysis.Results show that there are seven types of pores in the Longmaxi Formation shale,including interparticle pores,intraparticle pores,intercrystalline pores,dissolution pores,fossil pores,organic pores,and microfractures,among which the interparticle pores and organic pores are best developed.According to the nitrogen adsorption/desorption curves,the pore structures can be classified into three types,including the cylindrical pores with both opening ends,narrow parallel-plate pores and tapered parallel-plate pores with four opening sides.Diameter of micro-pores ranges from 2 to 64 nm,mainly at 2-6 nm.The diameter of micro-pores(less than 2 nm)ranges from 0.4 to 1.8 nm,The micro-pores with a diameter of 0.4-1.0 nm contribute most to the pore volume.Pore volume is dominated by meso-pores(2-50 nm)with a proportion of 83.1%.The micro-pores and meso-pores make major contribution to the specific surface area of pores with proportions of 20.1%and 79.3%,respectively.The total organic carbon(TOC)is the major factor controlling development of nanopores.Different pore types have different characteristics of gas occurrence and migration,indicating that nanopores provide favorable conditions for the occurrence and microscopic migration of gas in shales.

Establishing carrier transport channels based on Ti-S bonds and enhancing the photocatalytic performance of MXene quantum dots-ZnIn2S4 for ammonia

In the process of photocatalytic synthesis of ammonia,the kinetics of carrier separation and transport,adsorption of nitrogen,and activation of the N N triple bond are key factors that directly affect the efficiency of converting nitro-gen to ammonia.Here,we report a new strategy for anchoring MXene quan-tum dots(MXene QDs)onto the surface of ZnIn2S4 by forming Ti-S bonds,which provide a channel for the rapid separation and transport of charge car-riers and effectively extend the lifespan of photogenerated carriers.The unique charge distribution caused by the sulfurization of the MXene QDs further enhances the performance of the photocatalysts for the adsorption and activa-tion of nitrogen.The photocatalytic ammonia synthesis efficiency of MXene QDs-ZnIn2S4 can reach up to 360.5μmol g�1 h�1.Density functional theory calculations,various in situ techniques,and ultrafast spectroscopy are used to characterize the successful construction of Ti-S bonds and the dynamic nature of excited state charge carriers in MXene QDs-ZnIn2S4,as well as their impact on nitrogen adsorption activation and photocatalytic ammonia synthesis efficiency.This study provides a new example of how to improve nitrogen adsorp-tion and activation in photocatalytic material systems and enhance charge carrier dynamics to achieve efficient photocatalytic nitrogen conversion.