Pore Structure Of Shale Research Articles

A Modified Dent-Fractal Mathematical Model to Investigate the Water Vapor Adsorption on Nanopore Structure Heterogeneity from the Longmaxi Shale, Sichuan Basin, China

The study of water vapor adsorption (WVA) isotherms on shales is crucial to comprehend the adsorption–desorption behavior and deposit mechanism of water in shale pore systems. To systematically investigate the relationship between fractal dimension and WVA in shale reservoirs, a new Dent-fractal (DF) model was developed and the ability of different adsorption models to match with WVA experimental data was evaluated. The role of shale pore structure heterogeneity controlling the amount of WVA is also discussed. The results indicate that WVA on shale involves the monolayer–multilayer adsorption and capillary condensation. On the one hand, the GAB and Dent and DF models were found to be the best models for fitting and predicting WVA isotherms in Longmaxi shale. On the other hand, the DLP and DS models had the worst fitting qualities for WVA adsorption data. The pore structure of micropores has a more significant effect on WVA adsorption than that of meso-macropores. The larger surface and pore volume of micropores can provide more adsorption sites and space, which is favorable for WVA. In low-Rh conditions, the higher surface area, the pore surface complexity rises, resulting in higher water vapor monolayer adsorption. Under high-Rh conditions, for shale reservoirs with a highly heterogeneous pore structure, the relationship between the heterogeneous pore structure of the shale and the amount of water adsorbed in multiple layers is not obvious due to the formation of clusters of water molecules.

Response of pore network fractal dimensions and gas adsorption capacities of shales exposed to supercritical CO2: Implications for CH4 recovery and carbon sequestration

The injection of CO2 into shale reservoirs potentially increases rates and masses of CH4 recovery and simultaneously contributes to the sequestration of CO2. At typical reservoir conditions (T≥31.08 °C, P≥7.38 MPa) the CO2 will be supercritical. We compile, analyze, and supplement experimental data of shales from several basins across China, and use X-ray diffraction, scanning electron microscopy and low-pressure gas adsorption to characterize variations in shale pore structure before and after supercritical CO2 (ScCO2) treatment, and supplement these with CH4/CO2 adsorption experiments to characterize changes in shale adsorption capacity. The results show that clay and carbonate contents significantly decrease, and the relative content of quartz is increased after ScCO2 treatment. Pore structure changes significantly after ScCO2 treatment, with the majority of the shales showing a decrease in total specific surface area and total pore volume and an increase in average pore size — indicating the transformation of some micropores and smaller mesopores into mesopores and macropores. After ScCO2 treatment, the experimentally derived absolute adsorption volumes of both CH4 and CO2 decrease, and the volumes of both CH4 and CO2 fitting a Langmuir isotherm decrease with an increase in treatment pressure and increase with an increase in temperature. The adsorption selectivity factors αCO2/CH4 all remain greater than 1 with αCO2/CH4 primarily controlled by the pore structure. The fractal dimension is positively correlated with Langmuir volume and negatively correlated with Langmuir pressure while the fractal dimensions are negatively correlated with αCO2/CH4. The selectivity factor αCO2/CH4 decreases rapidly above a fractal dimension threshold (D1>2.65, D2>2.80). This paper further reveals critical interactions between ScCO2 and shale and defines controls on and of pore structure and adsorption capacity to speculate on physical and chemical storage mechanisms of CO2 in shale reservoirs. This provides several theoretical bases for shale gas recovery and the sequestration of CO2.

Study of pore structure of shale formation for CO2 storage

Carbon Geosequestration is considered a trending research area as it helps in mitigating the greenhouse gas effects on the environment by controlling global temperature. Recently, geo sequestration is considered one of the alternative ways to store CO2 for long periods. The shale is considered as the most suitable geological formation for CO2 storage due to its high adsorption capacity, microfractures, etc. Adsorption capacity in shale formation is highly influenced by pore structure, mineral composition, geomechanical properties, pressure, depth, etc. The shale pore structure is mainly dominated by mesopores (2–50 nm) and micropores (less than 2 nm). Storage of CO2 in the pores matrix is generally studied by using low-pressure N2 and CO2 adsorption isotherms, FE-SEM, etc. These techniques help in identifying types of pores, and their sizes, morphology, etc. Evaluation of pore microstructures helps to estimate the CO2 storage capacity in the formations. The present article discusses the importance of pore size, pore diameter, and pore volume distribution in shale formation for CO2 storage. A detailed review is carried out to analyze the methods of CO2 storage.

Characterization of Shale Porosity and Permeability by Nuclear Magnetic Resonance

To rapidly evaluate and characterize the porosity and permeability of shale reservoirs, this study uses Nuclear Magnetic Resonance (NMR) technology to classify the T2 spectrums of different shale reservoirs, analyzes the correlation between the T2 cut-off values and the main peaks of the T2 spectrums of the shale reservoirs, and establishes a rapid identification and evaluation method for different fluid signals of the reservoirs. Also, the correlation between the area of signals and shale permeability in different signal intervals is explored, and finally, a rapid evaluation method of shale pore structure characteristics based on NMR technology is developed. This study can provide methodological guidance and theoretical support for the rapid characterization of the porosity and permeability of shale reservoirs based on NMR technology.

A comprehensive assessment of image processing variability in pore structural investigations: Conventional thresholding vs. machine learning approaches

Broad ion beam - scanning electron microscopy (BIB-SEM) is an important technology for quantitative description of pore space and detailed investigation of pore structure in mudstones and shales. To obtain quantitative data from BIB-SEM images, image processing is required. However, the effect of different image processing approaches on the segmentation results has not been well documented yet. Particularly the interpretation of pore boundaries approaching the maximum image resolution can represent a major challenge. This study addresses the variability in image processing by comparing conventional thresholding with advanced machine learning methods and analysing the sensitivity of results to different segmentation. The test dataset consists of large area BIB-SEM maps of 12 Middle Miocene mudstone samples from the Vienna Basin, Austria. Segmentation of pore space and organic matter was performed using the thresholding method in ImageJ and the machine learning-based pixel classification in ilastik. The results show that the selected greyscale thresholding algorithm in ImageJ significantly affected the resolved porosity (1.7% to 2.8% variation) relative to the average total porosity of 4.5%. Compared to ImageJ, ilastik provides comparable results with better efficiency in separating pores from organic matter. However, the obtained pore information is inevitably influenced by variations in boundary delineation and feature identification due to inherent differences in the image processing methods and varying representativeness of image maps themselves. Overall, the study findings highlight the importance of carefully selecting image processing methods and considering their potential impact on the accuracy and reliability of pore characterization in mudstones and shales.

Characteristics and Governing Factors of Pore Structure and Methane Sorption in Deep-Marine Shales: A Case Study of the Wufeng–Longmaxi Formations, Weirong Shale Gas Field, Sichuan Basin

Characteristics and Governing Factors of Pore Structure and Methane Sorption in Deep-Marine Shales: A Case Study of the Wufeng–Longmaxi Formations, Weirong Shale Gas Field, Sichuan Basin

Comparison of Change in Nanopore Structure of Oil Shale after Anhydrous and Sub/Supercritical Water Pyrolysis

Oil shale is an important part of unconventional resources, which is considered as an important replacement for traditional oil resources. At present, the underground in-situ conversion method of oil shale is mainly aimed at shallow reservoirs, and it is difficult to apply to in-situ exploitation of oil shale with a depth of more than 1000m. In view of the portability, fracturing and extraction characteristics of high-pressure sub/supercritical water, it has become the research direction of in-situ mining of deep oil shale. The evolution of pore structure of oil shale is the basis for exploring the production mechanism of sub/supercritical water pyrolysis of oil and gas, which has important theoretical and scientific significance for improving the development and utilization of in-situ injection of sub/supercritical water in deep oil shale. Therefore, in this paper, the oil shale of Jijuntun Formation of Paleogene in Fushun Basin is taken as the research object. The pyrolysis experiments of oil shale in three ways, namely, water-free (electric heating), subcritical water and supercritical water, are carried out by means of high temperature and high-pressure source rock pyrolysis simulation device. The evolution characteristics of nano-pore structure under different pyrolysis methods are studied by XRD and low temperature nitrogen adsorption technology. The results show that, compared with anhydrous pyrolysis at the same temperature, the pore size of 3-30 nm of oil shale increases significantly after sub/supercritical water pyrolysis. The phenomenon of kaolinite illite fossilization increases, and the overall weak acidic environment is more conducive to the transformation of illite into Illite and montmorillonite mixed layer minerals, easier to increase the pore space in oil shale, and conducive to the migration and accumulation of produced oil and gas.

Study on microscale pore structure and bedding fracture characteristics of shale oil reservoir

Accurately characterizing the cross-scale structure of pores and fractures in shale reservoirs is the theoretical basis for efficient exploration and development of shale oil and gas. In this paper, (a) Based on the research background of different shale maturity and sedimentary characteristics, a three-dimensional model of cross-scale pore-fracture structure is established; (b) Based on a large number of CT characterization data of core pore-fracture structure, the characteristics of pore-fracture development under different geometric dimensions (25 mm and 2 mm) are statistically analyzed; (c) Pore and fracture structures of mixed shale, felsic shale and laminated felsic shale is comparatively studied, and the pore structure and bedding fracture characteristics of shale oil reservoir is revealed. Results show that: (a) for the bedding fracture porosity, felsic shale in Group I is 0.36 %–0.89%; mixed shale in Group I is 0.34 %–0.47%; mixed shale in Group III is 0.12 %–0.86%; mixed shale gravelly dolomite in Group III is 0.13 %–0.15%. (b) Compared with block 1, the laminated felsic shale bedding in block 2 is more developed. (c) CT imaging is based on density difference. Different components of shale samples are distinguished by gray scale.

Study on the Sedimentary Environments and Its Implications of Shale Reservoirs for Permian Longtan Formation in the Southeast Sichuan Basin

Marine–continental transitional shale is one of the most promising targets for shale gas exploration in the Lower Yangtze region. To investigate the sedimentary environments and the regularity of the enrichment of the Longtan shale, multiple techniques including core and thin-section observations, geochemical and elemental analyses, X-ray diffraction, scanning electron microscopy (SEM), and low-pressure nitrogen adsorption (LPNA) were used to analyze the sedimentology, mineralogy, and pore structure of the Longtan shale. The core descriptions and thin-section observations showed that the Longtan shale was deposited in marine–delta transitional environments including delta-front, shore swamp, mixed tidal flat and shallow shelf environments. The Sr/Cu, V/Cr, CIA, EF (Mo), EF (U), and other major and trace element results indicated warm and moist climates and water-reducing conditions in the Longtan period. Both the climate and water conditions were favorable for organic matter production and preservation. The geochemical results showed that the Longtan shale was in the overmature stage (Ro values ranging from 2.4% to 3.57%) and that the average total organic carbon (TOC) content was 5.76%. The pore system of the Longtan shale consisted of inorganic pores with a small number of organic pores and microfractures. The porosity and specific surface area were mainly affected by the TOC and clay mineral contents. An effective combination of brittle mineral particles, organic matter, and clay minerals provided the necessary conditions for pore preservation. The organic pores, intergranular pores in clay minerals, and brittle mineral pores formed the main network system for the Longtan shale. In summary, the lithological combinations, organic geochemistry, and pore structure system were all affected by the sedimentary environments.

Hydration of Transitional Shale and Evolution of Physical Properties Constrained by Concurrent NMR and Acoustic Observations

Hydration state is a key factor affecting the pore structure and mechanical properties of shale that directly impacts the migration and then production of geofluids. We quantify the impact of the hydration state on physical properties through spontaneous imbibition experiments constrained by concurrent nuclear magnetic resonance (NMR) and acoustic observations as diagnostics of response. The long-duration experiments (24 days) are on samples from the Permian Shanxi Formation of the Ordos Basin, complemented by measurements of composition and physical properties. These shales primarily comprise quartz and clay minerals with intraparticle pores present within the clay minerals. The NMR T2 spectra indicate that mesopores (2–50 nm) comprise ∼80% of the total pore space. The imbibition rate decreases by a factor of 3 from 0.08 g/h to a stable rate of 0.03 g/h from the 1st to the 24th hour. This is accompanied by a 10% decrease in the deformation modulus from 44 to 40.3 GPa over 24 days. The reduction in the modulus is consistent with observations of the shedding and spalling of hydrophilic quartz particles from the unconstrained surface and the generation of fractures. Reduction in the imbibition rate is consistent with the swelling followed by squeezing of clay minerals and the corresponding reduction in pore diameters in the confined core of the sample, together with the reduction in saturation gradients with the progress of imbibition. Understanding the mechanisms and effects of hydration on the pore structure and mechanical properties is important in understanding the shale hydration process and defining diagnostic (acoustic and NMR) signatures that may be used in reservoir surveillance.

Impact of tectonic deformation on shale pore structure using adsorption experiments and 3D digital core observation: A case study of the Niutitang Formation in Northern Guizhou

Tectonic deformation has an evident impact on the pore structure of organic-rich marine shale. To study the impact of tectonic deformation on the pore structure of shale, cores were taken from different tectonic regions in the Niutitang Formation. The paleotectonic stress, pore structures, mineral composition, and geochemical parameters of the shale were determined by conducting the following tests: paleotectonic stress test, core thin-section observation, focus ion beam-scanning electron microscopy (FIB-SEM) observation, vitrinite reflectivity test, low-temperature nitrogen adsorption test, total organic carbon (TOC) test, X-ray diffraction (XRD) test, and nano-CT. The results showed that the shale in the deformed region mainly developed meso-macropores, and the pores were mainly slit-type and conical. The shale in the transition and stability regions primarily developed mesopores and micropores, which were primarily elliptical, slit-type, and columnar. The pore volumes of the shale in the deformed region were 9.54 cm3/kg and 13.9 cm3/kg higher than those in the transition and stability regions, respectively. The shale in the stability and transition regions had average microporous volumes 8.67% and 7.93% higher than that in the deformed region, respectively. The shale reservoir in the stability region developed more micropores and had a higher specific surface area and microporous volume. Furthermore, the organic matter had a greater impact on the micropores than on the meso-macropores. Thermal maturity had a greater impact on the microporous volume than on the macroporous volume. The shale pore connectivity of Well TX 1 was 86.48% and that of Well FC 1 was 22.33%. The pore connectivity of the shale in the stability region was higher than that of the shale in the deformed region. The shale samples from the deformed region had larger pore volumes and primarily developed macropores. The difference in the meso-macropores is significantly greater than the difference in the microporous volume. Hence, the geological tectonism had a greater impact on the meso-macropores than on the micropores in the shale cores.

Multi-scale pore structure transformation of shale under mixed acid acidification method

Most of the acid fluids used in the existing acidfrac process have the shortcomings of plugging pores, polluting formation and poor effect. The method of adding nitric acid in a specific proportion to conventional acid can avoid the above problems to a certain extent. A mixture of HCl: HF: HNO3 (3:1:1) was used to acidize the shale of the Longmaxi Formation in Sichuan and the Piyuancun Formation in Jiangxi for 24 h. The mineral fractions and pore structures of the samples were then analyzed using scanning electron microscopy-energy dispersive spectrometry, X-ray diffraction, low-pressure nitrogen adsorption, and nuclear magnetic resonance. The results showed that by reducing the mineral constituents of shale, the 2–15-nm sized pores decreased, 1–10-μm sized pores increased, and fractal dimensions decreased. The mixed acid solution caused cross-scale porosity expansion in the shale and increased gas circulation within the matrix. The process of acid mixing in the shale pore structure was divided into four stages: surface dissolution, macroporous dissolution, meso–microporous dissolution, and feedback dissolution.

Geochemical and Microstructural Characteristics of Clay Minerals and Their Effects on the Pore Structure of Coal-Measure Shale: A Case Study in Qinshui Basin, China

As the essential component of shale, clay minerals have a vital influence on the pore structure and the gas content of reservoirs. To investigate the compositional characteristics of coal-measure shale and its effects on pore structure, a total of thirteen Taiyuan formation shale samples were collected from the Qinshui Basin and were analyzed using a combination of X-ray diffraction analysis, X-ray fluorescence spectrometry, Fourier transform infrared spectroscopy (FE-SEM), polarized optical microscopy, and field emission scanning electron microscopy. The results show that the principal minerals of the samples are quartz, kaolinite, and illite. Most of the kaolinite was an original terrigenous detrital material with low crystallinity and a low degree of ordering, whereas the illite was mainly composed of 1Md resulting from diagenesis. Clay minerals developed slits, irregularly-shaped or multisized pores during diagenesis, which can be classed into interlayered pores, intergranular pores, and microfractures. Eight micro-morphological forms of clay minerals were summarized based on FE-SEM observations, such as compacted, parallel, bent, tilted, mutually supporting structures, etc., which are mainly formed by the mechanical compaction of clay minerals with different sizes, shapes, and contact relationships. The diversity and complexity of the micro-morphological forms of clay minerals contribute to the strong heterogeneity, low porosity and high permeability anisotropy of shale.

Evaluating Pore Structure of Shale Reservoirs through Pore Space Multifractal Characterization

Characteristics of pore structure are crucial to oil exploration and development in shale reservoirs, but practical evaluation of shale pore structure remains a longstanding challenge. In this study, based on nuclear magnetic resonance transverse relaxation time (T2) spectra under water-saturated and irreducible conditions, total pore space and moveable fluid space were skillfully determined. Meanwhile, multifractal spectra were used to characterize T2 distributions of total pore space and movable fluid space. The results show characteristic parameters of high probability measure areas of multifractal spectra (multifractal parameters) have strong negative correlations with T2 geometric mean value, and average radii of pore and throat increase with a decrease in the multifractal parameters, indicating the shale reservoirs with low values of the multifractal parameters may have good pore structure. Moreover, clay mineral content has strong positive effects on the multifractal parameters. In contrast, (quartz+feldspar) mineral content negatively influences on the multifractal parameters. Finally, new accurate models for permeability estimation in core scale and well-logging scale of shales are both proposed based on the multifractal parameters, which are also applicable for the other reservoirs. Overall, this study is important and timely for evaluating pore structure and predicting permeability in shale reservoirs.

Fractal Perspective on the Effects of the Acid–Rock Interaction on the Shale Pore Structure

Fractal Perspective on the Effects of the Acid–Rock Interaction on the Shale Pore Structure

Microscopic pore structure and connectivity of lacustrine shale of the Shahejie Formation, Zhanhua Sag, Bohai Bay Basin

As an emerging source of global energy supply, shale reservoirs have extremely low porosity and permeability which are often associated with their highly heterogeneous and poorly connected pores. This is due to peculiar pore structure system and morphology of the shale reservoirs. The focus of this study is to evaluate the pore structure, connectivity and heterogeneities of lacustrine shale reservoirs of the Shahejie Formation within the Bohai Bay Basin using a combination of three methods. These are low temperature gas adsorption experiment (for pore morphology and structure), spontaneous imbibition experiments (for pore connectivity) and nuclear magnetic resonance (NMR) to characterize the structure and fluid saturation of the entire shale sample. The results showed that, high clay content blocked the pore throat and caused significant reduction in connectivity which results in the collapse of pores. However, high content of brittle minerals (e.g. calcite and quartz) promotes the formation of intergranular and intercrystalline pores of inorganic minerals as well as micro-fractures, which favours good pore connectivity in shale. Increase in available space for fluid movement in shales with higher porosity results in a beneficial increase in the mobility of the fluid within the pores. Hence, mineral composition and organic matter content are the two main factors affecting the pore structures within the Zhanhua Sag of the Jiyang Depression in the Bohai Bay Basin.

Effect of thermal oxidant stimulation on the alteration of shale seepage channel: Implication from the change of macromorphology and microstructure

Alteration of the seepage channel through the shale-oxidant interactions is critical for using oxidant stimulation to enhance fluid mobility in tight shale reservoirs. Although many studies reported the effect of oxidant stimulation on the macro-crack morphology of shale, little is still known about its impact on shale microstructure, especially under reservoir temperature. In this study, we explored the influence of three oxidants, including H2O2, NaClO and Na2S2O8, on the alteration of shale pores and fractures at reservoir temperature (60 °C), and illustrated the underlying mechanism based on water chemistry and solid characterization including X-ray diffraction, Fourier-transform infrared spectroscopy, low-pressure nitrogen gas adsorption, scanning electron microscopy coupled with energy dispersive spectroscopy analyses. Results showed that the reaction temperature and oxidant type both led to an inconsistent response of shale macromorphology and microstructure during oxidant stimulation. Three oxidants processed different shale matrix alteration, with the degree shown as follows: Na2S2O8 > H2O2 > NaClO. Even though gas generated from the thermal and catalytic decomposition of H2O2 and dissolution of carbonate were both favorable for the shale plug disintegration, the rapid consumption of H2O2 limited the change of shale pore structure. After being treated with NaClO, the organic nanopores were transformed into larger mesopores and macropores with a decrease in pore surface roughness and structural complexity. Therefore, other methods should be combined with NaClO simulation to generate fractures and improve the fluid fluidity of shale. The serious dissolution and precipitation of minerals by Na2S2O8 causes significant alteration in the shale matrix, resulting in a large number of pores and induced fractures. Our research confirms the distinct effects of different oxidants on shale fractures and micropores, and the most promising Na2S2O8 for improving shale permeability, but still needs to be demonstrated by core-flooding experiments in the future.

High-Temperature-Induced Pore System Evolution of Immature Shale with Different Total Organic Carbon Contents.

The pyrolysis process of source rock, especially organic-rich immature shale, is required for oil and gas extraction, during which the evolution of the pore structure system in the immature shale determines the heat conduction and fluid flow under the heating treatment. Although some sound achievements have been made regarding the pyrolysis of immature shale, the effect of the total organic carbon (TOC) content on the pore structure evolution of immature shale remains unclear. With respect to this issue, in this work, a series of N2 adsorption/desorption and nuclear magnetic resonance (NMR) experiments were conducted, and fractal dimension theory was also introduced to analyze the pore structure evolution of immature shale subjected to heating treatment in a quantitative manner. The results indicate that the adsorption branch of the nitrogen adsorption-desorption isotherm can be divided into three stages. The pore structure of different TOC immature shales does not change significantly, and they are all slit-shaped. In addition, immature shale with a higher organic content has a higher hydrocarbon expulsion strength and a higher pore volume growth rate, which indicate that the pyrolysis of organic matter greatly affects the pore structure of immature shale during heating. This phenomenon shows that the pyrolysis of organic matter greatly influences the pore structure of immature shale during the heating process. The pores of immature shale in the study area have significant fractal characteristics, the fractal dimension is between 2.397 and 2.636, the pore space of the sample is extremely small, the pore structure is extremely complex, and the heterogeneity is strong.

Coupled pressure-driven flow and spontaneous imbibition in shale oil reservoirs

Coupled pressure-driven (viscous) flow and spontaneous imbibition are the main regimes during shale oil production. Revealing the unclear mechanisms of this coupled flow is a major concern for scholars and field engineers. In this work, the oil–water flow mechanisms within shale pore structures are investigated by pore-scale modeling methods in focused ion beam scanning electron microscopy digital rocks enhanced by applying super-resolution reconstruction (SRR). More small pores are identified with SRR, and the connectivity is improved. The enhanced pore size distribution is consistent with the nitrogen adsorption measurement; hence, more representative capillary pressure and relative permeability curves are obtained with essential experimental measurements. Then, an analytical solution of coupled pressure-driven (viscous) flow and spontaneous imbibition is derived, and a corresponding algorithm is proposed. Based on the pore-scale calculated relative permeability and capillary pressure curves, the analytical solution is applied to investigate the variations in water saturation profiles and conductance of the oil phase during the shale reservoir development. The results demonstrate that most of the shale oil is recovered by pressure dropdown-induced viscous flow and that imbibition is a minor factor. The overall oil-relative permeability decreases due to imbibition invasion. When the fracture spacing increases, the impairment of the overall oil-relative permeability decreases.

Fractal dimension changes of shale pore structure and influence on mechanical properties, relative permeability under different hydration degree

Fractal dimension changes of shale pore structure and influence on mechanical properties, relative permeability under different hydration degree