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Copyright of Petroleum Geology & Recovery Efficiency is the property of Petroleum Geology & Recovery Efficiency and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Based on physical property, casting thin section, scanning electron microscope, high-pressure mercury injection and other test analysis data, the fractal dimension of pore structure of tight sandstone reservoir was calculated by the method of mercury saturation and water saturation in the first Member of Lower Shihezi Formation in Hangjinqi area, and the relationship between fractal dimension and the physical properties of reservoir was analyzed.The results have shown that the average porosity and permeability of the Lower Shihezi Formation reservoir were 9.83% and 1.03×10-3 μm2, respectively.The reservoir space is mainly composed of intergranular dissolved pores, intragronular dissolved pores and residual intergranular pores.The overall fractal dimension calculated by the mercury saturation method is distributed in 2.138 4-2.829 2 with an average value of 2.396 5 while calculated by the water saturation method is distributed in 2.529 4-2.879 7 with an average value of 2.679 1.Compared with the water saturation method, the fractal dimension calculated by the mercury saturation method has a better correlation with the porosity, permeability and pore structure parameters, because the water saturation method tends to produce deviation on samples with smaller pore throat.The pore structure was divided into four types based on fractal dimension: Type Ⅰ(Df≤2.31), Type Ⅱ(2.31 < Df < 2.4), Type Ⅲ(2.4≤Df < 2.52), Ⅳ(Df≥2.52).Fractal dimension(Df), averoged radius of pore throct(Rm) and porosity(φ) were selected to calculate permeability by multiple nonlinear regression.The calculated permeability by multiple nonlinear regression shows strong correlation with measured permeability, whose correlation coefficient squared is more than 0.9, which means the permeability estimation model is suitable for the study area.

• SDS synergistic acidification increased the proportion of seepage pores by 5.99%. • SDS synergistic acidification promotes the pores connectivity of coal. • The average size of corrosion hole is distributed at the nanometer scale. • Coal quality characteristics have a great influence on the pore fractal dimension. In order to study the influence of surfactant on pore fractal characteristics of composite acidized coal. In this paper, the coal samples were acidified by solution constituted with the Sodium Dodecyl Sulfate (SDS), hydrochloric acid (HCL) and hydrofluoric acid (HF). And the proximate and ultimate analysis, X-ray diffraction (XRD) testing, scanning electron microscopy/energy dispersive spectroscopy (SEM-EDS) and nuclear magnetic resonance (NMR) testing were carried out. The fractal dimension of pore in coal was evaluated by fractal theory, and the evolution relationship between coal quality, NMR fractal features and coal pore structure parameters were characterized. The results show that the addition of surfactant has a great influence on the coal characters. The mineral content in coal is exponential positively correlated with the fractal dimension (D s) of seepage pores. The relative content of ash is linear positively correlated with the fractal dimension. Conversely, the volatiles is linear negatively correlated with the fractal dimension. SDS synergistic acidification compared to without SDS, the fractal dimension (D w) of the total pore distribution and the fractal dimension (D s) of the seepage pores are both reduced, indicated that the addition of SDS promotes the improvement of coal pore connectivity by acidification treatment. The results showed that after SDS synergistic acidification, the proportion of seepage pore volume was increased by 5.99%, and the fractal dimension is linear negatively correlated with the porosity. The changes of these parameters are beneficial to the seepage, migration and extraction of coalbed methane. Therefore, SDS synergistic composite acid fracturing technology has practical guiding significance for improving acid fracturing effect and increasing coal seam permeability. The research results are of great significance for improving the efficiency of coalbed methane mining and reducing coalbed methane pollution. [ABSTRACT FROM AUTHOR]

Highlights • Multiscale image-based shale pore structure fractal characteristic is studied. • The real gas permeability fractal model considering second order slip is derived. • Image resolution for accurate permeability prediction should be less than 50 nm. Abstract The heterogeneities of shale are manifested in the complex pore spatial configurations and the wide distribution of pore sizes. The recent advances of high-resolution imaging techniques such as Scanning Electron Microscope (SEM) and Focussed Ion Beam Scanning Electron Microscopes (FIB-SEM) enable accurate characterization of shale pore structure in the limited imaging area. Due to the nature of multiscale pore size, image-based petrophysical properties are highly dependent on the selection of image resolution. Fractal theory proves to be an effective approach to characterize pore structure as well as calculate fluid transport properties. In this study, the image-based fractal characteristic of shale pore structure at multiscale resolutions is investigated and its impact on the accurate prediction of gas permeability is analyzed. The fractal dimensions of pore phase in 100 SEM images at resolutions ranging from 15.5 nm to 420 nm are calculated by the box counting method and Sierpinski carpets analytical solution. The real gas permeability model in consideration of second order slip is derived based on the fractal theory. Two groups of gas permeabilities at different resolutions are estimated respectively based on the fractal dimensions obtained from the box counting method and Sierpinski carpets analytical solution. The results found that fractal dimensions obtained from the box counting method at different resolutions are more close to the exact fractal dimension compared with that obtained from the Sierpinski carpets analytical solution at low resolutions and gas permeabilities calculated at different resolutions based on the box counting estimated fractal dimensions are more close to the exact gas permeability. The image resolution for accurate calculation of shale pore structure properties and gas permeability should be less than 50 nm based on our analysis results. [ABSTRACT FROM AUTHOR]

The heterogeneity of shale pore structure is a vital problem that cannot be ignored in the process of shale oil exploration and development. The organic-rich laminated shale (ORLS) and organic-poor layered shale (OPLS) show significantly different pore system heterogeneity. In this study area, the porosity and permeability of ORLS (2.7% and 0.00053mD) are higher than those of OPLS (2.29% and 0.00041mD). The heterogeneity of the pore system in ORLS is reflected by the wider pore size range of intercrystalline pores (pore size of 0.002–5.5 μm), the stronger holistic distribution heterogeneity of intergranular pores (average △α of 1.61) and organic pores (average △f of 2.65) at local locations. The wider range and discontinuity of the pore size distribution for intergranular pores and organic pores, and the instability of the morphological structure for intercrystalline pores and microfractures, all lead to the heterogeneity of the pore system in OPLS. The micropore (<2 nm) volume and macropore (>50 nm) volume of ORLS are higher than that of OPLS, while the surface area and volume of mesopores (2–50 nm) are lower than those of OPLS. The macropore has the most obvious fractal characteristics and three fractal dimensions. The macropore volume of ORLS is positively correlated with reservoir physical and pore multifractal properties, while the mesopore volume of OPLS is positively correlated with reservoir properties and negatively correlated with fractal properties. Pore system evolution patterns of shale reservoirs are obviously controlled by the mineral composition, laminae, TOC content, thermal evolution degree and fluid pathways. The macropore volume in ORLS shows a clear positive correlation with the pyrite content. The high TOC content and high thermal evolution degree are more likely to produce erosive fluids. Laminar interfaces, microfractures and primary pores provide channels for the fluid migration. The development degree of secondary dissolution may be an important factor leading to the difference of reservoir qualities between ORLS and OPLS. • Pore structures of multi-type pores and multi-scale pores in the ORLS and OPLS have strong heterogeneity. • The integration of CGA, NGA, HPMI and fractals can characterize shale pores more finely. • The dominant pore volume is helpful to physical properties, while pore fractal shows dual effects. • The reservoir qualities of ORLS and OPLS are the function of five main controls. • ORLS and OPLS have different pore evolution patterns. [ABSTRACT FROM AUTHOR]

CO 2 intrusion has a crucial effect on the pore structure of mineral-bearing coal. In this study, we selected long flame coal, lean coal, and anthracite after CO 2 adsorption at different pressures and tested the coal samples using X-ray diffraction, mercury intrusion porosimetry, and N 2 (77 K) adsorption methods. The tests were conducted to determine the variations in mineral content, pore structure, and fractal characteristics. The results showed that supercritical CO 2 had a greater ability to dissolve minerals in coal than that of subcritical CO 2. Although the total pore volume and BET specific surface area gradually increased with the increase in CO 2 intrusion pressure in coal, the transformation of different pores and partial new pores caused by the dissolution of minerals and the adsorption swelling of coal matrix caused the micro-macropores in the three coal samples to exhibit different trends. The pore surface roughness and pore structure complexity of seepage pore in the long-flame coal after CO 2 adsorption increased while those of the lean coal and anthracite decreased. Meanwhile, CO 2 intrusion caused the surface of the adsorption pore in coal to become smooth, and the pore structure was more regular, except for the lean coal. A conceptual model of the mineral-bearing coal was developed to describe the relationship between the mineral composition and pore structure induced by CO 2 intrusion. These findings help to understand the transformation effect of CO 2 on coal seams. Thus, a higher CO 2 injection pressure should be used to obtain a larger injection volume and shorter injection time during CO 2 storage implementation. • Supercritical CO 2 has a greater ability to dissolve minerals than subcritical CO 2. • The pore volume in coal gradually increases with increasing CO 2 intrusion pressure. • CO 2 intrusion smoothes the surface and regularizes the structure of adsorption pore. • A conceptual model of the effect of mineral on pore structure was established. [ABSTRACT FROM AUTHOR]

To quantitalively prove the effect of temperature on RPC internal pore structure changes,the characteristics of pore structure and the fractality of RPC after high temperatures were tested by mercury intrusion porosimetry ( MIP). The development of the characteristics of pore parameters, such as the pore volume, the threshold pore size, the most probable pore size,etc.,with increasing temperature was analyzed. The volume fractal dimension of RPC in the effective region of capillary pores and transition pores were calculated. The results indicate that the pore structure degrades at high temperature, which lead to the porosity and pore volume increase with the temperature rising. The damage process of RPC can be described by means of the fractal method,at the temperature 150 °C, RPC starts to show fractal effect, the volume fractal dimension of RPC in the range of capillary pores and transition pores increase with the rising temperature. [ABSTRACT FROM AUTHOR]

In the process of tectonic evolution, a comprehensive understanding of pore structure and fractal characteristics of coals with different tectonic degrees is critical. For this paper, the tectonic evolution process of coal is divided into two stages (Stage 1 and Stage 2) and N 2 (77 K)/CO 2 (273 K) adsorption methods were conducted. The results show that the original coal-bearing strata underwent multiple periods of stress action, the original stratified strip-band structure of the coal was destroyed gradually and formed pulverized coal in the Stage 1. In Stage 2, the original structure of pulverized coal disappeared, fractures in the coal body developed intensively and the internal structure changed greatly, forming tectonic coal with low strength and weak cohesion. The mesopore, macropore volumes and BET specific surface area of coal increase gradually during the tectonic evolution due to the closed and open pores that are connected by constricted pore openings may be destroyed in the Stage 1, and some chemical changes may have occurred in Stage 2, resulting in relatively developed pore structure. However, the micropore structure characteristics of coals first suffered from the closed pores being opened and then collapsed in the Stage 1, and the changes of micropore were not significant in Stage 2. Moreover, the fractal dimension D 1 of coal samples shows a general trend of increasing (from 2.075 to 2.359) and D 2 decreases from 2.683 to 2.469 during the Stage 1 of tectonic evolution, illustrating the pore surface becomes rougher and the irregularity of pore structure gradually decreases. The Stage 2 of tectonic evolution shows the same results. The variations of pore structure characteristics of coal are of great significance for understanding desorption and diffusion characteristics of coal in the process of tectonic evolution. • The tectonic evolution of coal seam is divided into two stages. • The pore structure characteristics of coal are analyzed during tectonic evolution. • The effect of tectonic evolution on fractal characteristics of coal is acquired. [ABSTRACT FROM AUTHOR]

The pore structure and fractal characteristic of different types of tectonically deformed coal (TDC), were investigated by the low-temperature nitrogen adsorption method. The adsorption loop curves of low rank TDCs were divided into three types, i.e, Ⅰ-Ⅲ. The pores in primary-and cataclastic coals mainly consist of cylinder pores with one or two sides open and that in schistose-and scaly coals are mainly constituted by cylinder pores with two sides open. There are also some slit type-and ink bottle type pores in schistose-and scaly coals. The pores in wrinkle coals mainly consist of ink bottle type-and slit type pores. From primary-to wrinkle coals, the specific surface area in macro-, meso-and micropores manifests as a significant decreasing, slight changing and gradual increasing with the enhancement of tectonic deformation respectively. With the increasing adsorption ability, the medium diameter of nano-pores gradually decreases whereas fractal dimension increases, indicating that pore system becomes more complicated. The coal body destructive intensities and the heterogeneity of pores can be effectively characterized by fractal dimension. Suffered from stronger tectonic deformation, the wrinkle coal with higher pore fractal dimensions (D>2.9) are characterized by more diversities in pore morphology, and larger amount specific surface area. The coals with low pore fractal dimensions (2.6