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A series of experiments including porosity and permeability measurements, thin section and scanning electron microscopy (SEM) observations, incremental pressure mercury injection (IPMI), and nuclear magnetic resonance (NMR) were conducted to systematically characterize the pore structure of tight sandstone from the Lower Shihezi Formation of Permian (P2x) in the northeastern Ordos Basin, China. The influences of pore types, pore size distribution, and fractal characteristics on reservoir quality of tight sandstones are also investigated. Results show that the studied tight sandstones generally possess poor quality and complex pore structure. The porosity and permeability range from 4.08% to 17.56% (average 9.22%) and from 0.05 to 16.66 mD (average 2.49 mD), respectively. Five pore types were observed in thin section and SEM images: primary intergranular pores, intergranular dissolution pores, intragranular dissolution pores, micropores within clay aggregates, and microfractures. The pore throats are mainly hairy/fibrous, inhibiting the connectivity between pores. Three types of pore structures were identified in the mercury-injection curves and pore size distribution curves from the IPMI experiment and in the T2relaxation time spectrum obtained by NMR. Both experiments yielded consistent classifications, and their combination was necessary to analyze the pore structure effectively. In general, permeability and porosity are positively related and depend on pore types. Large numbers of small pores confer high storage capacity, whereas small numbers of larger pores improve the flow capability. In the high porosity–permeability zone, larger pores also determine the storage capacity. The P2x tight sandstone is fractal, and macropores are more heterogeneous while micropores are more homogeneous. The fractal dimensions of macropores are good indicators of the reservoir quality of the P2x tight sandstone as larger fractal dimension values of macropores reflect poor reservoir quality.

To characterize the pore structure and quantify fractal dimensions of tight gas sandstones, a case study is performed on the Lower-Middle Permian tight sandstones in the Ordos Basin in China by conducting a series of experiments including X-ray diffraction (XRD) analysis, routine petrophysical measurements, thin section and scanning electronic microscope (SEM) observations, and nuclear magnetic resonance (NMR) experiment. The studied tight sandstones mainly consist of quartz and clay minerals, and pore types include primary intergranular pores, inter- and intragranular dissolution pores, as well as micropores associated with clay aggregates; T2spectra reflect three types of pore size distributions in the studied samples, indicating a rather irregular pore distribution pattern in tight sandstones; NMR can estimate porosity of tight sandstones accurately, and movable-fluid porosity from NMR can better reflect the permeability of tight sandstones than total porosity. Two fractal dimensions, Dbnd(with respect to bound-fluid pores) and Dmov(with respect to movable-fluid pores), are calculated to be 1.1135–1.8116 (average 1.4750) and 2.6816–2.9932 (average 2.8921), respectively. Dbndincreases with the decrease of detrital quartz content and the increase of clay mineral content, whereas Dmovincreases with the increase in authigenic quartz content and the decrease of detrital quartz content; fractal dimensions can reflect the physical properties of tight sandstones, as large Dbndand Dmovvalues typically result in low movable-fluid porosity and permeability; the pore network of tight sandstones can be considered as a dual-scale pore system based on fractal theory, whereas Dbndand Dmovcan reveal the roughness of bound-fluid pore surface and the distribution of movable-fluid pores, respectively. This study shows that NMR fractal dimension can be employed as an effective indicator to characterize the pore network of tight sandstones.

Describing the characteristics of shale pore structure is vital for the assessment of shale reservoir, which has significant influence on the storage and seepage mechanisms of gas shale. To profoundly understand the shale pore structure characteristics of continental shale reservoir, fractal analysis was performed on 45 continental shale samples from the Ordos Basin, NW China, via low-pressure N2adsorption experiments. The characteristics of N2adsorption isotherms revealed that slit-shaped shale pores are dominant among the geometric shapes of shale pores. During N2molecules adsorption process, different characteristics were displayed at two regions where relative pressures (P/P0) were 0–0.45 and 0.45–1. The Frenkel–Halsey–Hill (FHH) method was used to calculate fractal dimensions (D) at these two regions. In addition, the fractal exponents “(D– 3)/3” and “(D– 3)” were compared adequately. The results show creditable fractal characteristics for continental shale. Fractal exponent D– 3 is more suitable for the calculation of the fractal dimension in the study area. The surface fractal dimension (D21) and pore structure fractal dimension (D22) were further investigated. Results indicate that D21, ranging from 2.04 to 2.50, was affected by shale constituents and provided a site for gas shale adsorption. D22reflects the irregularity and heterogeneity of the shale structure, varying from 2.20 to 2.65, and is higher overall than D21. Furthermore, the value of D22negatively correlates with the average diameter of the shale. In addition, the comparisons of shale pore structure characteristics between the reservoirs Chang-7 and Chang-9 show that the shale pore structure of Chang-9 reservoir is more irregular and nonhomogeneous and is favorable for gas shale storage but unfavorable for seepage.