Effects of Pore Water on the Heterogeneity in Nanopore Structures of Deep Marine Shales: A Multifractal Study on As-Received Shale Samples from the Southern Sichuan Basin, China

Abstract

Shale strata generally contain a certain amount of pore water that accompanies the whole diagenetic and thermal evolution processes of shales. The distribution and occurrence characteristics of pore water significantly influence the nanopore structures of shales, which subsequently affects the accumulation and production of shale oil and gas resources. Based on a set of fresh Wufeng−Longmaxi (WL) shale samples from the deep marine strata in the southern Sichuan Basin, this study investigated the effects of pore water on the pore heterogeneity of shales by a multifractal method. The results show that (1) the deep WL shales have low pore water content (C PW ) and water saturation (S EW ), with average values of 7.47 mg/g and 35.21%, respectively, and the C PW and S EW of the shales are mainly controlled by clay mineral content. (2) Under dried conditions, the pore heterogeneity of high-probability measure areas has negative relationships with the clay mineral content and pore volumes with pore widths of 4−10 nm, and the pore connectivity is mainly controlled by clay mineral content. Under as-received conditions, however, the pore heterogeneity of low-probability measure areas and pore connectivity have positive relationships with clay mineral content and pore volumes with pore widths less than 4 nm. (3) Under as-received conditions, the pore water mainly occupies the nanopore with pore widths less than 4 nm and reduces the effective pore spaces and connectivity, which reduces the pore heterogeneity of high-probability measure areas and enhances the pore heterogeneity of low-probability measure areas.

Response of Coal Nanopores and Their Complexity to Igneous Intrusion

“…Multiple fractals based on the mercury pressure experiment can be used to treat the experimental pressure as an interval as a whole and divide the object of study into N ( N = 2 k , k = 1, 2, 3) boxes with equal sides based on the logarithmic value of the relative pressures, with the sides of the boxes being ε . The multifractal parameters are calculated as follows. ,, …”

“…The pore connectivity is expressed using the Hurst index H : H is ( D 2 + 1)/2. The larger H is, the better the pore connectivity is. , …”

“…There are many kinds of fractal models, and the commonly used fractal models mainly include the Sierpinski Fractal Model, Menger Fractal Model, Thermodynamic Fractal Model, Li Kewen Model, Brooks–Corey Model, etc. Different fractal models reflect different levels of pore complexity due to different principles. − All of the above fractal theories have their own different problems and are insufficient in describing the nonhomogeneity of coal rocks, while the multiple fractal theory can describe multiple factors of fractal characteristics and has unique advantages in studying the nonhomogeneity of coal rocks. , Multiple fractal theories have been applied in various types of research. For example, multiple fractal theory can be used to verify the applicability of other fractal theories .…”

“…For example, multiple fractal theory can be used to verify the applicability of other fractal theories . Some scholars have also used fractal theory to solve the effect of moisture on the pore structure of shale . Meanwhile, the change of pore structure during matrix compression can be solved by using the fractal theory …”