Microdistribution and mobility of water in gas shale: A theoretical and experimental study.

Abstract Multiphase (adsorbed and free) liquid water that is accumulated in shale matrix pores (i.e., pore water) substantially affects the storage and transport of gas in shale. Traditionally used macroscopic parameters (e.g., initial, irreducible, mobile and clay bound water saturations) are not sufficient to describe the microdistribution characteristics and mobility of pore water in the complex porous shale matrix. In this paper, a new classification was proposed in which the pore water is composed of adsorbed and free parts, of which the latter is subdivided into capillary bound and mobile water. The components of pore water have the following features: during the centrifugation process, (a) adsorbed water is essentially immobile, while free water is potentially mobile; (b) capillary bound water is gradually transformed into mobile water with increased centrifugal pressure difference; and (c) the maximum mobile water amount is numerically equal to the free water amount. Based on the classification, an effective technique, which integrates centrifugation, water imbibition and low temperature nitrogen adsorption methods with the modified Li's adsorption ratio equation (2018), was established to quantitatively evaluate the microdistribution characteristics (adsorbed and free amounts and their ratios) and the mobility (mobile amount, potential and capacity) of pore water in marine shales from the southern Sichuan basin in China. Furthermore, the mechanism of controlling the microdistribution and mobility of pore water is analyzed with respect to the microscopic pore structure characteristics and material compositions of shales. The primary results demonstrate that (1) adsorbed and free water amounts obtained by centrifugation tests range from 5.145 to 31.282 mg/g (adsorbed) and from 7.880 to 49.751 mg/g (free); the weight ratio of adsorbed water is 0.268–0.629. (2) The mobile potential and mobile capacity of pore water are represented by the free water amount and mid-value pressure difference, respectively. A positive relationship between these two parameters occurs in the studied shales. (3) Pore microstructure characteristics of shale mainly impact the microdistribution and mobility of pore water in the shale matrix. The adsorbed water amount is closely associated with the specific surface area of pores, while the free and mobile water amounts are controlled by the pore volume. The ratios of adsorbed and free water depend on the size and morphology of pores for certain state parameters. The mobile ratio is impacted by pore complexity, which indicates that as the complexity increases, the mobile ratio decreases. The study provides significant insight into the microdistribution and mobility of pore water in gas shale matrix. Highlights • A new classification was proposed for the components of pore water. • A technique was established to evaluate the microdistribution characteristics and the mobility of pore water in shales. • Mechanism of controlling the microdistribution and mobility of pore water was analyzed. [ABSTRACT FROM AUTHOR]