Water Distribution in Marine Shales: Based on Two-Dimensional Nuclear Magnetic Resonance and Low-Temperature Nitrogen Adsorption

Abstract

A significant amount of foreign fluid is kept in the marine shale reservoirs after hydraulic fracturing. The water distribution characteristics in the pore space are quite complex because of the shale’s significant inhomogeneity. The change in reservoir water status brings new challenges to studying the adsorption and flow capacity of shale gas, which also brings difficulties to assessing the geological reserves of shale reservoirs and capacity prediction and development. In this study, based on a two-dimensional nuclear magnetic resonance technique and low-temperature nitrogen adsorption technique, clay minerals, organic matter, and marine shale were used as the study objects to get the distribution characteristics of water in shale pores. The results show that after entering the shale reservoir, most of the water is adsorbed on the surface of inorganic pores, mostly clay pores, to create a bound water film with a thickness of around 0.96 nm. Shale pores’ diameter, volume, and surface area are all smaller in the wet state compared to the dry, falling by 10.11, 47.02, and 71.00%, respectively. Small-scale pores (D < 10 nm) are better at absorbing water, and the water in these pores fills the spaces between them to create capillary water besides being adsorbed on the pore surface. Bound water can aid in the development of gas reservoirs and the desorption of adsorbed gas, but the capillary water column obstructs the flow of gas and restricts the flow of shale gas in microscopic pores. This study will lay a theoretical foundation for evaluating the water-bearing characteristics of marine shale reservoirs, which will help analyze the modification effect of fracturing fluid retention on reservoir water-bearing characteristics and then guide gas well development.