Dynamic wetting mechanism of coal under different CO2 pressure: A new analytical basis through T2 cutoff

Abstract

The technology of injecting carbon dioxide into coal seams to reduce greenhouse gas emissions and has brought attraction on the wettability of coal. As a fast and non-destructive method, nuclear magnetic resonance (NMR) is widely used in the evaluation of coal wettability, but the inconsistency of analytical basis makes the quantitative evaluation of coal wettability changes uncertain. In this study, based on NMR combined with centrifugal experiments, a novel analytical basis using irreducible water defined by T2 cutoff value was proposed to quantitatively analyze the dynamic wetting mechanism under different CO2 pressures. The results show that when T2 < 72.32 ms, the water in coal is irreducible water; when T2 > 72.32 ms, the water in coal is movable water. CO2 pressure hindered free water conversion to adsorbed and capillary water, but impacted adsorbed water more. Increased CO2 pressure caused the wetting porosity of coal to decrease from 1.62 % to 1.08 %, and coal gradually changed from water-wet to CO2-wet. The increase in CO2 pressure enlarged the CO2 aggregations in the seepage channel, thus weakening the wetting rate of water to coal. CO2 preferentially occupied small pores, which will hinder the migration of water into small pore spaces. Molecular diffusion controls water to achieve wetting equilibrium more than gravity/pressure difference and capillary force. As CO2 pressure increases, the wetting equilibrium time decreases.