Abstract
The mechanisms by which CO2 and water interact in coal remain unclear and these are key questions for understanding ECBM processes and defining the long-term behaviour of injected CO2. In our experiments, we injected helium/CO2 to displace water in eight water-saturated samples. We used low-field NMR relaxation to investigate CO2 and water interactions in these coals across a variety of time-scales. The injection of helium did not change the T2 spectra of the coals. In contrast, the T2 spectra peaks of micro-capillary water gradually decreased and those of macro-capillary and bulk water increased with time after the injection of CO2. We assume that the CO2 diffuses through and/or dissolves into the capillary water to access the coal matrix interior, which promotes desorption of water molecules from the surfaces of coal micropores and mesopores. The replaced water mass is mainly related to the Langmuir adsorption volume of CO2 and increases as the CO2 adsorption capacity increases. Other factors, such as mineral composition, temperature and pressure, also influence the effective exchange between water and CO2. Finally, we built a quantified model to evaluate the efficiency of water replacement by CO2 injection with respect to temperature and pressure.
Highlights
Spaces that would otherwise have been available for CO2 adsorption[10]
Interactions within pores in a CO2–coal–H2O system are extremely important for CO2 storage and enhanced coalbed methane recovery (ECBM)
The underpinning principle of the method is that proton Nuclear magnetic resonance (NMR) transverse relaxation time (T2) is affected by bulk, diffuse and surface relaxation according to the basic characteristics of NMR measurements in rock, characterized by: 1= 1 +1 + 1
Summary
Spaces that would otherwise have been available for CO2 adsorption[10]. competitive adsorption exists between CO2 and water molecules in addition to that between methane and CO2. Some previous studies have investigated methane-coal-water interactions on the coal surface, such as contact angles and wetting behaviour in CO2–coal–H2O systems. Nuclear magnetic resonance (NMR) provides a fast, convenient and non-destructive method for detecting hydrogen-bearing fluids[20] This technique has been used to determine various petrophysical characteristics such as porosity and permeability of the formation and viscosity and saturation of fluids in conventional reservoirs, and for well logging in petroleum exploration[21,22,23]. Gas and water exchange processes are followed as a function of time, temperature, pressure and coal properties using changes in the configurations of transverse relaxation time (T2) distributions of the water in the coal as a proxy. Surface relaxivity and the ratio of the pore surface area to the pore volume are proportional to the surface relaxation and can be described by the following equation:
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