Abstract:

The research on coal wettability is significant in coal chemical industry, safety of coal mining, and coalbed methane (CBM) production. The conventional methods for coal wettability determination are difficult to be performed and unsuitable for an in-situ measurement, which make it difficult to be used as in-situ analysis method for determining wettability of CBM reservoirs. Based on a series of low-field nuclear magnetic resonance (LFNMR) experiments, this paper discussed the practicality of the application of LFNMR in evaluation of the wettability of CBM reservoirs. It was found that moistures in the coals have different phases including the free water, interparticle capillary confined water, and adsorption water that can be distinguished by the water transverse relaxation time (T₂) spectra in a LFNMR measurement. For the hydrophilic powdered coal, there are adsorbed water and interparticle water that correspond to two distinct peaks in the T₂  spectra. For hydrophobic powdered coal, there exist adsorbed, interparticle and free waters, corresponding to three distinct peaks in the T₂ spectra. The stronger the hydrophilicity is, the faster the relaxation time of capillary water and free water is. In an equilibrium system of the coal-water wettability, the geometric mean value of T₂ for free and capillary waters is found to be logarithmically correlated with the contact angle between coal and water that is determined from a conventional method. We suggest that the low-field NMR is quantifiably or semiquantifiably applicable for estimation of coal wettability, which provides an important theoretical foundation for evaluating the water-gas interactions in a CBM reservoir by using LFNMR.

Key words: coal wettability, contact angle, NMR, T₂ relaxation time, coalbed methane