Assessing the Water Migration and Permeability of Large Intact Bituminous and Anthracite Coals Using NMR Relaxation Spectrometry

Abstract

Although extensive literature has emerged on the use of nuclear magnetic resonance (NMR) relaxation spectrometry for the determination of various petrophysical properties of clastic rocks, relatively few papers have reported the use of NMR to determine the moisture migration process and permeability of large intact coals. In this study, evaporation experiments simultaneously with NMR measurements were conducted for seven coal core plugs. Differences in the relaxation time distribution at the various saturation stages provide both qualitative and quantitative information on the saturation state and migration of moisture in the coal samples. Moisture migration accrues first in the larger pores, whereas the smaller pores appear to remain water-saturated. The evaporation of moisture in large intact coals undergoes three continuous processes: Migration of free water, macro-capillary water, and micro-capillary water, among which, the migration rates of both macro-capillary water and micro-capillary water are related to the coal pore size distribution and porosity. The results of the NMR measurements also indicate that coal permeability has a relationship with the moisture migration characteristics, which make it possible to develop an NMR-derived permeability model. The Timur–Coates and the Schlumberger-Doll Research (SDR) equations, which are commonly used in NMR logging evaluations of the permeability of clastic rock reservoirs, were discussed concerning their application to coals. It is found that a modified SDR permeability evaluation model $$[K_\mathrm{SDR}=0.0224(\hbox {T}_\mathrm{2gm}^\mathrm{b})^{0.182}(\hbox {T}_\mathrm{2gm}^\mathrm{a})^{1.534}]$$ , which is a double-exponential relationship with the $$T_{2}$$ geometric mean in the full water-saturated state and in the bound water state, fits the analyzed coal samples well. The NMR-derived methods provide a framework for the determination of the moisture migration and permeability of coals.