Classifying Coal Pores and Estimating Reservoir Parameters by Nuclear Magnetic Resonance and Mercury Intrusion Porosimetry

The widely used coalbed methane (CBM) flow model-triple porosity/dual permeability (TPDP) model indicates that coal pores can be divided into micro-trans-pores, meso-macro-pores, and fractures, while CBM can flow via meso-macropores and fractures. The mechanism to obtain the reservoir parameters of these two flowing systems has been given little attention in the TPDP model. In this study, nuclear magnetic resonance (NMR), mercury intrusion porosimetry (MIP), and other routine core analysis methods were conducted on nine coal samples to classify coal pore types, transform transverse relaxation time to pore radius, and estimate porosity and permeability of meso-macro-pores and fractures. Results show that the two referenced relaxation times of T-2C1 and T-2C2, which were identified from the curves of irreducible and full water saturations obtained from NMR experiments, can classify coal pores into fractures (T-2 > T-2C2), meso-macro-pores (T-2C1 < T-2 < T-2C2), and micro-trans-pores (T-2 < T-2C1). The dividing point of micro-trans-pores and meso-macro-pores, which was obtained from NMR and MW (mercury intrusion porosimetry) experiments, provided another method for transforming transverse relaxation time to pore radius. Based on the classification results and routine core analysis methods, the porosity of meso-macro-pores and fractures were calculated, and the relationship between air permeability and porosity of meso-macro-pores and fractures was finally proposed. This paper provides an effective method for obtaining the permeability and porosity of meso-macro-pores and fractures using the TPDP model to simulate the performances of CBM wells.