A novel pore size classification method of coals: Investigation based on NMR relaxation

Nuclear magnetic resonance (NMR) severs as a nondestructive and relative new technique that has been widely used in characterizing reservoir fluids and pore size distribution (PSD) of coals. Conventionally, pore fluids in coals are classified into movable fluid and irreducible fluid based on a single NMR T-2 cutoff value (T-2C). However, the single NMR T-2 cutoffs model has some apparent defects in pore fluid/size classification, and few researches have reported the limitation of the single T-2 cutoffs model. In contrast, the dual T-2 cutoffs model may provide an accurate quantified model to classify different pore fluid types in coals. In this study, fifteen coal samples with different ranks were conducted in systematic NMR and centrifugal experiments to investigate the characteristics of pore fluid typing and PSD. Results show that when tried applying the single NMR T-2 cutoffs model to classify the pore fluid typing, there are still some movable fluids when T-2 < T-2C. At the same time, when T-2 > T-2C, there is remaining some irreducible fluid in pores after high pressure centrifugal experiments. These results indicated the limited application of single NMR T-2 cutoffs model in pore fluid classification. By introducing a novel pore fluid classification method (i.e. the dual T-2 cutoffs model), a typical T-2 spectrum under fully-saturated condition, the absolute irreducible fluid T-2 cutoffs (T-2C1) and absolute movable fluid T-2 cutoff (T-2C2) can re-divide the pore fluid typing of coal into three types: absolute irreducible fluid (T-2 < T-2C1), partial movable fluid (T-2C1 < T-2 < T-2C2), and absolute movable fluid (T-2 > T-2C2). The results show that the T-2C1 is in the range of 0.10-0.32 ms, while the T-2C2 has a wider range from 36.12 ms to 89.07 ms. Finally, a conceptional model were proposed to clarify a full-scale PSD classification that includes the absolute irreducible fluid pores, partial movable fluid pores and absolute movable fluid pores. The model established in this study, can also be applicable for other rock types (e.g., sandstones, carbonates and shales).