Pore characterization of coal cuttings based on low-field nuclear magnetic resonance: A new method

Abstract

Coal reservoir pore complexity governs coalbed methane occurrence and migration, making core analysis essential for pore architecture characterization. However, obtaining intact cores from soft, low-permeability coal seams with weak mechanical strength and dense fractures is challenging. Consequently, drilling cuttings are often used for pore structure evaluation, necessitating assessment of their substitutability for porosity and pore size distribution analysis. To address this, we developed a low-field nuclear magnetic resonance (LF-NMR) methodology for cuttings porosity measurement. Bituminous coal samples from Shenmu’s Zhangminggou Mine were analyzed using a 1H probe and 5 % CuSO4 centrifugation. Comparative testing employed two artificial crushing methods to prepare cuttings particles. Porosity and pore size distributions across varying particle sizes were compared with intact core data. Key findings indicate: (1) Cutting and core T2 spectra exhibit high similarity, with micropores (<2 nm) and mesopores (2–50 nm) dominating pore structures; (2) Maximum porosity deviation is 3.03 %, minimized at > 3.35 mm particle sizes, confirming cuttings reliability for basic reservoir characterization; (3) Fragmentation intensity critically affects measurements—total porosity increases from 10.13 % to 14.26 % as particle size decreases from 3.35 mm to 0.85 mm due to exposed occluded pores and semi-closed pore transformation, while micropore proportion rises from 53.84 % to 58.00 %. We propose a ≥ 1 mm particle size threshold, with optimal accuracy at 1.7–2.36 mm cuttings, balancing precision and closed-pore preservation. This non-destructive method enables efficient unconsolidated coal reservoir evaluation with significant engineering applicability.