Abstract
On the basis of the complexity of the pore structure characteristics of a coal reservoir, coal samples with different ranks were selected to study the difference in pore structures and permeability using nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and permeability measurement. Porosity and pore size distribution (PSD) above 20 nm can be analyzed by the improved NMR equation, and the results were basically consistent with that of SEM and MIP. The NMR spectra of the coal samples from the same location were close, but the difference between the coal samples from different locations was quite large, which indicated that the heterogeneity of a coal reservoir was strong. An empirical equation of movable fluid porosity was proposed, which can be used to evaluate the fluid migration characteristics of the coal reservoir, and the porosity of movable fluid mainly came from the contribution of fissures and micro-fissures. The average movable fluid porosity of the coal samples from the Chengzhuang (CZ) coal mine, Wuyang (WY) coal mine, and Yujialiang (YJL) coal mine was 1.37%, 0.67%, and 4.26%, respectively. Although the permeability is related to the NMR porosity and movable fluid porosity, it was difficult to establish a widely used mathematical equation correlating permeability and porosity based on the experimental data.
Highlights
Coalbed methane (CBM), as a type of clean and efficient unconventional energy, is mainly adsorbed in the coal matrix micropores
Based on the morphology of the T2 spectra and the size and shape of the pores and fissures observed by scanning electron microscopy (SEM), the pores and fissures in the YJL coal samples can be divided into three types: nanopores, micro-fissures, and fissures
The pore structure and permeability of the different coal rank samples were analyzed by nuclear magnetic resonance (NMR), SEM, and mercury intrusion porosimetry (MIP) experiments
Summary
Coalbed methane (CBM), as a type of clean and efficient unconventional energy, is mainly adsorbed in the coal matrix micropores. As a typical porous material, there are many test methods for studying the pore structure They are divided into three types: fluid injection, image observation, and physical detection. The pores corresponding to the T2 peaks are divided into adsorption pores, seepage pores, and fissures by Yao et al [11] and Cai et al [12] They think that the patterns of unimodal, bimodal, and multi-peak in the T2 spectra represent different pore types and PSD, and the pore characteristics of NMR were compared with those of MIP and CT scanning [13,14]. MIP, SEM, and pulse-decay-permeability tests were carried out, the variation of the T2 spectra of the different coal samples were analyzed, and the classification of the pores and fissures in the coal were divided based on the shape of the T2 spectra. Based on the experimental data of porosity and permeability, the accuracy and applicability of the different permeability calculation models were analyzed and validated
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