Experimental study of the interplay between pore system and permeability using pore compressibility for high rank coal reservoirs

Abstract

Different from the United States where coalbed methane (CBM) is produced from low rank coal seams, the most productive CBM field in China is mainly distributed in the regions with high rank coal seams, particularly in the Qinshui Basin, where coal seams distributed in shallow areas. Pore system, permeability, and interplay between them are the most important factors for CBM reservoir evaluation. In this study, laboratory measurements, including low-field nuclear magnetic resonance (LF-NMR), confining pressure nuclear magnetic resonance (CP-NMR), and pulse attenuated permeability were used to describe the properties of pore systems and permeability for high rank coal reservoirs. A newly proposed approach was also used to gain insights into the interplay between pore systems and permeability. The results show that the LF-NMR transverse relaxation time (T2) spectra of different coal samples were characterized by a bimodal distribution, with a significant distance between the main peak and sub-peak, meaning nanoscale micropores are well developed in coals with poor pore connectivity. Total pore volume (TPV) was divided into two portions, the immovable pore volume (IPV) and the movable pore volume (MPV), based on the distribution of T2cutoff values. Both pore system and permeability are highly stress dependent, decreasing exponentially with increasing effective stress. Further, two kinds of pore compressibility models (i.e., a constant pore compressibility model and a variable pore compressibility model) for TPV, MPV, and IPV, were proposed based on the results of CP-NMR. Coal permeability was highly correlated with the MPV pore compressibility, suggesting that the MPV reflects large scale pores with good pore connectivity, substantially contributing to coal permeability. Additionally, the relationship between the measured permeability and variable MPV compressibility is stronger than that between the measured permeability and constant pore compressibility. Permeability evaluations using constant pore compressibility are substantially lower than those based on variable MPV compressibility for deep coal reservoirs. Thus, pore compressibility is a variable parameter under in situ conditions, and a variable pore compressibility model is more accurate for evaluating deep high rank coal reservoirs. The findings of this study are particularly meaningful for evaluation of deep high rank CBM, such that in the deep areas of the Qinshui Basin.