Abstract
Mercury intrusion porosimetry (MIP) as a practical and effective measurement has been widely used in characterizing the pore size distribution (PSD) for unconventional reservoirs (e.g., coals and shales). However, in the process of MIP experiments, the high mercury intrusion pressure may cause matrix compressibility and result in inaccurate estimations of PSD. To get a deeper understanding of the variability and heterogeneity characteristics of the actual PSD in coals, this study firstly corrected the high mercury intrusion pressure data in combination with low-temperature N2 adsorption (LTNA) data. The results show that the matrix compressibility was obvious under the pressure over 24.75 MPa, and the calculated matrix compressibility coefficients of bituminous and anthracite coals range from 0.82 to 2.47 × 10−10 m2/N. Then, multifractal analysis was introduced to evaluate the heterogeneity characteristics of coals based on the corrected MIP data. The multifractal dimension Dmin is positively correlated with vitrinite content, but negatively correlated with inertinite content and mercury intrusion saturation. The multifractal dimension Dmax shows negative relationships with moisture and ash content, and it also emerges as a “U-shaped” trend with efficiency of mercury withdrawal. It is concluded that multifractal analysis can be served as a practical method not only for evaluating the heterogeneity of coal PSDs, but also for other unconventional reservoirs (e.g., shale and tight sandstone).
Highlights
The pore size distribution (PSD) properties of coals play an important role in characterizing coal reservoirs since it has significant effects on the exploration coalbed methane (CBM) [1,2,3,4]
mercury intrusion porosimetry (MIP) measurement has been proven as an effective method to estimate the PSD of coals
Based on the corrected PSDs from MIP measurement, the heterogeneity characteristics of coals were investigated by the introduced multifractal analysis
Summary
The pore size distribution (PSD) properties of coals play an important role in characterizing coal reservoirs since it has significant effects on the exploration coalbed methane (CBM) [1,2,3,4]. Energies 2019, 12, 4743 to characterize the two- or three-dimensional distribution of coal PSDs. Indirectly inferred methods include mercury intrusion porosimetry (MIP) [11,12], gas adsorption (N2 and CO2 ) [13,14,15,16] and nuclear magnetic resonance (NMR) [17,18,19]. Previous studies have shown that it is a feasible method to evaluate coal matrix compressibility by combining low-temperature N2 adsorption (LTNA) and MIP data [22,23,24]. Zhao et al [29] combined multifractal analysis with NMR technique to investigate the heterogeneity distribution of tight sandstone reservoirs. Multifractal analysis was applied in describing the heterogeneity characterizations for the corrected coal PSDs
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