Nanoscale Pore Structure Characterization and Permeability of Mudrocks and Fine-Grained Sandstones in Coal Reservoirs by Scanning Electron Microscopy, Mercury Intrusion Porosimetry, and Low-Field Nuclear Magnetic Resonance

Abstract

Porosity and permeability of two typical sedimentary rocks in coal bearing strata of underground coal mines in China, i.e., mudrocks and fine-grained sandstones, were comprehensively investigated by multiple experimental methods. Measured porosity averages of the helium gas porosity ( ), MIP porosity ( MIP ), water porosity ( ), and NMR porosity ( NMR ) of the twelve investigated rock samples range from 1.78 to 16.50% and the measured gas permeabilities ( ) range from 0.0003 to 2.4133 mD. Meanwhile, pore types, pore morphologies, and pore size distributions (PSD) were determined by focused ion beam scanning electron microscopy (FIB-SEM), mercury intrusion porosimetry (MIP), and low-field nuclear magnetic resonance (NMR). FIB-SEM image analyses showed that the mineral matrix pores including interparticle (interP) and intraparticle (intraP) pores with varied morphologies are the dominant pore types of the investigated rock samples while very few organic matter (OM) pores were observed. Results of the MIP and the full water-saturated NMR measurements showed that the PSD curves of the mudrock samples mostly present a unimodal pattern and nanopores with pore diameter less than 0.1 m are their predominant pore type, while the PSD curves of the fine-grained sandstone samples are featured by a bimodal distribution. Furthermore, comparison of the full water-saturated and irreducible-water-saturated NMR measurements indicated that pores in the mudrocks are solely adsorption pores (normally pore size < 0.1 m) whereas apart from a fraction of adsorption pores, a large part of the pores in the sandstone sample with relatively high porosity are seepage pores (normally pore size > 0.1 m). Moreover, the PSD curves of NMR quantitatively converted from the NMR 2 spectra by 2 and weighted arithmetic mean (WAM) methods are in good agreement with the PSD curves of MIP. Finally, the applicability of three classic permeability estimation models based on MIP and NMR data to the investigated rock samples was evaluated.

Investigation of the Effect of Confining Pressure on the Mechanics-Permeability Behavior of Mudstone under Triaxial Compression

“…Note that the ordinate value of the starting point of each data set is 1. 8 Geofluids (approximately 2.15 to 3.15 GPa) of the mudstone significantly lower than the unloading modulus. As the confining pressure increases, more pore and microcrack compressions occur during the exertion of the confining pressure; therefore, the irreversible strain in this stage decreases and the secant modulus increases slightly with the increase in the confining pressure.…”

“…Therefore, mudstone formations are an effective barrier to geofluid accumulation and play an important role in petroleum systems and in deep energy and waste disposals. Although not always the case, the permeability of mudstones in a well-compacted and undisturbed state is less than 1 μD (≈10 -18 m 2 ) based on laboratory measurements of intact mudstone samples [3][4][5][6][7][8][9]; the reason for this result is that mudstone is rich in fine-grained and clay sediments and usually has a mean pore radius of less than 100 nm [4,10,11], which is more than several orders of magnitude smaller than that of coarser-grained rocks such as sandstone that retains geofluids [12,13]. Since the clay grains are platy shaped, mudstone has higher compressibility than other sedimentary rocks and its permeability, which is related to porosity, decreases greatly with increasing diagenesis depth or consolidation pressure [14][15][16][17][18].…”

Variable Pore Structure and Gas Permeability of Coal Cores after Microwave Irradiation

“…NMR has been widely used for the characterization of complex porous media, such as the petroleum reservoir rocks and coal [32][33][34][35][36]. For example, a recent research by Zhang et al demonstrated that the successful use of the NMR to obtain pore size distribution of mudrock and fine-grained sandstone samples [37]. When the magnetic field is small, few paramagnetic minerals within the coal could influence the measured results and the magnetic information on solid state proton ( 13 C and 1 H) can be shielded [31,38].…”

Evaluation of Coal Seam Gas Drainability for Outburst-Prone and High-CO2-Containing Coal Seam

“…Coal with extremely complex pore structure can store a large amount of CSG; CSG has also become an energy resource of global significance. In addition to USA, countries such as Canada, Australia, China, and India practice CSG drainage activities [5][6][7][8][9][10]. In Australia, the CSG drainage industry is growing rapidly and is becoming one of the significant energy sources in both Queensland [6,11] and New South Wales [5].…”