Pore Structure Analysis of Coals Using Low Field Nmr Measurements and Thermogravimetry Analysis

Four different coals were studied using a combination of T2, T1, FID and solid echoes measurements on saturated plugs and degassed cuttings. These measurements were performed on two NMR spectrometers working at a proton Larmor frequency of 2 and 23 MHz. The measurements on saturated plugs allow the characterization of the cleats, and the measurements on degassed cuttings allow the characterisation of the matrix. For the latter, thermogravimetry coupled with mass spectral analysis were also performed. The results indicate a coherent measurement of liquid protons from NMR and thermogravimetry when the FID decay is analysed using a combination of Gaussian and exponential functions. This allows the distinction between solid and liquid fractions and consequently an accurate estimation of the porosity of the samples. For the four types of coal studied, microporosity represents a significant fraction of the total porosity (from 0.41 up to 0.91) and can be grossly underestimated when using usual NMR petrophysical analysis. INTRODUCTION With the prospect of geological storage of CO2, there is an increasing need for detailed characterisation of coal structure. Coals are generated by accumulation of dead plant debris and minerals such as clays, alumina or silica. Almost immediately after deposition and burial, biochemical and geochemical processes are initiated. With continued burial, these processes progressively cause coalification of the organic matter to form the maturational sequence of peat, brown coal, bituminous coal and finally anthracite (Tissot and Welte, 1978). These processes result in a very complex chemical composition and textural structure for coal. Coals typically have micro pores but also exhibit meso and macroporosity covering four decades in pore sizes. Due to its complex nature, a variety of techniques have been used to determine the pore structure of coals and the problems well identified. For example, the adsorption of nitrogen at 77 K is a standard method used in determining the surface area of coal. However Anderson and co-workers (Anderson et al., 1965) have demonstrated that this approach underestimates coal surface areas. Indeed, activated surface diffusion limits the ability of nitrogen to access the micropores. Another routine experiment used to