Characterizing Moisture and Gas Content of Coal by Low-Field NMR

Abstract

Coalbed Methane (CBM) shows great potential to be an important energy source. One key factor for the successful development of CBM processes is to characterize coal on its moisture and gas content. Usually, the moisture and gas content of coal are determined from laboratory analysis. Low-field nuclear magnetic resonance (NMR) is a relatively new technique used in logging and in the analysis of fluids contained in reservoir rocks. This paper investigates the potential for coal characterization by low-field NMR. Low-field NMR detects hydrogen-bearing molecules and, in reservoir rock samples, distinguishes between ‘free’ bulk fluid and ‘bound’ surface fluid. Coal contains free water in the cleats as well as moisture that forms an integral part of the coal structure. Methane gas is a light hydrocarbon gas and coal contains free methane gas in fractures and adsorbed methane in internal surfaces. NMR characterization of moisture and adsorbed gas in coal and implications for moisture, adsorption isotherm and gas content measurements are explored. Experiments of moisture-free coal, moist coal and coal/water mixtures indicated drastically different spectra. From these spectra, free and bound water could be estimated using a methodology that is currently applied in clay-rich formations. In this paper, two sets of data are presented. First, measurements at ambient conditions provided a reference to other conventional moisture and cutoff data. Second, a high-pressure cell for the measurement of adsorbed coal was used and comparisons were made. Coal samples in the form of powder and chunk were used. The paper focuses on the methods and results to date. Introduction Coalbed methane (CBM) has evolved into a commercially profitable source of natural gas. Canada has vast resources of coal and it has been estimated that the total in-place reserves are 36 ? 1012 m3. Over 60% of Canada's CBM assets are in Alberta(1). Coalbed methane has the potential of contributing a significant portion of Canadian natural gas production in the near future. One key factor for the successful development of CBM processes is to characterize coal on its moisture and gas content. Usually, the moisture of coal is determined from coal proximate analysis and the gas content of coal is determined by the desorption measurement in the laboratory. Low-field NMR is a relatively new technique used in logging and in the analysis of fluids contained in reservoir rocks. Nuclear magnetic resonance occurs when the nuclei of certain atoms (i.e. hydrogen proton) are immersed in a static magnetic field and exposed to a second oscillating magnetic field(2). NMR provides a non-destructive analytical method of detecting hydrocarbons in reservoirs(3) and characterizing the hydrocarbon gas(4). Generally, the NMR spectrum can provide three types of information: the quantities of the fluids in the rock, the mobility (viscosity) of these fluids and information about the pores that contain these fluids. As a source rock and at the same time a reservoir rock, coal contains large amounts of water and methane. Water exists as free water in the cleats as well as the moisture that forms an integral part of the coal structure.

Application of a Modified Low-Field NMR Method on Methane Adsorption of Medium-Rank Coals

“…As shown in Figure 5(a), the relaxation time of bulk methane is relatively large, which exhibits an obvious peak at 70 ms-2000 ms. The relaxation area and relaxation times of bulk methane gradually increase with the increase in pressure, which is consistent with the work of Guo et al [48] and Yao et al [49,50]. Morriss et al [51] proved that the mean free path of methane molecules declines with the increase in pressure, which results in the increase in bulk methane relaxation time.…”

“…The CPMG pulse sequence measurement is used to obtain the T 2 relaxation under fast diffusion condition. The influence of diffusion relaxation can be ignored as the magnetic field is uniform [48,49]. The bulk relaxation is an intrinsic property of the hydrogencontaining fluid and affected by the physical properties.…”

“…There are, however, quite few studies involving low-field NMR relaxation characteristics of adsorbed methane in coal. Guo et al [48] used a low-field NMR amplitude index from bulk methane to quantify the mass and volume of adsorbed methane in two low volatile bituminous coals, without distinguishing the bulk methane and adsorbed methane relaxation properties. Yao et al [49] and Xie [43] built an NMR transparent isotherm adsorption experimental setup to evaluate the capacity of methane in coal.…”

Unconventional gas reservoir evaluation: What do we have to consider?

“…Guo et al (2007), for example, used the T 2 spectra from low-field NMR to detect adsorbed (fast relaxation, T 2 < 5 ms), capillary-bound (T 2 ¼ 5e200 ms) and bulk (free) water (slow relaxation, T 2 > 200 ms). NMR was recently used to study shale wettability (Odusina et al, 2011).…”

Experimental study on the influence of aerated gas pressure and confining pressure on low-rank coal gas adsorption process

“…According to the experimental results and NMR test theory, the lateral relaxation time of coal gas is proportional to the pore radius of coal measure shale. Referring to the results of previous scholars (Guo et al, 2007;Tang et al, 2017;Yao et al, 2014), the different states of methane can be further distinguished. From the T 2 spectrum curve of coal gas adsorption, each T 2 time corresponds to a gas signal amplitude on the vertical axis, and this amplitude is proportional to the number of gas molecules (Deng and Xie, 2010).…”