Coalbed Characterization Studies With X-Ray Computerized Tomography (CT) and Micro CT Techniques

Abstract Much of the focus of CBM reservoir assessment in Canada is based on understanding cleats and natural fractures, both in outcrop and in core taken from well bores. Coal characterization studies using imaging devices are presented. X-ray computerized tomography (CT) and micro CT are used on coal samples to provide a better understanding of fracture morphology and apertures. Images are collected in samples of approximately 10 cm in diameter with resolution of (0.4 mm)2 using X-ray CT, and samples of 1 cm in diameter with resolution of (5_m)2 using micro CT. Visualization at both resolutions allows for discussion and comparison of structural characteristics at both scales. X-ray CT images are processed to obtain density "logs" and maps under different overburden pressures. The pore space contained helium, and, in one set of scans, argon. Bulk densities increase with increasing overburden pressure. Fracture patterns in both cores were reconstructed from the images using a fracture identification algorithm. Manipulation of the density maps can also provide local strains as a function of increasing overburden pressure. Introduction The width of the natural fractures (cleats) and the corresponding permeability is typically a strong function of the net stress in the coalbed. Understanding stress-dependent permeability is essential as declining permeability during coalbed depletion would be detrimental to the productivity of coalbed wells. By using imaging techniques, the distribution of density, fractures and volumetric strain fraction can be defined and used to understand flow characteristics of gas in coals. Computerized tomography (CT) scanning is a non-destructive laboratory technique which can provide two and three dimensional image reconstruction of opaque objects using X-rays. It is relatively easy to apply, can offer fine spatial resolution, and is adaptable to many types of experimental procedures. Micro CT is also a non-destructive technique enabling virtual slicing of opaque objects. Stacking several slices enables 3D visualization of the object. The final images show the differences in the linear attenuation coefficient of X-rays. This linear attenuation coefficient depends on the density and the atomic number of the object. Consequently, components that differ in these parameters can be distinguished. EXPERIMENTAL SAMPLE PREPARATION Two coal cores, Sample 1 and Sample 2, both from the Manville Group in Alberta, were used in this work. Figure 1 is a photograph of Sample 1 (Figure 1 A) and Sample 2 (Figure 1 B). Table 1 presents the depth of burial, the coal rank and the formation from where the coal samples were taken. Because coal is a friable material, the coal core was not a smooth cylinder as in the case of a reservoir rock. In order to mount the coal core in a rubber sleeve inside a core holder for permeability measurements, it had to be made cylindrical first. This was accomplished by casting the core in urethane in a cylindrical mold. The urethane is solid enough to keep the core intact and confine it in a core holder, but retains enough flexibility to transmit overburden pressure to the coal.

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