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The characterization of porous rocks with nuclear magnetic resonance and the confocal laser scanning microscope Chapman, Alice Elizabeth


The characterization of porous media with the confocal laser scanning microscope (CLSM) and nuclear magnetic resonance (NMR) was investigated. Pore casts (epoxy replicas) of one sandstone, Berea 100, and two carbonates, S55 and S77, were imaged with the CLSM. Isolated two-dimensional images and serial sections, a series of two-dimensional images separated by a specific vertical distance, were collected. The isolated images were analysed and mean pore radii of 25.3, 17.5, and 90.2p.m were determined for Berea 100, S55 and S77 respectively. The serial sections were reconstructed to form three-dimensional images of the pore networks. However, the pores in the samples were large compared to the depth of the pore casts, resulting in images with insufficient depth to allow three-dimensional pore size analysis. Spin-lattice ( T1)measurements of twenty-two saturated carbonates (samples S31 to S49, S54, S55, S77) and six partially saturated carbonates were analysed to determine pore sizes and fluid distributions respectively. Pore sizes were calculated using the “two-fractions in fast exchange” model and the slow diffusion model of Brownstein and Tarr (1979). Samples S31 to S49, with pore radii of less than 41.Lm, were within the fast diffusion regime and thus the “twofractions in fast exchange model” produced reliable pore sizes. Samples S54, S55 and S77, with mean pore radii of 5.1, 19.3, and 85.3.tm, were not within the fast diffusion regime and so the accurate analysis of data for these samples required the slow diffusion model. The spin-lattice time constants calculated for the partially saturated samples were assumed to be proportional to the sizes of the saturated pores. Thus it was concluded that the larger pores within the samples were drained preferentially; however, large pores which were accessible through smaller pore throats remained saturated to low saturations. Further investigation into these methods could focus on the production of pore casts to enable reconstruction of an image with sufficient depth for three-dimensional analysis or on the development of a pore model which more closely approximate the shape of the pores.

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