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Abstract:

Quantitative models of the reduction of permeability in reservoir sandstones due to the growth of authigenic-clay minerals in the pore space are based on the ability to estimate the permeability of the original clay-free rock. Simple physical models based on Carman-Kozeny relations are used to calculate permeability for the idealized sandstone pore space. Values for the surface-area parameter in the models are determined from proton NMR longitudinal-relaxation times and area/perimeter ratios extracted by petrographic-image analysis. Although the magnitude of the difference between measured and calculated permeabilities is model dependent, the different models characterize relative behavior for each suite of sandstones. The normalized permeability differences correlate weakly with various measures of total clay abundance. This indicates that permeability reduction is influenced more by clay distribution than by clay abundance. Cation-exchange capacity (CEC) measurements made by flow through the intact rock are lower than values determined by standard methods on powders. As the ratio of flow to bulk CEC values decreases, fewer of the clays in the pore space are accessed by the fluid. Samples with increased fractal dimensions or surface roughness have lower CEC ratios, indicating that increased roughness limits the accessibility of exchange sites. Samples with lower fractal dimensions have more authigenic kaolinite than fibrous illite, in addition to greater differences in measured and calculated permeability. This suggests that physical constrictions caused by clay growth in the throats is more important than surface-roughness effects in reducing permeability in sandstones.

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