Understanding fluid flow in rough-walled fractures using x-ray microtomography images




Tokan-Lawal, Adenike O.

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Natural fractures provide fluid flow pathways in otherwise low permeability reservoirs. These fractures are usually lined or completely filled with mineral cements. The presence of these cements causes very rough fracture walls that can constrict flow and hinder the connectivity between the fracture and matrix/fracture pores thereby reducing porosity, permeability and matrix/fracture transfer. In order to accurately predict fluid transport in the unconventional reservoirs, I study the influence of diagenesis (cementation and compaction in particular) and fracture roughness on flow in artificial (fractured polyethylene) and naturally fractured carbonate (Niobrara formation outcrop) and tight gas sandstones (Torridonian outcrop and Travis Peak reservoir in particular). X-ray microtomography imaging provides information on fracture geometry. Image analysis and characterization of the connectivity and geometric tortuosity of the pore space and individual fluid phases at different saturations, is performed via ImageJ and 3DMA Rock software. I also use a combination of the level-set-method-based progressive-quasistatic algorithm (LSMPQS software), and lattice Boltzmann simulation (Palabos software) to characterize the capillary dominated displacement properties and the relative permeability of the naturally cemented fractures within. Finally, I numerically investigate the effect of (uniform) cementation on the fracture permeability as well as the tortuosity of the pore space and the capillary pressure-water saturation (Pc-Sw) relationship in the Niobrara. Permeability estimates in the different formations vary by several orders of magnitude with the different correlations that currently exist in the literature for all samples studied. The presence of cements increases the geometric tortuosity of the pore space and capillary pressure while reducing the permeability and porosity. Contrary to our expectation, the tortuosity of either wetting or non-wetting phase and their respective relative permeabilities show no clear correlation. Overall, pore scale methods provide an insight to flow characteristics in rough walled fractures at micron scale that are not well represented by existing correlations. The measured properties can be used as input in reservoir simulators.



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