Natural fractures in unconventional reservoirs are characteristically filled or lined with mineral cement. Despite the significance of natural fractures for stimulation and production, the effect of cement linings on fracture fluid flow is poorly understood. In this work, we focus on correlating permeability with geometric tortuosity of both pore (fracture) space and individual fluid phases for fractured Torridonian Sandstone, an outcrop analog for tight sandstone reservoirs. The input geometries for simulation are binarized, high-resolution microtomography images at two different resolutions. We use a combination of lattice-Boltzmann simulation and the level-set-method-based progressive-quasistatic (LSMPQS) algorithm to characterize the capillary dominated displacement properties (capillary pressure-saturation and relative permeability-saturation relationships) of the natural, partially cemented fractures within. We also use image analysis to characterize the connectivity and tortuosity of the pore space, as well as individual fluid phases at different saturations.
The partially cemented fractures of the Torridonian Sandstone are found to be very constricted, with many crystals bridging across the fracture but keeping large portions open to flow. The adjacent matrix, however, is almost completely cemented. We compare the tortuosity distribution in the Torridonian Sandstone with those in other porous media. We find that the fractures have considerably narrower tortuosity distribution when compared to other porous materials. Despite their cement lining, these fractures provide the most direct path across the material. In addition, we find that tortuosity in both consolidated porous media and partially cemented fractures increases with an increase in the amount of carbonate or quartz overgrowth cement. When analyzing tortuosity of different fluid phases, we find very weak correlation between fluid phase tortuosity and relative permeability. Relative permeability correlations and capillary pressure curves found here can be used in reservoir simulators to model recovery of hydrocarbons in fractured tight reservoirs.
URTeC 1581613
