Natural fractures provide preferential pathways for fluid migration, and their effect is especially high in rocks with low matrix permeability. These fractures are usually lined or completely filled with mineral cement. The presence of cement can hinder the connectivity between residual fracture pores, thereby reducing fracture permeability.
In order to better understand fluid transport in the Niobrara Formation, we studied the influence of cementation on flow in the fracture. We acquired the fracture geometry from x-ray microtomography (CT) scans, capturing the small-scale roughness of the mineral-lined fractures. The permeability and tortuosity of the fracture profile were determined from simulations of fluid flow through these geometries with impermeable fracture walls.
We used a combination of the level-set-method-based progressive-quasistatic algorithm (LSMPQS software), and Lattice Boltzmann simulation to characterize the capillary-dominated displacement properties and the relative permeability of the naturally cemented fractures. Finally, we numerically investigated the effect of increasing cement layer thickness on the fracture permeability as well as the tortuosity of the pore space and the capillary pressure-water saturation (Pc-Sw) relationship.
Pore space tortuosity and capillary pressure as a function of water saturation both increase with the numerically simulated increase in thickness of the fracture cement layer. Even when there are no cement contact points or bridges in the original fracture volume, numerical fracture cementation creates unevenly distributed apertures and cement contact points. This in turns causes the wetting and non-wetting fluids to impede each other, with no consistent trends in relative permeability with increasing saturation.