Understanding Flow in Unconventional Reservoirs Fractures: Influence of Compaction and Cementation

Abstract:

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 and permeability.

In order to accurately predict fluid transport in unconventional reservoirs, we study the influence of diagenesis, numerical cement and fracture roughness on flow in three different fractures: a carbonate outcrop from the Niobrara formation; and two distinct sandstones, from a core from the Travis Peak and an outcrop from the Torridonian. We use x-ray microtomography imaging to provide 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. We 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, absolute permeability and relative permeability of the naturally cemented fractures within. In addition, we 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 (P_c-S_w) relationship in the Niobrara. Finally, we create 3D prints of the fractures for visualization purposes.

Permeability estimates in the studied fractures vary by several orders of magnitude when computed with the different correlations that currently exist in the literature. The presence of cements increases the geometric tortuosity of the pore space and capillary pressure while reducing the permeability. Contrary to our expectation, the tortuosity of the wetting and non-wetting phases and their respective relative permeabilities show no clear correlation.