Completely-Cemented Natural Fractures in Mudrocks: Flow Barrier or Highway?

Abstract

It has been postulated that hydraulic fractures reactivate natural fracture networks, resulting in greater access to the host rock and increased rates of production. However, many of these natural fractures are completely-cemented, and currently there is very little evidence that completely-cemented natural fractures are anything but impermeable, and thus would block production. Among natural fractures observed in core of Eagle Ford Shale, Texas, tall sub-vertical calcite-cemented fractures were the focus of this investigation. Similar sub-vertical, completely-cemented, opening-mode fractures are very common in mudrocks. We used SEM imaging on broad-ion-beam (BIB)-milled samples of a calcite-cemented fracture to study the microstructure of the calcite for any indication that completely cemented fractures are permeable. In the fracture calcite cement, we observed open flow-paths between calcite grains that are generally well-connected with an average aperture between ~25 and ~100 nm. The permeability of these flow-paths was determined by lattice Boltzmann methods to be between 60 to 660 μD. These flow-paths have a spacing between 200 and 400 μm; therefore, a square centimeter (length*height) of fracture cement will contain on average more than 500 flow-paths. Using simple effective medium upscaling the fracture cement studied here is found to have a permeability in the range of 30 to 630 nD. Although this is a very low permeability, it is within the range of the permeability of typical mudrocks; therefore, these calcite cements would have almost no effect on flow orthogonal to the plane of the fracture. These flow-paths are also connected within the calcite cement creating a flow-path network along the fracture. Although the flow-path network has a bulk permeability close to that of typical mudrocks, due to the much lower porosity of the calcite cement (< 0.05%) in comparison to the host rock (2% < typical mudrock porosity < 15%) the actual velocities in the flow-path network are much greater than the host rock. During production, the significantly greater actual velocities of flow in the cemented natural fracture results in the lower pressure of the wellbore-hydraulic-fracture being quickly translated into the natural fracture. This will effectively increase the hydraulic fracture/host rock interfacial area, and production rates. Therefore, completely-cemented natural fractures in mudrocks can act as flow highways.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015