Permeability of calcite-cemented fractures in mudrocks: Flow highway or hindrance?

Abstract:

Among natural fractures observed in core of Eagle Ford Shale, Texas, tall sub-vertical calcite-cemented fractures are the most spatially extensive. Similar sub-vertical opening-mode fractures are found in many mudrocks, and far more often than not they are completely cemented. Currently there is very little evidence that these calcite cements are anything but impermeable, and thus would hinder flow across the fractures while having an insignificant effect on flow parallel to the fractures. This suggests that natural fractures in mudrocks are barriers to flow, which goes against the general consensus that natural fractures in mudrocks enhance flow. We used scanning-electron microscopy (SEM) on broad-beam argon-ion milled samples of a calcite-cemented fracture with a kinematic aperture (width) of 200 µm to study the pore space of the calcite for any indication that completely cemented fractures are permeable. In the fracture calcite cement, we observed primary porosity between calcite grains that is 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 50 to 200 µD. These flow-paths have a spacing between 50 and 300 µm, therefore a square centimeter (length*height) of fracture cement will contain on average more than 100 flow-paths. Thus for flow across the fracture cement (orthogonal to the fracture) the overall cross-fracture permeability of the cement can be approximated using simple effective medium upscaling. The fracture cement studied here is found to have a cross-fracture permeability in the range of 25 to 100 nD. Although this is a very low permeability, it is within the range of the permeability of the host rock, and thus 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 cement creating the equivalent of a single tortuous flow-path along the plane of the fracture with a possible permeability upwards of 10 to 50 µD. This is far lower than the permeability of the fracture without the cement (~1500 D), but in comparison to the host rock permeability (< 200 nD) would still act as a highway for fluid flow.