Potentially Conductive Channels in Fracture Cements of Low Permeability Rocks

Abstract:

Natural fractures in shale reservoirs are frequently filled with mineral cement that lacks any residual fracture porosity that is visible under the petrographic microscope. These fully cemented fractures are generally interpreted to be impermeable for fluid flow. Scanning electron microscopy of calcite, dolomite, and barite fracture cements from a variety of shale reservoirs, prepared using broad ion beam milling, provides evidence of open to partially healed elongate pores that are on the order of hundreds of nanometers in aperture. In calcite fracture cement, these pores have consistent apertures of about 100 nm. In dolomite and barite, apertures are up to 500 nm. These pores have been previously overlooked because traditional thin sectioning and polishing destroys sub-micron details of the fracture cement pore structure. Ion milling preserves these details with a minimum of sample damage during sample preparation. Electron backscatter diffraction shows that these pores occur along grain boundaries within the blocky or columnar fracture cement. While partially healed, these pores, arranged along grain boundaries, are frequently sufficiently well connected acting as channels for fluid flow along and across fully cemented natural fractures. In shale reservoirs of ultra-low matrix permeability, these grain boundary channels may thus provide fracture permeability significantly contributing to reservoir production where intersected or indirectly reactivated by hydraulic fractures.