Fractures in Shale: a Study on the Roughness of the Vein-Rock Interface vs. Open Fractures

Abstract:

Gas recovery from methane-bearing shale rocks receives currently a lot of attention. The permeability of these rocks is controlled by induced and/or pre-existing fractures, where flow and thus gas recovery is partly controlled by the roughness of fracture walls. In this study, we determine fracture roughness and geometry both of veins and decompression fractures of the Pomeronian shale from Poland, recovered from ~4 km depth. Since vein formation occurs at depth, the topography of the vein-rock interface is preserved even when samples are extracted from the subsurface. Here, we have imaged a series of shale sample in 3D using X-ray microtomography performed on a laboratory tomograph and on the beamline ID19 at the European Synchrotron Radiation Facility. Several voxel spatial sizes were used, in the range 0.2 to 20 micrometers, allowing a multi-scale analysis of these rocks. We also compare fracture and veins roughness values from tomography scans to results obtained using white light interferometry, where these different measurements provides us with information on how scale, resolution and method affect roughness values. The low resolution CT scans show a small but clear difference between the roughness of the vein-rock interface and decompression fractures, implying fracture origin is important in controlling final roughness. The first results on scans made at different resolution furthermore indicate that higher resolution shows higher roughness values, which implies that low resolution tomography alone cannot capture the complexity of these fine-grained rocks. The results from this study provide input for (reactive) flow models, to study how efficient gas recovery can be.