51st U.S. Rock Mechanics/Geomechanics Symposium,
San Francisco, California, USA
2017. American Rock Mechanics Association
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ABSTRACT: We have investigated tectonic structures in several hundred meters of continuous core from five boreholes located in the Ordovician/Silurian shale formations of the Baltic Basin in Poland. Detail core examination of the natural tectonic structures and mechanically significant features comprised: mineral veins, fractured veins, bare fractures, slickensides, fault zones and bedding planes. All of our observations were compared with available, geophysical logging data. The most common tectonic structures are steep, rather small strata-bound fractures and veins with very low kinematic aperture. They create an orthogonal joint system locally disturbed by minor faults. For these regular joints, we have calculated average fracture intensity parameter by averaging fracture area per cubic meter of rock. We also constructed rose diagrams of fractures orientation weighted with height, aperture or percentage of cracked veins and also taking into account an orientation error. Obtained weighted diagrams enhance dominant sets of fractures and are more relevant for reservoir characterization than common diagrams considering only numbers of fractures. Finally, we compared results received from core with those from borehole scanner interpretation. This comparison reveals some interesting discrepancy between fracture profiles based on borehole scanner logs and direct borehole core profiles.
Natural discontinuities occurring in low permeable shale reservoir are important for borehole completion and hydrocarbon exploitation. Natural fractures and other tectonic structures control effective reservoir properties, such as: permeability and anisotropy (Hennings, 2009, Odling, 1992, Pijush et al., 2007), behavior of induced hydraulic fracture during fracturing treatment (Gale et al., 2006, Warpinski and Teufel, 1987) and size and shape of stimulated reservoir volume (Greenwood, 2015). Open minor- and moderate-scale penetrative fractures increase reservoir drainage and create additional space for free hydrocarbon accumulation. In contrast, large tectonic structures like seismic-scale faults can be harmful for exploitation, as they may capture fracking fluid, cause hydrocarbons escape or brine invasion into reservoir (Bratton et al., 2006).
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