An important aspect of exploration and development of unconventional reservoirs is understanding controls on the acoustic properties in organic rich shales to improve interpretation of prospective reservoirs through seismic. Dynamic elastic properties derived from velocities on a continuous interval can be used to target ideal facies for a more effective frac design (i.e. total organic content, thermal maturity and elastic properties). Studies that have examined organic rich shale properties at multiple scales have successfully correlated kerogen content and thermal maturity with acoustic velocities and anisotropy. However, discrete acoustic velocity variations associated with kerogen type and thermal maturity remain unclear.
A scanning acoustic microscope is used to measure high resolution acoustic velocities on discrete laminae of variable organic content, type and maturity in unconventional samples. We infer that for unconventional reservoirs, in which typical particle and pore sizes are substantially smaller than 20 microns (i.e. resolution of a 20MHz probe), the difference in travel time between the first arrivals from the top and bottom surfaces of the sample provides an accurate measure of the velocity. A Backus average of the measured velocities of each layer type agrees well with laboratory measurements made at the core plug scale.
Velocity measurements are integrated with micro-CT, thin section, XRF and SEM to identify presence and distribution of kerogen and mineral phases in the matrix (i.e. load bearing vs isolated). One-inch diameter core plugs are first micro-CT scanned and their acoustic properties are measured as received. After CT scanning, thin sections, acoustic microscope discs and SEM mounts are prepared. The end trim is ion milled in preparation for SEM and acoustic microscopy. Large area image mosaics are produced using low voltage SE imaging for characterizing porosity, and BSE imaging for characterization of organic content and mineralogy. Scanning CL imaging and image analysis are utilized to differentiate between detrital and authigenic phases. Energy dispersive x-ray mapping is also used for the identification of major mineral phases. The resulting suite of mosaic images are analyzed using UH-developed image analysis software. Segmented volumes of porosity, TOC, and mineral phases are determined for each layer type in the sample.
We illustrate the relationship between segmented porosity, TOC, and mineralogy on the acoustic properties of each layer type. Mineral phases included in the modeling are clay minerals, pyrite, carbonate, and quartz. We include, where possible, the differentiation of authigenic quartz and carbonate phases. Velocities for each layer type are mapped to the microCT data for the core plug. We illustrate the technique applied in several highly heterogeneous formations including the Niobrara, Haynesville, Barnett, Woodford, and Eagle Ford.