Summary
We present and analyze strongly anisotropic compressional and shear velocities in formations of interest to model the elastic response of anisotropic rocks in an unconventional reservoir. Although we recognize that there can be many possible sources of anisotropy, we focus on three sources: mineral orientation, layering, and microcracks.
Introduction
We collected a wide variety of data over multiple scales. The data range from well logs at meter scale to digital images at nanometer resolution. In between, we have ultrasonic velocity measurements at the centimeter scale, core-HD data at the millimeter scale, and X-ray synchrotron data that reflect crystal orientatin of grains with a wide range of sizes.
Data
The data we collected to estimate and model the elastic anisotropy is in descending order of the scale of measurement. The well logs in Figure 1 are from a vertical well. The sonic, shear, and density logs (in blue) span two zones of interest, Formations A and B. Ultrasonic core measurements from Samples A and B that correspond to the respective formations, show fairly good agreement with the log data in the bedding-perpendicular direction. The well logs measure lower shear-wave anisotropy than the core in the shallower formation.
Figures 2 and 3 show compressional and shear velocity measurements, plotted as a function of differential pressure.
The pore pressure was zero in the experiment. All five velocity measurements were made on a single horizontal plug from Formation A. We observe in Figure 2 that the velocities are anisotropic and stress sensitive. The same velocity versus pressure measurements were also made on a single horizontal core-plug from Formation B, and show similar charactersitics.
