Pressure-dependent elastic anisotropy: A Bakken petroleum system case study

Abstract

The complexity of the micropore fabric and organic texture in tight unconventional reservoirs often complicates their correlations with physical rock property parameters. Anisotropy, which is a common physical property of tight oil reservoirs, can affect directional deformation and permeability during the reservoir development and pressure depletion period. Thus, accurately measuring seismic anisotropy and its relationships with petrophysical properties can refine our understanding of vertical transverse isotropy rocks and ultimately improve fracturing design as well as production forecasting. We have developed a measurement technique of ultrasonic velocities (P and S) from multiple directions, that is, bedding-parallel, bedding-perpendicular, and at 45° to bedding under simulated in situ reservoir pressure. Following these measurements, we also correlate these findings to pressure-dependent porosity and permeability, in addition to geologic attributes and textural anisotropy. The samples that we investigate are tight carbonaceous siltstone and organic-rich mudrock cores originating from the Bakken petroleum system, which include Lodgepole, upper Bakken shale, middle Bakken, lower Bakken shale, and Three Forks. Depending on the rock fabric and mineral constituents, each formation exhibits distinctive anisotropic behavior in response to pressure changes. The results indicate that the elastic parameters and petrophysical properties are inversely related under pressure for all samples. We also find that P-wave anisotropy and SH-wave anisotropy parameters decrease gradually with increasing confining pressure. The anisotropic behavior can be related to volumetric concentration and directional connectivity of clay layers, kerogen veins, and subparallel microfractures.