Tight oil formations such as the Bakken system have complex micropore fabric which often causes difficulties in characterizing physical rock properties. These rocks demonstrate anisotropic features that may affect the direction of fluid flow during depletion of the reservoir. The main goals of this study are to develop a multi-physics understanding of not only the reservoir section but also the shale member, while incorporating anisotropic behavior and correlating pressure-dependent geophysical characteristics to geological attributes.
In this study, four samples were collected from the Middle Bakken and Lower Bakken Shale Formation for multi-physical rock measurements that include induced polarization (IP) and acoustic waves. To capture anisotropic features of the rocks, we used a single core measurement technique with the ability to record simultaneous ultrasonic velocity and electrical conductivity from multiple directions, i.e., 0° (parallel to bedding), 45°, and 90° (perpendicular to bedding) at elevated confining stresses. We also obtained backscattered electron images and Micro Computed Tomography (μCT) images.
Based on the pressure-dependent ultrasonic velocity measurement results, we categorized the evolution of strain into two major deformation regimes: A (high strain) and B (low strain). The application of these regimes can be extended to other geophysical parameters that are related to pressure, such as Thomsen’s anisotropy parameter, real conductivity, imaginary conductivity, and anisotropy conductivity. Joint measurement of acoustic and IP results indicates that the properties and pore structure deformation in each formation behave differently under stress.
The successful results of this study can improve characterization of direction-dependent pore network, which can be used to monitor permeability anisotropy evolution as well as optimize stimulation planning. Multi-physics measurements to characterize a full petroleum system are rare, thus, the results presented in this paper can be very valuable for Bakken producers.
The study of geophysical anisotropy has gained increasing importance especially in producing hydraulically stimulated reservoirs. Transverse isotropic (TI) rocks, such as tight rocks and organic-rich shales, are known to exhibit stress dependent textural anisotropy (Vernik and Nur, 1992; Sayers, 1999) due to its laminated structure. The effort to understand anisotropic features in these rocks can improve characterization of directional mineral and pore morphology evolution when effective stress of the reservoir is changing (Allan et al., 2015). This structure deformation is measured, and the results can be tied to permeability anisotropy evolution, as well as predicting direction of fracture propagation, which affect directional fluid flow during depletion period.