Mechanical response of the Middle Bakken rocks under triaxial compressive test and nanoindentation

The laboratory characterization of tight rocks is essential for reliable rock physics analysis. In this study, combined laboratory experiments and computational calculations were applied to specimens from the Bakken Formation to examine the mechanical properties of the unconventional petroleum resource in different scales. Multistage compressive tests were conducted on core plugs with several loading-unloading cycles under different confining pressures. Microscale mechanical test, nanoindentation, were performed to measure microscale elastic properties. Additionally, a comparative study was included using homogenization upscaling methods to model the modulus of the macro-scale sample from nanoindentation measurements and rock mineralogy, respectively. Results from the triaxial compressive tests showed nonlinear behavior for all samples and pronounced plastic deformation was observed during loading-unloading cycles when lower confining pressures were employed. Young's modulus increased with confining pressures increasing. The ultrasonic P- and S-wave velocities are sensitive to the changes of confining and differential stresses, which might be due to the open and closure of microcracks. Three mechanical phases can be found from nanoindentation results by deconvolution, and the average microscale Young's modulus obtained from nanoindentation tests showed a positive relationship with the theoretical Voigt and Reuss boundaries. The prediction of Young's modulus from mineral fractionand moduli information indicated a better agreement with laboratory data than the nanoindentation cluster dataset.