Homogenization Models to Determine the Change in Elastic Properties Due to CO₂ Injection

In this study, the elastic modulus and hardness of the middle Bakken were characterized by the nanoindentation technique and homogenization models for its pre- and post-CO₂ saturation conditions. This was done based on the analyses of mineralogical compositions from X-Ray Diffraction data and microstructure by optical microscopy. The analyzed samples identified three distinct microstructural zones: the carbonate matrix, the composite matrix of clay, and the embedded quartz grains and small mineral grains. Nanoindentation test was used to determine the elastic modulus and hardness of the distinct zones for pre- and post-CO₂ saturation conditions. The spatial mappings and frequency distributions of the elastic modulus and hardness of these zones were obtained based on the statistical analyses of nanoindentation effects. From the nanoindentation tests, the elastic moduli of the key constituent phases of the sample were further estimated. Our values were also used to evaluate the macroscopic elastic modulus using two distinct homogenization models: the dilute scheme and the Mori-Tanaka models. The projected values from homogenization are correlated with experimental data obtained from traditional triaxial compression experiments. The dilute scheme result is closer to the traditional triaxial compression experiment than the Mori-Tanaka model. The results showed that CO₂ saturation could impact the elastic properties of the Bakken shale reservoir, which can affect a proposed development, exploration, and production designs of reservoir characterization.

The middle Bakken (MB) is located in the North Dakota portion of the Williston Basin. Like most unconventional reservoir rocks, the MB is a heterogeneous rock with complex microstructures, lithofacies, and mineralogical composition (LeFever et al., 2011; Sorensen et al., 2017). The evaluation of the mechanical properties of the MB and changes in its properties after CO₂ exposure, estimation of mineral volume fraction changes, and mineral composition are critical factors to consider because rock brittleness index increases with an increase in the brittle mineral content in a rock containing minerals like calcite (Schumann, 2008).