Alteration in the mechanical properties of the Bakken during exposure to supercritical CO2

Carbon neutrality, a balance between emitting and removing carbon from the atmosphere, has become a global aspiration that has resulted in significant scientific advancements. The effective long term geological storage of CO2 as one of the efficient ways to achieve such goal requires a deep understanding of interactions between CO2 and geologic formations. In this study, a sample from the Bakken Formation in North Dakota, which is a target layer for both enhanced oil recovery (EOR) and storage of CO2, was incubated for 3, 8, 16, 30 and 60 days. Then, mineral assemblages and mechanical properties including fracture toughness and contact creep modulus were assessed following each reaction time using XRD analysis and the nanoindentation technique, respectively. Results showed that fracture toughness variation of all three mechanical phases that were recognized based on the force-displacement curves, exhibited an N-shape: an increase (from 1.48 to 2.43 MPa m0.5 after 8 days) proceeding by a decrease (1.26 MPa m0.5after 16 days) and then an increase (2.17 MPa m0.5 after 60 days). Similarly, contact creep modulus of these three different mechanical phases showed the similar N-shape variation pattern. Furthermore, the p-value of the t-Test of fracture toughness and the contact creep modulus values was found less than 0.05, verifying that these two mechanical parameters were truly affected as a result of exposure to ScCO2. These alterations in mechanical properties were attributed to mineral evolution, microstrain, and microstructural alterations which were observed in electron micrographs from the sample after 60 days. Collectively, findings from this study can enable us to predict physico-chemical response of the shale formations that simultaneously produce hydrocarbons, should undergo hydraulic fracturing, EOR and will become future CO2 storage sites.