Contrasting mechanical evolution of bakken shale under hydrous and anhydrous pyrolysis: A nanoindentation-based assessment

The Bakken Formation is one of the largest unconventional shale oil reserves, where hydraulic fracturing and horizontal drilling are key methods for enhancing recovery. A thorough understanding of its mechanical properties is crucial for designing these processes, especially since kerogen within the shale undergoes chemical changes during thermal evolution that alter its mechanical behavior. This communication primarily concentrates on evaluating how the mechanical behavior of Bakken shale, rich in Type II kerogen, evolves during controlled thermal maturation under anhydrous and hydrous pyrolysis conditions. The immature samples are pyrolyzed at 300, 325, 350, 365, 400, and 450 °C. To extensively investigate the mechanical characteristics of shale samples, a sum of 775 nanoindentation tests is performed on their surface. The results indicated that during the AHP, the initial increase of Young's modulus of the samples to more than 34 GPa was followed by a relatively large decline and it starts fluctuating between 20 and 25 GPa from 350 to 450 °C. The hardness value during the AHP, on the other hand, increased to almost 0.77 GPa at 300 °C and then started a steady decrease to approximately 0.25 GPa at 365 °C. It then experienced an uprise to 0.39 GPa at 400 °C and finally, fell to 0.32 GPa in the last temperature step. During the HP, the trends were quite different, which reflects the impact of the presence of water on kerogen maturation. The hardness values reduced constantly during the HP process reaching approximately 0.15 GPa at 450 °C. Young's modulus, on the other hand, showed a reduction to 21.6 GPa at 300 °C and then increased to almost 25 GPa at 325 °C. This increase was followed by a large decrease to nearly 11 GPa at 365 °C. Then, after a small increase to more than 12 GPa at 400 °C, it fell to 10.5 GPa at 450 °C. The existence of water during hydrous pyrolysis enhances thermal cracking phenomenon. This leads to the generation of more oil and bitumen and making the kerogen more porous. This was considered the main cause of the lower mechanical properties of HP samples. The deconvolution approach also showed that the distribution curves of the mechanical properties of samples were a combination of three different mechanical phases. This study provides new insights into the micromechanical evolution of shale during thermal maturation by explicitly distinguishing the effects of pyrolysis on kerogen-controlled mechanical behavior which establishes a direct link between mechanical properties and maturation pathways. This is basis for predicting changes in stiffness, brittleness, and deformation response in organic-rich shales in optimizing hydraulic fracturing strategies and improving geomechanical modelling of unconventional reservoirs.