Assessing the Effect of Thermal Maturation on Shale Organic Matter via PeakForce Quantitative Nanomechanical Mapping

Abstract

Organic-rich shales have gained significant attention in recent years due to their pivotal role in unconventional hydrocarbon production. These shale rocks undergo thermal maturation processes that alter their mechanical properties, making their study essential for subsurface operations. However, characterizing the mechanical properties of organic-rich shale is often challenging due to its multiscale nature and complex composition. This work aims to bridge that knowledge gap to fully understand the nanomechanical properties of Shale organic matter at various thermal maturation stages. This study employs PeakForce Quantitative Nanomechanical Mapping (PF-QNM) using Atomic Force Microscopy (AFM) to investigate how changes at the immature, early mature, and peak mature stages impact the mechanical properties of the Bakken Shale organic matter. PF-QNM provides reliable mechanical measurements, allowing for the quantification and qualification of shale constituents' elastic modulus (E). We also accounted for the effect of probe type and further analyzed the impact of probe wear on the nanomechanical properties of shale organic matter. In immature shale, the average elastic modulus of organic matter is approximately 6 GPa, whereas in early mature and peak mature shale, it decreases to 5.5 GPa and 3.8 GPa, respectively. Results reveal a mechanical degradation with increasing thermal maturation, as evidenced by a reduction in the Young's modulus (E). Specifically, the immature shale exhibits an 8% reduction in E, while the early mature and peak mature shales experience more substantial reductions of 31% and 37%, respectively. This phenomenon could be attributed to the surface probing of low-modulus materials like bitumen generated during heating. The findings underscore the potential of AFM PF-QNM for assessing the nanomechanical characteristics of complex and heterogeneous rocks like shales. However, it also highlights the need for standardized measurement practices, considering the diverse components in these rocks and their different elastic moduli.