--> Assessing Controls of the Woodford Shale Rock Strength at the Bed Scale, Ardmore Basin, Oklahoma

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Assessing Controls of the Woodford Shale Rock Strength at the Bed Scale, Ardmore Basin, Oklahoma


Despite the significant role that rock strength plays on the development of natural and induced fractures in self-sourced reservoirs, its estimation and study remain limited to the availability of well cores and its high cost to acquire. This work presents a comprehensive study of the rock strength of a Woodford Shale outcrop in Oklahoma, which not only analyzes this mechanical property as individual numbers, but also addresses multi-scale factors that affect this measurement, such as rock fabric, lithofacies, organic content and porosity. The exposed succession studied is about 320 feet thick, comprising the entire Woodford Shale. Lithologically it consists of dark-colored, organic-rich and fine-grained strata, arranged in interbeddings of cherty and shaly beds, and some scattered dolomitic beds. Around 500 samples were collected (two samples per foot), recording the typical hard/soft interlayering of this unit. A micro-rebound hammer was used to evaluate the rock strength, which was calibrated with uniaxial compressive strength lab tests. In addition analytical techniques such as X-Ray Diffraction (XRD), X-Ray Fluorescence (XRF), Organic richness (TOC), Porosity, bulk density, petrography (Thin sections), and Scanning Electron Microscopy (SEM), were conducted to assess the factors controlling the mechanical behavior of the Woodford shale. Overall, rock hardness confirms the brittle/ductile behavior of this stratigraphic succession. However, small-scale characteristics reveal that mineralogical composition is not the only factor affecting the rock strength, the rock fabric, organic content and porosity may also affect the rock strength. For example, beds with similar mineralogical composition, but with subtle changes in grain size, laminations, structure and porosity present variations in hardness/rock strength. From these observations, we hypothesize that at the bed scale: 1) smaller grain sizes, abundant laminations, higher porosities, and higher organic richness contribute to decreasing the hardness/strength of a rock. 2), coarser grains, structureless beds, lower porosities and lower organic richness are factors correlating with an increase in hardness/strength values.