A key component to success with low-permeability, organic-rich rock resource plays is targeting rocks that contain natural fractures or that can be effectively hydraulically stimulated. Hardness and brittleness are key mechanical properties for rock types that are susceptible to fracturing. Typically, these rock properties are measured by sonic wireline logs or through triaxial tests on rock core. Core-based rock measurements are costly and therefore usually provide limited data sets. Sonic-log-derived properties have limited bed resolution, which is a significant issue in many shale reservoirs where rock properties can vary significantly over small stratigraphic intervals. An alternate method using hand-held devices such as rebound hammers has recently been shown to produce reliable, reproducible rock strength results that correlate to laboratory and logging techniques. The advantage of hand-held equipment is significant cost savings and the ability to take measurements at a single-bed scale (less than 4 cm2 [0.62 in2]), thus more data can be gathered within a particular rock unit.

A low-energy rebound hammer was used in this study to characterize rock strength (confined compressive strength [CCS]) in Niobrara and Frontier Formation cores from the Piceance Basin in Colorado. Leeb strength values that were obtained from the rebound hammer were converted to unconfined compressive strength (UCS) values using the method described by Zahm and Enderlin, 2010. UCS values were then converted to CCS using an empirically derived correlation of UCS and CCS from Core Lab's North America Shale Consortia data set. The results show a strong correlation between the laboratory-measured triaxial CCS and the micro-rebound hammer CCS measurements when plotted against depth. Furthermore, there is a good correlation between the log-derived Young's modulus, bulk modulus and Poisson's ratio with the micro-rebound hammer data. This study shows that hand-held rock-strength tools such as the micro-rebound hammer serve as an economical and reliable option for assessing rock brittleness and hardness, thereby providing critical mechanical properties for resource play targets.

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