MICRO-MECHANICAL ROCK STRENGTH TESTS AND THE EFFECTS OF SAMPLE VOLUME – AN EXAMPLE FROM THE BARNETT SHALE, FORT WORTH BASIN, TEXAS

We used two different micro-mechanical, hand-held devices to assess rock strength in the Barnett Shale in the Fort Worth basin, Texas. The advantage of these highly-portable tools over traditional triaxial load cell tests is that tests can be completed at very high resolutions and are generally non-destructive to the analyzed samples. Furthermore, the tools can be used on varying sample sizes from small rock cuttings circulated to the surface during a drilling operation to direct sampling on the face of sampled core or outcrop. We also completed high-resolution ED-XRF analyses to evaluate geochemical variations and assess which minerals might control rock strength.

Here, we present data collected from two cores from the Barnett Shale, which was one of the first successful unconventional resource plays. Data were collected using a micro-rebound hammer (Equotip Bambino 2) and a point load penetrometer. We sampled each core with both devices at a 7.5 cm sampling interval. Results from both devices are supplemented by ED-XRF data that was collected at the same interval and in the same locations. In addition, we calculated the volume of material that was analyzed.

Our detailed dataset provides insight into the chemical make-up and strength of the analyzed cores. Hierarchical cluster analysis of the XRF data resulted in the identification of seven different chemofacies in these two cores. Three of these chemofacies are present in both cores and two are unique to each core. Overall, some of the subtle chemical differences would not have been detectable by traditional rock description alone. In addition, the data from the micro-mechanical devices allows assessment of the unconfined compressive strength (UCS) variations in these rocks, which can be used to evaluate mechanical stratigraphy throughout the interval of interest. UCS derived from the micro-rebound hammer ranges from ~0.3-216 MPa. A similar range of UCS values is determined with the point load penetrometer. One important result is that the micro-rebound hammer appears to be sensitive to the analyzed rock volume and we suggest that a minimum volume of 85 cm³ is required to produce reliable and repeatable results. Similar volume-related problems are not observed in data obtained by the point load penetrometer.