USING XRF ANALYSES TO UNDERSTAND THE EFFECTS OF COMPOSITIONAL VARIATIONS ON ROCK STRENGTH DETERMINED FROM MICRO-MECHANICAL DEVICES

The exploration of unconventional resources and search for “sweet spots” requires understanding the geology and geologic variation of the formation of interest. Variations include stratigraphic and structural (including strength) variations, which affect the deformation behavior and may be important when it comes to the rock’s response to hydraulic fracture stimulation. The fine-grained nature of many unconventional plays complicates traditional stratigraphic and structural analyses. Also, the high cost and time-consuming nature of many chemical analyses and rock deformation experiments using triaxial load cells may present an obstacle to adequately evaluate significant variations.

We present data from three hand-held devices used to analyze a Barnett Shale core from the Fort Worth basin. We used energy-dispersive XRF to assess the chemical and mineralogical variation and a micro-indentation tool and micro-rebound hammer to evaluate strength variations. These tools are easy to use, quick, relatively inexpensive, portable and non-destructive and reveal mineralogical and rock strength changes throughout the core. Furthermore, we can assess how chemical variations affect rock strength in this rock.

The ~60 m cored interval was sampled every 30 cm, where we completed XRF analyses and rock strength test. Based on XRF analyses of both major and trace elements, the Barnett in this core is mostly a siliceous mudrock, although more quartz- and carbonate-dominated facies are locally present and overall the core can be broken into eleven chemostratigraphic facies. The unconfined compressive strength (UCS) based on the micro-indentation tool varies from ~38 to ~138 MPa with an average UCS of ~72 MPa. These values are similar to UCS values from the micro-rebound hammer. However, UCS results using the micro-rebound hammer and micro-indentation tools appear to vary systemically as mineralogy changes. While UCS values show a closer agreement in more siliceous facies, more calcareous facies appear to show greater divergence in UCS values, which suggests that the micro-rebound hammer is sensitive to the presence of carbonate material in the rock. Overall, our approach provides a quick, reliable assessment of the mineralogy and strength variations of the core and shows how the former can influence the latter.