Woodford shale is emerging as one of the major gas formations in the US Midwest. Despite its tremendous potential, existing data on the Woodford shale geomechanics characterization are limited at best. In this work, a well in the Woodford shale formation, 200 feet deep, was cored and logged in Oklahoma, USA. The resulting retrieved preserved cores were lab tested using standard acoustic techniques and triaxial testing for shale mechanical and poromechanical characterization in terms of compressibility, strength, pore pressure coefficient, Young's modulus, Poisson's ratio, etc. In addition, shale mechanical parameters alteration when in contact with drilling muds and fracturing fluids were measured using Brazilian tests and the innovative Inclined Direct Shear Testing Device (IDSTD™). Finally, mechanical Woodford shale parameters were also measured and correlated with field log results, using samples a tiny as drill cuttings (a few millimeters in size) with the newly emerging nano-indentation rock characterization techniques developed in the GeoGenome™ Industry Consortium. This newly developed methodology for rock testing and shale chracterization, part of the nanotechnology wave, showed excellent results when compared with shale acoustic laboratory measurements and log data and results.
Despite a relatively high quartz content as shown by XRD and Elemental Capture Spectroscopy (ECS) log results, the Woodford shale does exhibit clear transversely isotropic mechanical characteristics, from Young's moduli to Poisson's ratios and other mechanical parameters. Moreover, IDSTD™ and Brazilian tensile tests on the preserved Woodford samples exposed to different drilling and hydraulic fracturing fluids showed that fluid effects play an important role on both compressive strength and tensile strength of the shale despite the fact that the Woodford clay content mainly composes of illite and chlorite.
The mechanical and poromechanical properties of Woodford shale were measured at four different scales, from field well logs to standard rock testing to the penny-size samples of IDSTD™ and down to drill cuttings scales using the nano-indentation. Furthermore, the innovative nano-indentation techniques for rock testing have allowed the construction of a GeoGenome™ simulation model which can estimate and determine macroscopic rock properties based on porosity, packing density, and mineralogy. The simulated moduli and parameters using this model showed excellent agreement when compared to both lab and field log results.