Lithologic Controls on Shale Rheology: A Lookback on 7 Years of Mechanical Testing

Abstract

In 2010 ConocoPhillips Technology Structure and Geomechanics group initiated a research program named “Lithologic Controls on Shale Rheology”, with the main goals to (1) investigate the applicability of the elastic assumption for reservoir geomechanical analyses, and (2) to better model mechanical stratigraphy along the wellbore. We soon determined that we first needed to overhaul existing mechanical testing practices and design our own quality-controlled systematic protocol (Amendt et al., 2013) that could be repeated for all incoming core. The focus was on obtaining reliable static-dynamic transforms, multi-plug triaxial testing for Mohr Coulomb parameters, and co-located lithologic characterization. Seven years later, we have systematically processed 35 cored wells with mechanical testing at over 500 discrete depth points, co-located with >2300 single-stage triaxial tests, >800 Brazil tensile tests, >300 XRD and poro/perm analyses, and >12,000 ultrasonic velocity measurements. Here we highlight our top five takeaways from this scientific endeavor.

1. Co-location and full integration of robust mechanical data with lithologic and petrophysical data is critical to move beyond simplistic geomechanical idealizations.

2. There is no universally valid empirical formula linking lithologic constituents (e.g., quartz, clay, or porosity content) to mechanical properties, whether stiffness or strength/failure.

3. Static and dynamic Young’s modulus are well-constrained and empirically relatable, whereas Poisson’s ratio is less reliable.

4. Non-linear strain-hardening and softening, corresponding to brittle inelastic and plastic deformation mechanisms are predominant in mudstones and “shale”.

5. We are now fully convinced that elastic “Brittleness”, as commonly as invoked in industry to screen for favorability to hydraulic fracturing is, at least in a strictly mechanical context, dubious at best.

In addition to these key technical findings, we have noticed that despite our ability to acquire robust laboratory mechanical properties and understand realistic rheological behaviors, most mainstream industry workflows are still quite rudimentary in terms of rock mechanics integration. It remains a question how closely tied the success of unconventional plays is to uptake of the fundamental geomechanics research.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018