Unconventionals just keep getting more, well, unconventional. That seems to be the finding of a paper given at the recent Unconventional Resources Technology Conference in Denver.

The paper, authored by Gang Han of Aramco Services Co., Peter Schmitz of Hess Corp., and Marte Gutierrez of the Colorado School of Mines, outlined a study that was meant to determine whether or not standard hydrostatic testing conditions can reveal true rock properties under in situ nonhydrostatic stress environments. This, according to Han, “challenges the lab status quo and raises the necessity to establish new standards for unconventional core tests.”

While in situ rock is under different overburden and horizontal stresses, challenges in sample preparation and lab setup limit most unconventional tests to hydrostatic and isotropic stress conditions.

By using triaxial stress conditions, loading stress was increased from 2,500 psi to 10,500 psi while maintaining the confining stress at 2,500 psi. It was noted that once the differential stress passed 3,160 psi, the rock plastic alteration accelerated significantly. Scanning electron microscope (SEM) measurements indicated that this was likely dominated by the deformation of inter-particle cementation and pore-filling minerals as opposed to the rock matrix itself. As confining stress increased, post-yield stresses were extended significantly beyond failure, and the rock was transient from brittle to ductile. This challenges the current definition of rock brittleness, which often overlooks stress effect.

When the study investigated heterogeneity, four samples were taken at the same depth with angles of 0°, 30°, 60°, and 90° to the bedding planes. The variations of both strength and modulus were as high as 10 times in magnitude and dominated by the bedding planes. This has implications for hydraulic fracturing design and well planning.

In the acoustic and anisotropy measurements, the authors noted that the shear (S)-wave velocities might be more sensitive to rock fabric changes during yield and development of shear-induced damage. Most interestingly, it appeared that after all three Thomsen coefficients increased briefly at the low differential stress, the anisotropies of both compressional-wave and S-wave velocities generally reduced with the stress increase, indicating rock anisotropy increased before it became more homogeneous. Even though it was not related to the mechanical properties, the rock flow properties might potentially relate to acoustic anisotropy.

Overall the authors concluded that the unconventional rock properties were strongly dependent on nonisotropic stress environments and should be measured under triaxial stress conditions.

“The core data are much more realistic and show significant impact on reserve estimates, production profiles, and well construction,” Han said.

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