AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Geomechanical, Petrophysical and Rock Physics Characterisation of a Global Suite of Shales: Results and Learnings
(1) Earth Science and Resource Engineering, CSIRO, Kensington, WA, Australia.
(2) Ian Wark Research Institute, University of South Australia, Adelaide, SA, Australia.
(3) Land and Water, CSIRO, Adelaide, SA, Australia.
(4) Earth Science and Resource Engineering, CSIRO, Clayton, VIC, Australia.
Preserved shale samples were collected from locations around the world, including the North Sea and the Australian Margin, which ranged in age from Proterozoic to Tertiary. An extensive experimental programme was undertaken to evaluate geomechanical, rock physics, petrophysical and geomechanical properties of those shales and to establish links between them.
A combination of high field and low field nuclear magnetic resonance were used together with conventional interfacial tension determinations to evaluate wettability of shales. Natural shales show significant variation in surface affinity for oil versus water dependent on mineralogical composition. Hydrophilic shales have a higher cation exchange capacity (CEC), such that illitic and smectitic mudrocks are more hydrophilic whereas kaolinitic mudrocks are potentially hydrophobic, being wetted preferentially by oil and retaining that tendency after exposure to brines. Trends in conductivity and dielectric constant of shale powders treated with oil and brines are consistent with these observations.
Porosity and cation exchange capacity correlate well with strength properties and dielectric constant measurements on intact shales and pastes made from powdered shales show strong relationships between high frequency electrical properties, mineralogy and CEC, and mechanical strength. Dynamic elastic properties were strongly related to both imposed stress conditions and the orientation of the maximum principal stress in relation to shale microfabric. Anisotropy of velocity was significantly impacted by stress orientation as well as the presence of preferred orientation of particles and the presence of laminations.
One well from the Officer Basin in Western Australia was fully cored. Preserved shales from this well when experimentally deformed in the laboratory show stiffening of static and dynamic elastic properties with depth of burial and in addition both strength and friction also increase. While shales are known to compact during burial, this is the first time that absolute values of stiffness and strength and their change during burial have been physically documented in the laboratory. These changes are attributed to both mechanical compaction and diagenesis.