Elastic Anisotropy of Unconventional Shale at Reservoir Conditions - New, Faster Laboratory Characterization Technique

Abstract

Abstract Most unconventional shale formations are transversely isotropic with a vertical axis of symmetry. To fully characterize the elastic anisotropy of a transversely isotropic rock, five independent elastic constants are required. In the laboratory, this tranditionally translates to measuring the wave velocities on at least three separate adjacent core plugs with various orientations - one parallel, one perpendicular, and one at 45° to the bedding. However, it is difficult to drill multiple adjacent core plugs with good quality from a whole core of gas shale because of its weakness and brittleness. This paper presents a simpler, alternative technical lab procedure of characterizing the elastic anisotropy of transversely isotropic rocks from one horizontal core plug only. The plug is cut parallel to the bedding of the whole core. In the test, it is jacketed in a core holder specifically designed to simultaneously measure six wave velocities at each pressure step: axial compressional, two axial shear, radial compressional and two radial shear. By adjusting the relative position of the sample bedding to the radial velocity transducers, variations of wave velocities with angles is determined; thereafter, the five elastic constants and three Thomsen parameters are calculated. For validation, one synthetic rock sample was constructed comprising thin layers of various materials. Three plugs were cut adjacently from the artificial rock in various orientations: one parallel, one perpendicular, and one at 45° to the bedding. The ultrasonic velocity measurements showed that elastic constants derived from the horizontal plug match very well with those from three plugs, confirming that only one horizontal core plug is necessary to analyze the elastic anisotropy of the transversely isotropic media. The method's feasibility and applicability was demonstrated by performing the elastic anisotropy analysis at room temperature and various confining pressures on two real rock samples: one tight sand plug and one unconventional shale plug. Both plugs are cut horizontally and exhibit characteristics of layers/beddings or a horizontal zone filled with different minerals. For the tight sand plug, wave velocities measured along the axial direction are in good agreement with those from the radial direction. However, the similar phenomenon was not observed for the shale plug. We analyzed the discrepancy using the micro-CT imaging technique. The three-dimensional geometrical structure of the plug was obtained, and the difference was found to be caused by the non-parallelism of the zone to the symmetry axis of the plug.