Single Plug Protocols

One popular example is our single plug protocol for geomechanical characterization in anisotropic rocks. Guided by the high-resolution core scanning, core plugs are taken via smart sampling to sample key rocktypes and cover the range of properties sampled by the core. Core plug testing is performed with NER AutoLab systems to provide geomechanical properties at reservoir conditions. The AutoLab testing suite utilizes a very efficient single-plug workflow that relies primarily on a single horizontal plug from each depth. This plug is used to measure both static and dynamic stiffness tensors and anisotropic Biot poroelastic coefficients.

Static Moduli: The testing for static moduli is performed on horizontal plugs taken from locations identified by the high-resolution scanning. Analysis of the data is performed assuming that the axis of VTI symmetry was known based on visual inspection of fabrics within each sample.  NER has developed a method to measure 4 of the 5 independent static moduli for a single plug with the plug axis perpendicular to the symmetry axis. The static elastic moduli are measured by fitting the stress vs. strain response to a sequence of loading cycles including four different stress trajectories (3B). The protocol consists of a sequence of confining and axial load perturbations from a starting effective stress state typically chosen to be the net mean stress of interest. The measured strain data are fit to the equations described in graphic C below to obtain the best fitting compliance tensor Sij. The Sij’s are then converted to stiffnesses and engineering elastic constants.

Biot Coefficients: Biot coefficients tests are performed as a continuation of the static moduli tests without removing the sample from the apparatus. The Biot portion of each test is performed by introducing a helium driven pore pressure to the sample. Both pore and confining pressure are cycled in a specific sequence. The Biot parameters are determined by fitting the measured equilibrium condition pressure vs. strain responses to the poroelastic model.

Dynamic Moduli: Dynamic moduli are determined by measuring ultrasonic velocities during a separate set of tests on the same plug.

For dynamic moduli tests, compressional and 2 polarized shear velocities were measured at each of three orientations (0, 45, and 90 degrees to bedding) at the specified net confining pressure. A schematic diagram of the experimental setup is shown. The phase velocities as a function of propagation direction and measured sample densities are used to constrain a VTI elastic stiffness tensors for each sample.