Determining the transverse isotropic rocks’ static elastic moduli with cylindrical plugs: Shortfalls, challenges, and expected outcomes

Abstract

Cylindrical-shaped plugs can be tested using a Hoek-cell-like apparatus that allows for efficient and inexpensive measurements of a rock’s static elastic properties. However, when it comes to transverse isotropic material, this approach has a natural limitation due to the isotropic radial stresses; particular attention to the boundary conditions and the proper design of pressurization steps is warranted. Typical attempts to constrain the complete set of compliances ([Formula: see text]), using multiple plugs of different orientations, are impeded by the heterogeneity and pressure-dependent elasticities inherent to sedimentary rocks. Through stepwise pressure increases, we can constrain four normal compliances [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text], describing two Young’s moduli and three Poison’s ratios, using a single horizontal plug drilled parallel to the rock’s isotropic plane, contrary to the common assumption that both horizontal and vertical plugs are needed. The measurement of the shear modulus [Formula: see text] needs to be obtained using a plug that is drilled oblique to the isotropic plane; replicating the in situ stress environment is not possible using this approach. Finally, the specimen’s anisotropic plane’s geometry is elliptical under isotropic radial stress; this causes a discrepancy between the strain gauge’s contraction and the actual strain. We have developed an iterative inversion approach to account for this issue and calculate the exact strains useful for inferring [Formula: see text] from measurements reported by strain gauges. The example included in this writing indicates that, without correction, inferred values of [Formula: see text] may suffer errors of 20%.