Quantifying Anisotropic Viscoelastic Parameters Using In-Situ Pressuremeter Testing in Shale

Shale exhibits a certain degree of anisotropy in its deformation properties. This anisotropy is important to consider in rock engineering problems. Pressuremeter (or dilatometer) testing, as an in-situ method to determine geomechanical parameters, has revealed its potential in characterizing this anisotropy with the borehole deformation measured by calipers oriented at multiple diametric axes. In this work, a transversely isotropic viscoelastic model is proposed to predict the anisotropic borehole deformation for pressuremeter testing in the boreholes drilled perpendicular and parallel to the bedding of the shale. The determination of the model parameters is demonstrated using the data obtained from two pressuremeter test cases in Opalinus Clay, a shale with strong anisotropy. The elastic parameters of the model are determined with the model fit on the data from the unloading step while the stiffness relaxation parameters are determined using the model fit on the borehole creep deformation measured during the pressure hold step. The stiffness relaxation parameters determined from the two test cases are considerably different, implying the altered viscoelastic properties in the local borehole damage zone induced in the test case when the borehole is drilled parallel to bedding. 1. INTRODUCTION Shales play a critical role in various subsurface engineering projects, including petroleum extraction, geological sequestration of CO2, and construction of underground gas storage and radioactive waste disposal facilities. Accurate measurement of their geomechanical properties is important for the successful design and implementation of these projects. A key aspect of shale behavior under subsurface conditions is its tendency to undergo creep deformation when subjected to loading or unloading (Sone & Zoback, 2013). The principle of linear viscoelasticity has been used to model the creep of shale (Trzeciak et al., 2018). Due to its fine-grained texture and laminated structure, shale also exhibits a certain degree of anisotropy in its geomechanical properties. This anisotropy is important to consider in predicting and managing the behavior of shale formations in various engineering applications such as wellbore stability and caprock integrity assessment.