A mechanistic model for anisotropic thermal strain behavior of soft mudrocks

Under drained heating, soft mudrocks can expand or contract depending on its mineralogy, composition, structure, stress history, and the imposed temperature. Previous research on the physics behind the thermally induced deformation behavior is limited. The impact of clay minerals on the overall anisotropic deformation behavior has not been quantitatively considered. In this study, a compositional thermal strain model is proposed to quantify the thermally induced deformation in soft mudrocks through a homogenization approach. The intrinsic fabric of soft mudrock was examined and considered in the model. Theoretically, thermally induced deformation in a soft mudrock is contributed by the expansion of solid minerals and interlayer bound water, the removal or dehydration of clay-bound water, and thermal plastic strain (grain rearrangement). The overall deformation of soft mudrocks is governed by the thermal deformation behavior of individual constituents and their interactions. The interactions among non-clay minerals and clay-water composites can be considered by applying a structural state coefficient. The proposed thermal strain model was validated by a series of experimental results using reconstituted and natural soft mudrock samples with different clay fractions. The results indicate that soft mudrocks with a structural state of clay matrix-supported are on the risk of having thermal contraction behavior which comes from clay dehydration or thermal plastic strain. The oriented fabric in soft mudrocks contributes to the anisotropy in thermal strains. The proposed thermal strain model can be applied to estimate critical parameters in phenomenon-based thermal elastic-plastic models for constitutive modeling.