Thermally induced deformation in soft mudrocks may pose severe issues to thermal projects such as thermal heavy oil recovery and radioactive waste disposal. 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 behaviour is limited. The impact of clay minerals on the overall 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. 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 proposed thermal strain model is validated by a series of experimental results using natural and laboratory prepared soft mudrock samples. The results indicate that clay fraction is an important factor affecting the expansion and contraction behaviors. High clay fraction soft mudrocks 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.
Thermally induced deformations in saturated clay soils or mudrocks result from the thermal expansion of solid minerals, rearrangement of the material skeleton structure, and pore fluid behavior (Baldi et al. 1988; Campanella and Mitchell 1968). Phenomenon-based theoretical models have been proposed by researchers using thermal elastic and plastic theories to account for reversible and irreversible strains, respectively (Cui et al. 2000; Hueckel and Borsetto 1990; Laloui and Cekerevac 2003; Zhang et al. 2012). Such a phenomenon-based approach requires specific thermal tests to obtain model parameters, which depend on the clay fraction of mudrock. Large amount of clay minerals presented in soft mudrocks have contributed to their complex nano-scale (intra-particle) and micro-scale (inter-particle) pores (Li and Wong 2016). Soft mudrock samples retrieved from the same geological formation could have significant variations in clay fractions with burial depths. Variations in amount and type of clay minerals present in soft mudrocks will contribute to the differences in thermal strain behaviors. Previous research on the physics behind the thermally induced deformation behaviour is limited. In this study, the impact of clay minerals on the overall deformation behavior of soft mudrocks is investigated using a compositional thermal strain model.