Thermal spallation of dry rocks induced by flame parallel or normal to layering: Effect of anisotropy

Thermal spallation drilling is a prospective technique in many strategic projects involving energy. To investigate the effect of rock anisotropy on thermal spallation and provide reference for thermal rock-drilling methods, we conducted flame jet experiments on dry shale samples and finite element modelling. Thermal spallation is produced by the growth of pre-existed cracks under compressive thermal stress. These cracks separate thin layers from the bulk of the rock; the layers buckle under compressive stress producing the spalls. Simultaneously, tensile fractures are formed that can inhibit the thermal spallation process. Rock anisotropy can change the thermal spallation zone making it larger (over seven times) or smaller compared to the isotropic case, or even exclude spallation. Comparison with experimentally observed spallation zone in shale samples with different bedding orientations suggests that bedding planes do not induce anisotropy sufficient to affect thermal spallation. Strong anisotropy of rocks can generate thermal tensile fractures directly from the heating surface, the situation impossible in isotropic rocks. During progressive thermal spallation rock anisotropy can make the spallation zone larger or remain unchanged. The results can help understanding thermal spallation of rocks and are instrumental in designing thermal spallation-based drilling technique for strongly anisotropic rocks.