Abstract: The in-depth understanding of stress evolution and failure mechanism of the mudstone layer, in a thermal recovery environment, is helpful for scientifically and effectively solving the problem of cap rock cracking and interlayer blocking in the process of steam injection for heavy oil reservoirs. Firstly, the heat transfer and pressurization mechanism of the mudstone layer, in the reservoir thermal recovery process, are studied quantitatively, based on theory. The different stress evolution trends and failure modes of the mudstone layer are analyzed under variable temperature and pressure conditions. Then, using the cap rock fracture event as an example, which occurred in one of the thermal recovery projects in Canada, the heat-water-stress coupling numerical simulation of mudstone cap rock is performed. Thereby, the failure mechanism and key influencing factors of the mudstone layer, in the thermal recovery environment, are revealed. The results show that the stress evolution of the mudstone layer, in the thermal recovery environment, is mainly affected by heat-induced pressurization. Pore pressure is much higher than injection pressure and the peak value is located near the temperature corresponding to the maximum thermal pressure coefficient of pore water. The rock layers adjacent to the peak of the pore pressure are possibly subjected to tensile and shear damage. The development of a reasonable steam injection scheme, for the thermal recovery of reservoirs, must take into account the heat-water-stress coupling effect of the mudstone strata.

Key words: thermal recovery, mudstone, heat-water-stress coupling, failure mechanism, heavy oil reservoir injection