Stimulation characteristics of pore-fracture system in shale reservoirs under combustion-explosion loads

Combustion-explosion fracturing is a revolutionary technology developed in recent years for the fracturing of shale gas reservoirs. The research on the transformation effect of explosive impact load can provide a basis for the application of explosive fracturing. Shale samples from theLongmaxi Formation in the Luzhou Block, South Sichuan were selected. Specially designed equipment for combustion and explosion impact load was used to carry out blast impact test. A μ-CT scanning on the samples was conducted before and after the explosion. The pore space was extracted and reconstructed by digital image processing technique, and the pore size distribution was obtained by statistical pore data analysis to study the change characteristics of the pore fracture system before and after the impact load of combustion and explosion. The findings demonstrate that:(1) The thermal stress generated by instantaneous temperature rise destroys the laminated weak medium in the shale, causing destabilization, expansion and even penetration of the internal micro-fractures. Due to the effect of combustion explosion impact load, the central location of the shale stress concentration, and the impact of stress wave causes shale to form many cracks and a large number of pores.(2) The transformation of pore fracture system of shale reservoir under the action of combustion explosion impact loading is significantly affected by bedding and original micro-fractures. The more developed the bedding and the original micro fractures are, the more complex the fracture network developed after the transformation. With the reduction of plastic minerals such as clay minerals, the application range of reservoirs is wider.(3) Both the rise in instantaneous temperature and the combustion explosion impact loading significantly contribute to the formation and expansion of complex shale pore and fracture networks, where the combustion explosion impact loading is more effective in modifying the pore and fracture system of shale reservoirs, generating a large number of connected pore and fractures, and the connectivity pore increases by approximately 67%. This research demonstrates at the microscopic level that the effect of combustion-explosion impact loading can effectively modify shale reservoirs.