Numerical simulation on characteristics and mechanisms of steam-assisted in-situ combustion in extra-heavy oil reservoirs

Extra-heavy oil reservoirs, especially post-steam injection, continue to challenge performance of in-situ combustion (ISC), resulting in problems such as poor effect and slow startup. A novel enhanced oil recovery process, steam-assisted in-situ combustion (SAISC), is proposed. Using the Hongqian 1 Block as a case study, the model analyzes and elucidates production performance, combustion characteristics, oil displacement mechanisms, and mutual coupling effects associated with SAISC. Heat relationship analysis confirms feasibility of SAISC, while its applicability hinges on viscosity and the steam-air ratio, with economic limit calculations further demonstrating technique's superiority. The results show that injected steam during the SAISC process absorbs retained heat in the rock of the combustion zone through heat exchange, transforming into superheated steam. Steam condensation is delayed, and the condensation zone is expanded by about 10 m. The water and oil phase velocities in the condensation zone are both increased by 33 %, resulting in a production increase of over 40 %. Steam is more suitable than water as the heat-carrying fluid for SAISC, with a recommended steam-air ratio of approximately 4.0. When the oil price exceeds 60 USD/bbl, developing with SAISC technology yields an economic surplus. This study provides insights into improving the development effect in similar reservoirs.