Modeling Study on Reviving Abandoned Oil Reservoirs by In Situ Combustion Without CO₂ Production While Recovering Both Oil and Heat

A large number of oil wells have been or will be abandoned around the world. Yet, a very large amount of oil and energy is left behind inside the rocks in abandoned reservoirs because of technological and economic limitations. The residual oil saturation is usually more than 40%, and in shale reservoirs it can be more than 90%. There have been many enhanced oil recovery methods developed to tap the residual oil and improve the oil recovery. Interestingly, a concept has been proposed to transfer abandoned oil and gas reservoirs into exceptional enhanced geothermal reservoirs by oxidizing the residual oil with injected air (Li and Zhang, 2008, “Exceptional Enhanced Geothermal Systems From Oil and Gas Reservoirs,” 43rd Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA). This methodology was referred to as an exceptional enhanced geothermal system (EEGS). However, zero CO₂ production has not been achieved during the process of EEGS. To this end, numerical models of EEGS in abandoned oil reservoirs configured with vertical wells were established in the present study. Numerical simulations in different well configurations were conducted. The effects of well distance, perforation position, and formation permeability on the CO₂ production and the reservoir temperature have been investigated. The numerical simulation results showed that when the depth difference between the production and the injection well perforation positions reaches a specific value, the daily CO₂ production rate could be kept at almost zero for over 50 years or even permanently while producing oil and thermal energy continuously. This implies that we realized the concept of EEGS with no CO₂ successfully using numerical simulation.