CO2 enhanced oil recovery and storage using a gravity-enhanced process

CO2 flooding offers a means to recover significant amounts of oil while simultaneously sequestering CO2. Recent methods for CO2 geological storage have focused on CO2 injection into deep brine aquifers, or by water-alternating-gas (WAG) injection in a miscible gas flooding process using vertical wells. There is significant uncertainty in the amount of CO2 that can be stored using these methods owing to reservoir heterogeneity and variations in reservoir/fluid parameters. It would be useful therefore to have a more robust process that can also increase both CO2 storage and oil recovery in a symbiotic relationship, where increased storage leads to greater oil recovery. This paper considers an alternative process that maximizes both storage and oil recovery simultaneously using only horizontal wells in a gravity-enhanced miscible process. A reduced-order model (ROM) is developed to consider a wide range of reservoir heterogeneities and fluid properties. Monte-Carlo simulations using the ROM show that achieving very high storage and oil recovery is possible using the gravity-enhanced process and that the approach is very robust. For example, after 2.0 moveable pore volumes injected (MPVI), probabilistic forecasts show that CO2 storage efficiency across two standard deviations ranges from about 81% to 93%, indicating that nearly all of the available pore space (excluding immobile water) at the end of injection is occupied by CO2. Oil recoveries after 2.0 MPVI varied from 79% to 93% of the original mass of oil-in-place (OOIP). These storage and recovery efficiencies are significantly greater than any process reported to date. Response functions developed can also be used to estimate the maximum amount of stored CO2 and corresponding oil recoveries for a wide range of reservoir and fluid properties. Such estimates are critical for regional and national assessment of CO2 storage potential. (C) 2015 Elsevier Ltd. All rights reserved.