A pore-scale study of foam-microemulsion interaction during low tension gas flooding using microfluidics- secondary recovery

The most commonly used methods for secondary oil recovery have been waterflood and immiscible gas flood. However, these methods leave behind substantial amount of residual oil. Past laboratory scale Low-Tension-Gas (LTG) coreflooding experiments have shown secondary oil recovery up to 80% OOIP (Original Oil in Place) for low permeability (sub 10mD) reservoirs. LTG flooding combines the well-known concepts of Surfactant EOR, where a surfactant solution is injected to lower oil-water IFT to ultra-low values (10(-3) dyne/cm), and Foam EOR, where foam is generated in-situ by the co-injection of gas to provide mobility control to the oil displacement. While past LTG coreflooding experiments for secondary recovery have demonstrated promising oil recovery, the exact pore-scale mechanisms involved during secondary LTG flooding are yet to be studied. The most important questions to be addressed are the oil-displacement mechanisms in equilibrated and non-equilibrated microemulsion systems (Winsor Type I and Type III), the impact of foam on the microemulsion equilibration, and the overall oil recovery value and rate as functions of these factors. The study was conducted in a borosilicate waterwet micromodel with a porous network reflective of actual rock pore network. Alternate slugs of microemulsion/surfactant solution and gas were injected into the micromodel to re-create secondary LTG flooding on the pore-scale. Unique oil displacement flow mechanisms were identified for Type I and Type III microemulsion systems. Oil displacement flow mechanisms also varied depending on whether the injected liquid was equilibrated microemulsion or non-equilibrated surfactant solution. Oil-in-water macroemulsions were characteristic of non-equilibrated Type I systems, and water-entrained macroemulsions were characteristic of non-equilibrated Type III systems. Foam generated in-situ was observed to improve microemulsion equilibrium rate and oil recovery rates for secondary LTG flooding.