Investigating Water Trapping Effects During Tertiary Gas Injection in Mixed-Wet Rocks

Abstract

Abstract Gas trapping induced oil recovery has been explored in the past and has known to have occurred particularly in water-wet systems during tertiary gas injection. As the gas is the most non-wetting phase in a three-phase system (water-wet), the gas notably has the highest capacity to get trapped within the pore space. However, trapping of the intermediate wetting phase (i.e., water in a mixed-wet system during three-phase flow) is also possible and can be quite substantial under certain circumstances. In this work, we have carried out three displacement experiments on mixed-wet Berea rocks utilizing X-ray microtomography to assist visualization of pore scale mechanisms and fluid occupancies in order to illuminate the potential trapping of the intermediate wetting phase during tertiary gas injection. The experiments began with waterflooding followed by gasflooding and another waterflood on mm-scale sandstone cores. Mixed-wet rocks were aged with crude oil for 40 days and were initialized with different connate water saturation ranging between 0.15 to 0.18 fraction. The oil-water contact angle measured manually during waterflooding ranged around 100 to 130 degrees. During gasflooding in mixed-wet rocks, water occupied all pore sizes, instigated by the double displacement mechanism of gas displacing oil, that in turn displaces water. We also observed a significant water trapping in large pores, leading to the gas invading the pores via squeezing in between oil and water phases. Water trapping also increases the oil recovery. Hence, an engineering design to optimize the water trapping during tertiary gas injection will significantly stimulate the incremental oil recovery in field development projects. The outcome of this research will influence the method to model the hysteresis of water relative permeability for mixed-wet rocks.