Low pH and Salinity Induced Wettability Modification of Hybrid EOR as Polymer-Assisted Carbonated Low Salinity Waterflood in Calcite Cemented Sandstone Reservoir

Hybrid polymer-assisted carbonated low salinity waterflood (PCLSWF) modifies both oil and brine viscosities and wettability in calcite-cemented sandstone reservoirs. In core- and pilot-scaled systems, PCLSWF increases ion-exchange of Ca²⁺ modifying reservoir wettability toward to water-wet condition. In the process, dissolved CO₂ in brine is transferred into oil. The interphase transport reduces both oil viscosity and density. As well, the dissolved CO₂ in brine reacts with water and produces H⁺ in brine. This low pH of PCLSWF dissolves calcite minerals and provides Ca²⁺ into brine resulting in increased ion-exchange of Ca²⁺. Due to significant mineral reaction rate in pilot-scaled system, enhanced wettability modification by mineral dissolution is observed in pilot-scaled system rather than core-scaled system. In addition, PCLSWF increases the viscosity of displacing fluid. Though it undergoes mechanical degradation and slight chemical degradation, it is still effective to improve mobility. The improved sweep efficiency of PCLSWF allows injecting fluid to flow in less permeable area. In heterogeneous pilot-scaled reservoir, the combined effects of PCLSWF enhance oil recovery up to 12%, 6%, and 2% over low salinity waterflood (LSWF), low salinity polymer flood (LSPF), and carbonated low salinity waterflood (CLSWF). Optimization process maximizes the efficiency of PCLSWF and has recommended to deploy PCLSWF with CLSWF preflush. The optimized injection design of PCLSWF increases NPV up to 18% more than LSWF.