In Enhanced oil recovery methods, CO2 is an excellent injecting fluid than water because of its prominent properties which includes non-flammability, non-polarity and non-toxicity. The reservoir pressure is over minimum miscibility pressure (MMP), it also provides for miscible displacement process of multi-contact. The CO2 injection process has been widely practiced in oil industry in the last decades. In the fractured or heterogeneous reservoirs, the injected gas will mostly passage through the fractures or high permeability layers which present the minimum flow resistance, leaving much of crude oil unrecovered in the low permeability rock-matrix.

In this research, a composite polymer gel was proposed for harsh reservoir conditions, such as high salinity (NaCl 15.0 wt%, Ca2+ 0.8wt% and Mg2+ 0.8wt %), acid conditions (pH 3.25) and analyzed systematically at laboratory scale, consisting of polymer, cross linker and nanoparticles (silica dioxide). The gel system was characterized by different parameters like viscosity (μ), loss modulus (G″) and elastic modulus (G′) by varying polymer concentration, cross-linker concentration, silica dioxide (SiO2) concentration by using macro-rheology. The micro-rheology was used to calculate mean square displacement (MSD) and elasticity index (EI) of composite gel system. The bottle testing method was used for characterizing the gelation time of composite, gelation code and synergistic mechanism.

The macro-rheological results of composite polymer gel proved that elastic modulus (G′) and loss modulus (G″) of composite gel system tends to increase with increment in silica dioxide (SiO2), polymer, cross linker and salinity concentrations. The results of G′ and G″ showed that composite gel have good elasticity at high salinity and acidic conditions. The gelation time observed was 6 days, which is sufficient to pump the gel into targeted zone. The gel code obtained was G at the temperature of 60°C. The elasticity index (EI) in micro-rheological results showed that gel has good elasticity, and MSD curve showed gelling fluid is elastic fluid and bears good stability. The composite polymer gel bears very low syneresis in acid conditions for 60 days at the temperature of 60°C, and having good thermal stability.

The laboratory scale research indicates that newly formulated composite polymer gel can be efficiently applied to heterogeneous reservoirs for harsh reservoir conditions and CO2 shut-off problems, and can decrease the water or gas breakthrough problems by plugging the highly permeable channels and increasing the sweep efficiency from low permeable reservoir to produce the residual oil and ultimately increase the production of oil.

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