Increasing oil production is necessary due to the rise in energy demand and the lack of competitive alternative energy sources. Enhanced oil recovery (EOR) technology aims to retrieve the oil trapped in reservoirs after conventional methods have been exhausted. Many EOR processes are available based on thermal, gaseous, and chemical approaches. One of the most successful EOR methods is polymer flooding, where polymers are dissolved in flooding water to increase sweep efficiency, reduce viscous fingering, and produce more oil. Despite some progress that has been made in polymer EOR, many drawbacks hinder their wide applications due to polymer degradation and instability in harsh reservoir conditions such as high temperature and high salinity (HT-HS).
This project aims to improve the stability, rheology, and flooding performance of polymers in HT-HS conditions by two approaches, i.e., reinforcing polymer chains with desirable nanoparticles to form nanocomposites and engineering new temperature-resistant polymers. In addition, a novel synthesis approach to produce suitable nanoparticles in-situ in polymers is developed to further improve the polymer performance. Newly formed materials were carefully characterised, and their stability and rheological properties under HT-HS conditions were investigated, as well as their EOR performance in a pilot core-flooding facility.
The composites of partially hydrolysed polyacrylamide (HPAM), seeded with different nanoparticles (NPs), i.e., graphene oxide (GO) and carbon quantum dots (CQDs) was prepared using direct mechanical mixing approach but appears to be unstable in American Petroleum Institute (API) brine and complex formation brine (FB) solutions. Five different modified acrylamide copolymers (i.e., polymers synthesized from two different monomers) and terpolymers (i.e., polymers synthesized from three different monomers) are produced via free-radical polymerization. Among them, the polyampholytic terpolymer and polyelectrolyte copolymer containing negative sulfonate groups were found to be stable when multiwall carbon nanotubes (MWCNTs) were introduced in the presence of API brine, but not in the FB solution.
Consequently, a novel modification and in-situ synthesis approach were used to alleviate the instability challenge. Whereby covalent functionalization of the copolymer of acrylamide (COPAM) with the partially reduced GO (rGO) was successfully conducted, followed by the addition of 1,3-Propane sultone to further functionalize the obtained rGO−COPAM composites to accomplish the zwitterionic character on the rGO−COPAM surface with excellent stability. Similarly, SiO2 NPs were modified using (3-aminopropyl) triethoxysilane (M_SiO2) to create positively charged active groups that enabled stronger interaction with COPAM functional groups. These functionalisation’s and in-situ synthesis approach led to the formation of highly stable composites with excellent dispersibility and temperature stability, as well as salinity tolerance.
The HPAM rheological properties improve with the addition of GO but behaved differently when CQDs were added. The addition of GO significantly increased the viscosities and high-temperature stability of the base polymer fluid, as well as the elastic properties of the dispersion, due to the formation of covalent linkage and electrostatic hydrogen bond between GO and HPAM functional groups. Contrarily, CQDs addition into HPAM solution decreased both its viscosity and elasticity, demonstrating a phenomenon, which contradicts to the Einstein–Batchelor law, resulting in increased flow activation energy and decreased yield point. The solution became more sensitive to both temperature and shear rate. The mechanism behind the reduced viscosity behaviour of HPAM/CQDs composites appears to be the formation of free radicals and the elimination of ammonia molecules, leading to the deterioration of the polymer backbone. In addition, the viscosity of polyampholytic terpolymer and polyelectrolyte copolymer containing negative sulfonate groups increase in the presence of MWCNTs in both alkaline and saline conditions. The M_SiO2/COPAM and SiO2/COPAM composites synthesised in-situ via free radical polymerisation showed improved rheological properties compared to pure polymer solutions after 90 days aging at 80 °C.
The pilot core-flooding experiments showed that zwitterionic-rGO-COPAM composite showed more oil recovery enhancement compared to HPAM and HPAM/GO in both API and complex FB conditions, respectively, with even lower pressure drop. Similarly, M_SiO2/COPAM and SiO2/COPAM showed better recovery performance than NP-free COPAM solutions, and the addition of MWCNTs to polyampholytic terpolymer and polyelectrolyte polymers improved the oil recovery in both alkaline and API brine conditions with a lower pressure drop.
Conclusively the work demonstrated that the use of appropriate nanoparticles can reinforce the stability, rheology, and flooding performance of polymers in HT-HS conditions. Among various approaches investigated, nanocomposites synthesised via in-situ produced NPs in polymers showed better performance than those prepared via a direct mixture of polymer/NPs solutions, showing great promise for future EOR applications.
