Effective mobility control mechanisms for EOR processes in challenging carbonate reservoirs

Mobility control mechanisms are key to the success of any enhanced oil recovery processes due to their ability to provide favorable mobility ratio of the injected fluids, thus improving the sweep efficiency during the process. This work is focused on developing effective mobility control mechanisms in challenging carbonate reservoirs that are typically high temperature and high salinity and low permeability formations. The first half of the dissertation is focused on investigating novel foam technology using anionic and cationic surfactants to improve the gas enhanced oil recovery process. Typically, gas injection processes suffer from poor volumetric sweep efficiency due to viscous fingering, channeling, and gravity override. Foam helps to improve the sweep efficiency of the gas floods significantly by reducing the mobility of the gas by orders of magnitude, blocking the high permeability channels and diverting fluids to the bypassed lower permeability channels. Carbonate reservoirs, which are typically oil-wet heterogeneous and low permeability, pose additional challenges for an effective foam EOR process. Crude oils destabilize foam rapidly and the thin oil film on oil-wet rock surfaces makes in-situ foam generation difficult as well. Hence, wettability alteration from oil-wet to water-wet using a surfactant was one of the necessary mechanisms for in-situ foam stability. Low permeability of the carbonates makes strong foam generation challenging due to higher entry capillary pressure in small pore throats that exceeds the critical capillary pressures usually. On the other hand, low interfacial tensions (IFT) of the surfactant formulations helps to lower the entry pressure and stabilize the foam better. This work demonstrated the benefits of two different chemical systems – one that includes use of anionic surfactants for low IFT formulations and the other that includes blends of cationic, non-ionic and zwitterionic surfactants for non low IFT formulations in combination with wettability alteration and foaming to improve oil recovery in oil-wet carbonates after a secondary gas flood process. The second half of the dissertation is focused on developing a novel polymer treatment protocol for successful injection in low permeability carbonate reservoirs through mechanical shear degradation and aggressive filtration tests. The behavior of shear degradation of high molecular weight polymers of different chemistry in varying brine salinities performed with a laboratory blender at a constant speed and varying shearing times followed an exponential decay until a steady state was obtained. Master curves for degraded viscosity predictions were developed to estimate the degraded viscosity of any given polymer in any brine salinity at any given shearing time, given the shearing speed was kept constant. A superimposed master curve for the degradation for all kinds of polymers investigated was established to predict the rate of degradation at any given time. A robust approach of comparison of polymer size distribution from dynamic light scattering (DLS) method and pore throat distribution from mercury injection capillary pressure (MICP) was established for injection qualification of high molecular weight polymers in low permeability carbonates. A novel class of hydrophobically modified acrylamides, also known as associative polymers, were investigated as an alternative to conventional HPAMs and synthetic polymers for injection in low permeability carbonates. The thermo-thickening properties of the associative polymers at elevated temperatures and salinities (with high divalent ions) and higher resistance to shear degradation makes them promising for carbonate reservoirs in comparison to HPAMs, where high polymer dosages are required due to significant viscosity loss in shear degradation. The apparent high viscosities generated from high resistance factors during flow in porous media for associative polymers can be advantageous for optimization of polymer dosage in chemical EOR processes. This work demonstrated a significant potential for application of associative polymers as an effective mobility control agent in carbonate reservoirs, especially in low permeability formations. The novel polymer treatment method for low permeability reservoirs was combined with the development of alkaline-surfactant-polymer (ASP) and surfactant-polymer (SP) technology for improvement of oil recovery in carbonates. The successful polymer transport in low permeability carbonates showed great potential for application of chemical EOR processes like ASP and SP in tight formations. Development of robust SP technology for high temperature and high salinity reservoirs also showed promising results in phase behavior experiments and coreflood experiments. This work demonstrated the benefits of SP technology with optimization of surfactant formulation and coreflood design for lower surfactant retention and higher oil recovery, thus making the process economical