Chemical enhanced-oil recovery has been applied successfully in reservoirs with mild salinity and temperature conditions. Offshore reservoirs challenge chemical flooding, e.g. low-tension and foam flooding, because of the combined hardness and salinity of seawater along with characteristics of the reservoir connate brine. These physico-chemical conditions impose severe limitations to adequate phase behavior for most commercial surfactants. The purpose of the research described herein is to analyze surfactant phase behavior for scenarios with seawater as the main carrying fluid for surfactant systems.
Thermal stability and solubility were analyzed for a number of surfactants provided by two companies. Tests were conducted using a range of simulated connate brines and seawater compositions. Critical Micelle Concentration (CMC) values were estimated using a Nuclear Magnetic Resonance (NMR) protocol developed in our laboratory, recently published (Garcia-Olvera et al., 2016). NMR data were taken from solvents and co-solvents to analyze individual components in chemical blends. Phase behavior experiments were run to determine which blend composition and brine salinity would enable the desired phase behavior. A medium-gravity oil thoroughly studied in our lab was selected for phase behavior studies. Finally, a coreflood was conducted on a subset of surfactants to evaluate the additional oil recovery for one of the evaluated scenarios.
Some surfactants were disregarded for further analysis, due to their lack of thermal stability (dropout from solution was observed). In some cases, co-solvents were added to increase solubility in surfactant blends with a high divalent ion content, which in some cases was insufficient to stabilize the blends. Phase behavior experiments show that some surfactants did not yield Type III microemulsions, and therefore were disregarded for low-tension flooding applications. NMR data for surfactants and co-surfactants yielded good results even when the NMR spectra for different blend components overlaid significantly. Softening of high-salinity brines indeed improved micro-emulsion volume in the phase behavior tests, as demonstrated in the coreflood. However, this strategy is shown through geochemical analysis to be risky for more reactive lithologies, where dissolution and precipitation events are prompted by reduction in hardness in the injection brine.
Our use of an advanced spectroscopic technique (NMR) provides a more quantitative way of determining constraints associated with solubility limits and poor phase behavior under physico-chemical conditions of harsher environments. Geochemical considerations are important for mitigation strategies in reactive lithologies.