Combinations of surfactant-polymer (SP) and alkali-surfactant-polymer (ASP) have proven, in the laboratory and the field, to be good methods for enhanced oil recovery (EOR). However, field implementation of these methods has not been widely materialized because of economics, especially at the recent crude oil prices.
A more economical EOR alternative is surfactant-enhanced waterflood (SEW), achieved by injecting a small amount of an appropriate surfactant with the injected water. This method could be applied in reservoirs containing medium and light crude oil (less than 5 cP viscosity at reservoir temperature) that are currently under waterflood, and has the economic advantage of minimal additional CAPEX investment for surface facilities.
The main target of this method is to mobilize residual oil trapped at pore throats after an extensive waterflood. Increasing the Ca improves the efficiency of crude oil displacement from injectors to producers.
The workflow of surfactant formulation selection involves a comprehensive sequence of studies that cover:
Selection of candidate surfactants based on reservoir fluids properties and reservoir data conditions,
Winsor systems phase behavior studies, IFT measurement,
Evaluation of surfactant adsorption, and
Evaluation of the formulation in porous media (sandpack and coreflood tests).
Parameters involved in the phase behavior studies included a broad range of salinity, hardness, temperature, surfactants ratio in blends, and the equivalent carbon number (EACN) of the evaluated crude oils. After screening and selecting formulations, waterflood simulation with the surfactant formulation is required to estimate the recovery factor, the production profile, and the economics of the project.
Optimal surfactant formulations determined by phase behavior studies using surfactants-brine-crude oil systems were obtained for selected reservoir conditions (temperature, injection water salinity, specific crude oils, and rock properties). These custom-made formulations were Winsor III microemulsions with very high oil solubilization, and an IFT equal to or lower than 10−3 mN/m.
Evaluation of the selected formulations at low surfactant concentrations (around 0.15% in injection water) in sandpack and coreflood tests resulted in approximately 30% additional oil recovery. The results demonstrate that a high percentage of additional oil could be recovered after waterflood by adding a small concentration of a properly selected surfactant formulation to the water injected into cores saturated with light crude oil.
This paper discusses some aspects of the experimental work involved in various phases of the process for selecting surfactant formulations that meet the requirements of ultra-low IFT that result in a high capillary number (Ca) and high recovery of low-viscosity residual medium and/or light crude oil left behind in reservoirs that have been produced by waterflood.