Surfactant-Polymer flooding is usually not applied in low permeability reservoirs mainly due to injectivity problems in the presence of mobility control agents. Indeed, in the case of surfactant-polymer injection in low permeability formation, one of the major problem is to avoid the plugging of the core due to the jamming of polymer molecules in small pores. Hence, a compromise between molecular weight, concentration and viscosity must be determined. Here we demonstrate the feasibility of a surfactant-polymer EOR process in a sandstone reservoir core with a permeability around 1 mD.
Coreflood experiments were carried out on a sandstone reservoir core using a formulation consisting of a low molecular weight HPAM polymer and a typical oil recovery surfactant, that improved i) the mobility control as well as ii) the interfacial tension decrease between oil and the aqueous phase. The studied system consisted in a 1 to 3mD permeability and 15% porosity sandstone reservoir core, with dodecane as model oil and a 64g/L NaCl brine at 60°C. Specific petrophysical protocols were used to avoid non-uniform saturation profiles at irreducible water saturation (Swi) generated by strong capillary effects that are typical of low permeability media. In addition to the usual mass and volume measurements, we also used the X-Ray CTscan imaging in order to obtain saturation profiles at various steps of the EOR process.
The injection of a low molecular weight polymer solution was possible without plugging the core by choosing the polymer concentration so as to increase the mobility ratio but remaining under the critical entanglement concentration. A first slug of surfactant-polymer solution was injected followed by a polymer postflush. Due to the high amount of clays in the low permeability rocks we used a salinity gradient in postflush to minimize the important adsorption of the surfactant during the injection of the formulation and increase the oil recovery of the process. Saturation measurements were done after each experimental step (100% brine, Swi, ROS and Sor) and revealed a good correlation between the different methods. Thanks to this strategy it was possible to decrease the residual oil saturation (Sor) by at least a factor 2, increasing oil recovery in the range of 40–60% ROIP, compared to standard waterflooding.
The novelty of this work is to use the combination of surfactant and polymer in very low permeability sandstone rocks.