Interpretation of Differential Pressure in Laboratory Surfactant/Polymer Displacements

Experimental data were obtained for two surfactant/polymer systems in which fluid mobilities and mobility control were studied through the analysis of differential pressure measurements. One system used nonane as the hydrocarbon phase, while the other system used crude oil from the Madison field, Greenwood County, KS. During the experimental studies, capillary-pressure effects were observed in differential pressure data when floods were conducted at reservoir rates and small port spacing. Capillary-pressure effects interfered with the measurement of the "true" or viscous pressure drop in oil/water banks and the transition region between the oil/water bank and surfactant slug. Pressure ports were designed to permit injection of small quantities of fluid at the rock surface during permit injection of small quantities of fluid at the rock surface during displacements to verify and to eliminate capillary-pressure effects. Movement of fluid regions through the core was inferred from the differential pressure measurements along the core. Similar pressure perturbations at the ports were extrapolated to the end of the core and perturbations at the ports were extrapolated to the end of the core and correlated with effluent fractions. In the nonane system, the microemulsion surfactant slug changed to a more mobile macroemulsion within the first half of the displacement. This macroemulsion moved through the last half of the core as a stabilized, constant velocity bank indicating good mobility control even though the apparent viscosity of the macroemulsion was less than that of the stabilized oil/water bank. In the crude-oil surfactant system, pressure data collected along the core indicated capillary-pressure effects at the leading edge of the oil bank and the formation of a viscous, less mobile region resulting from mixing of the microemulsion slug with resident fluids. Phase-behavior studies indicated the formation of a viscous lower phase as the microemulsion was diluted with brine. The formation and propagation of this viscous region during displacement leads to favorable mobility control illustrated by the pressure/mobility curves as well as by good oil recoveries (80 to 95%). pressure/mobility curves as well as by good oil recoveries (80 to 95%). Introduction Mobility control is a necessary requirement for an effective surfactant flood. Mobility control is achieved when the displacing fluids' mobilities are less than or equal to the displaced fluids' mobilities. In a typical surfactant/polymer flood, the mobility of the stabilized oil/water bank must be greater than the surfactant slug's mobility, which in turn must be greater than the mobility of the trailing polymer bank. When these criteria are met, mixing between the different fluid banks will be minimized. This allows the surfactant slug to move as a stabilized bank, contacting a larger percentage of the reservoir. A procedure for selecting the mobility for the surfactant slug and polymer bank was presented by Gogarty et al. They obtain a design mobility polymer bank was presented by Gogarty et al. They obtain a design mobility of the stabilized oil/water bank generated ahead of a surfactant slug from one of the following two methods. 1. Total relative mobilities as a function of water saturations are calculated from relative-permeability curves. The minimum total mobility is selected as a safe value of the design mobility. Chang et al., pointed out that decreasing water-saturation (drainage for water-wet rocks) curves should be used. SPEJ p. 791