The basic micellar-polymer process uses a surfactant to lower the interfacial tension between the injected fluid and the reservoir oil. A surfactant is a surface-active agent that contains a hydrophobic (“dislikes” water) part to the molecule and a hydrophilic (“likes” water) part. The surfactant migrates to the interface between the oil and water phases and helps make the two phases more miscible. Interfacial tensions can be reduced from ~30 dynes/cm, found in typical waterflooding applications, to 10–4 dynes/cm, with the addition of as little as 0.1 wt% to 5.0 wt% surfactant to water-oil systems. Soaps and detergents used in the cleaning industry are surfactants. The same principles involved in washing soiled linen or greasy hands are used in “washing” residual oil off rock formations. As the interfacial tension between an oil phase and a water phase is reduced, the capacity of the aqueous phase to displace the trapped oil phase from the pores of the rock matrix increases. The reduction of interfacial tension results in a shifting of the relative permeability curves so that the oil will flow more readily at lower oil saturations.
When surfactants are mixed above a critical saturation in a water-oil system, the result is a stable mixture called a micellar solution. The micellar solution is made up of structures called microemulsions, which are homogeneous, transparent, and stable to phase separation. They can exist in several shapes, depending on the concentrations of surfactant, oil, water, and other constituents. Spherical microemulsions have typical size ranges from 10–6 to 10–4 mm. A microemulsion consists of external and internal phases sandwiched around one or more layers of surfactant molecules. The external phase can be either aqueous or hydrocarbon in nature, as can the internal phase.
Solutions of microemulsions are known by several other names, including surfactant solutions, soluble oils, and micellar solutions. Figure 11.5 can be used to represent the micellar-polymer process. A certain volume of the micellar or surfactant solution, fluid A, is injected into the reservoir. The surfactant solution is then followed by a polymer solution, fluid B, to provide a mobility buffer between the surfactant solution and the drive water, which is used to push the entire system through the reservoir. The polymer solution is designed to prevent viscous fingering of the drive water through the surfactant solution as it starts to build up an oil bank ahead of it. As the surfactant solution moves through the reservoir, surfactant molecules are retained on the rock surface due to the process of adsorption. Often a preflush is injected ahead of the surfactant to precondition the reservoir and reduce the loss of surfactants to adsorption. This preflush contains sacrificial agents such as sodium tripolyphosphate.
There are, in general, two types of micellar-polymer processes. The first uses a low-concentration surfactant solution (less than 2.5 wt%) but a large injected volume (up to 50% pore volume). The second involves a high-concentration surfactant solution (5 wt% to 12 wt%) and a small injected volume (5% to 15% pore volume). Either type of process has the potential of achieving low interfacial tensions with a wide variety of brine crude oil systems.
Whether the low-concentration or the high-concentration system is selected, the system is made up of several components. The multicomponent facet leads to an optimization problem, since many different combinations could be chosen. Because of this, a detailed laboratory screening procedure is usually undertaken. The screening procedure typically involves three types of tests: (1) phase behavior studies, (2) interfacial tension studies, and (3) oil displacement studies.
Phase behavior studies are typically conducted in small (up to 100 ml) vials in order to determine what type, if any, of microemulsion is formed with a given micellar-crude oil system. The salinity of the micellar solution is usually varied around the salt concentration of the field brine where the process will be applied. Besides the microemulsion type, other factors examined could be oil uptake into the microemulsion, ease with which the oil and aqueous phases mix, viscosity of the microemulsion, and phase stability of the microemulsion.
Interfacial tension studies are conducted with various concentrations of micellar solution components to determine optimal concentration ranges. Measurements are usually made with the spinning drop, pendent drop, or the sessile drop techniques.
The oil displacement studies are the final step in the screening procedure. They are usually conducted in two or more types of porous media. Often initial screening experiments are conducted in unconsolidated sand packs and then in Berea sandstone. The last step in the sequence is to conduct the oil displacement experiments in actual cored samples of reservoir rock. Frequently, actual core samples are placed end to end in order to obtain a core of reasonable length, since the individual core samples are typically only 5–7 in. long.
If the oil recoveries from the oil displacement tests warrant further study of the process, the next step is usually a small field pilot study involving anywhere from 1 to 10 acres.
The micellar-polymer process has been applied in several projects. The results have not been very encouraging. The process has demonstrated that it can be a technical success, but the economics of the process has been either marginal or poor in nearly every application.19,20 As the price of oil increases, the micellar-polymer process will become more attractive.
Leave a Reply