Surfactants for mobility control

Surfactants for Mobility Control. Water, which can have a mobihty up to 10 times that of oil, has been used to decrease the mobihty of gases and supercritical CO2 (mobihty on the order of 50 times that of oil) used in miscible flooding. Gas oil mobihty ratios, Af, can be calculated by the following (22) ... 

Early researchers sought to choose appropriate surfactants for mobility control from the hundreds or thousands that might be used, but very little of the technology base that they needed had yet been created. Since then, work on micellar/polymer flooding has established several phase properties that must be met by almost any EOR surfactant, regardless of the application. This list of properties includes a Krafft temperature that is below the reservoir temperature, even if the connate brine contains a high concentration of divalent ions (i.e., hardness tolerance), and a lower consolute solution temperature (cloud point) that is above the reservoir temperature. 

Both the use of one atmosphere foaming experiments and the technique of multiple correlation analysis have a common purpose minimizing the effort required to develop new surfactants for mobility control and other EOR applications. Proper use of these techniques with due consideration of their limitations can substantially reduce the number of experiments required to develop new surfactants or to understand the effect of surfactant chemical structure on physical properties and performance parameters. ... 

Apparatus and Procedure. It was necessary to design more definitive tests to further evaluate the better candidate surfactants. This was accomplished by means of a multi-phase dynamic-fiow test that consists of a small packed bed through which surfactant solution can be passed followed by gas to produce in situ foam. The pressure drop through the column is measured as the fiuid is drawn through the column at a constant volumetric fiow rate. From the recorded data, relative mobilities of the liquid and gas phases may be calculated. The change in gas mobility due to the presence of the surfactant is very closely related to the effectiveness of that surfactant for mobility control in oil core studies. A schematic drawing of the apparatus is shown in Figure 2. 

Polymers can be used for mobility control. The interaction between polymers and surfactants is shown to be affected by pH, ionic strength, crude oil type, and the properties of the polymers and surfactants [642]. 

Xanthan exhibits an interaction with anionic surfactants (petroleum sulfate), which is a beneficial synergistic effect for mobility control in chemical-enhanced oil-recovery processes [1115]. 

Coinjection of a low-concentration surfactant and a biopolymer, followed by a polymer buffer for mobility control, leads to reduced chemical consumption and high oil recovery. There may be synergistic effects between the surfactant and the polymer in a dynamic flood situation. The chromatographic separation of surfactant and polymer is important to obtain good oil recovery and low surfactant retention [1721],... 

When this pressure drops, it can be built-up again by water flooding. Unfortunately, after these primary and secondary processes, there still remains up to 70% of the oil adsorbed on the porous clays. Consequently, in recent years, there have been tremendous efforts made to develop tertiary oil recovery processes, namely carbon dioxide injection, steam flooding, surfactant flooding and the use of microemulsions. In this latter technique, illustrated in Fig. 1, the aim is to dissolve the oil into the microemulsion, then to displace this slug with a polymer solution, used for mobility control, and finally to recover the oil by water injection ( 1). 

Steps in the Development of Surfactant-Based Mobility Control. Although surfactant-based sweep and mobility control for gas flooding are still in the research stage, major advances have been made in several areas from which a pattern of past and probable future development can be inferred. In approximate historical order, the steps in this development include the following ... 

A key factor in the commercialization of surfactant-based mobility control will be the ability to create and control dispersions at distances far from the injection well (TJ ). Capillary snap-off is often considered to be the most important mechanism for dispersion formation, because it is the only mechanism that can form dispersions directly when none are present (39,40). The only alternative to snap-off is either leave-behind, or else injection of a dispersion, followed by adequate rates of thread breakup and division to maintain the injected lamellae. 

In actual use for mobility control studies, the network might first be filled with oil and surfactant solution to give a porous medium with well-defined distributions of the fluids in the medium. This step can be performed according to well-developed procedures from network and percolation theory for nondispersion flow. The novel feature in the model, however, would be the presence of equations from single-capillary theory to describe the formation of lamellae at nodes where tubes of different radii meet and their subsequent flow, splitting at other pore throats, and destruction by film drainage. The result should be equations that meaningfully describe the droplet size population and flow rates as a function of pressure (both absolute and differential across the medium). 

While mechanistic simulators, based on the population balance and other methods, are being developed, it is appropriate to test the abilities of conventional simulators to match data from laboratory mobility control experiments. The chapter by Claridge, Lescure, and Wang describes mobility control experiments (which use atmospheric pressure emulsions scaled to match miscible-C02 field conditions) and attempts to match the data with a widely used field simulator that does not contain specific mechanisms for surfactant-based mobility control. Chapter 21, by French, also describes experiments on emulsion flow, including experiments at elevated temperatures. 

Phase Behavior and Surfactant Design. As described above, dispersion-based mobility control requires capillary snap-off to form the "correct" type of dispersion dispersion type depends on which fluid wets the porous medium and surfactant adsorption can change wettability. This section outlines some of the reasons why this chain of dependencies leads, in turn, to the need for detailed phase studies. The importance of phase diagrams for the development of surfactant-based mobility control is suggested by the complex phase behavior of systems that have been studied for high-capillary number EOR (78-82), and this importance is confirmed by high-pressure studies reported elsewhere in this book (Chapters 4 and 5). 

These two branches, simulator development and materials selection, can then come together in well-engineered designs for field use of surfactant-based mobility control in gas flooding. 

Structure—Property Relationships for Mobility-Control Surfactants... 

Core floods and high pressure sight cell studies are unsuitable for evaluating large numbers of surfactants as mobility control agents because of the long duration of properly designed... 

The surfactant systems used for mobility control in miscible flooding do not form a surfactant rich third phase, and lack its buffering action against surfactant adsorption. Furthermore, for obvious economic reasons, it is desirable to keep the surfactant concentration as low as possible, which increases the sensitivity of the dispersion stability to surfactant loss. Hence, surfactant adsorption is necessarily an even greater concern in the use of foams, emulsions, and dispersions for mobility control in miscible-flood EOR. The importance of surfactant adsorption in surfactant-based mobility control is widely recognized by researchers. A decision tree has even been published for selection of a mobility-control surfactant based on adsorption characteristics (12). 

Surfactant-Induced Mobility Control for Carbon Dioxide Studied with Computerized Tomography... 

WELUNGTON VINEGAR Surfactant-Induced Mobility Control for CO. 349... 

Work previously reported has shown very significant reductions in gas mobility when in situ generated foam was used in laboratory core tests. Using unconsolidated core models, investigators measured ten to several hundredfold decreases in gas mobilityThe effectiveness of foam for mobility control is vitally dependent on the choice of the surfactant used. Early tests using 1% ammonium lauryl sulfate were not totally successful in demonstrating foam effectiveness. ... 

From approximately 130 carefully selected candidate materials, a relatively small number of surfactants emerged as being superior for mobility control applications. 

Surfactants with low foamability were found to be poor candidates for mobility control. In general, anionic surfactants appear superior in the static foam tests. Sulfate esters of ethyoxylated linear alcohols were slightly better than the other classes of surfactants. Most sulfonates do not appear compatible with even small amounts of calcium and, therefore, produce very little foam at the temperatures used, 75-120 F. 

Several field tests of injectivity and of various types of surfactant-based mobility control for gas flooding have been reported in the literature. These are briefly reviewed for clues to possible future directions for the technology and as a guide to the research and development needed for achieving technical success and commercialization of surfactant-based mobility control for gas flooding. 

In addition to the surfactant, a white oil was a component of the microemulsion. This oil was added in the minimum amount required to solubilize enough xanthan polymer to produce the target viscosity. In the absence of the white oil, the polymer could not be solubilized. The xanthan polymer itself was required for mobility control. To prevent biodegradation of the polymer, formaldehyde was added. Citric acid was also a component of the microemulsion, added to prevent the oxidation of ferrous ion present in the brine to ferric ion. The presence of ferric ion would lead to precipitation of iron compounds as well as cross-linking of the biopolymer. 

Addition of oil yields an oil-in-water microemulsion with nearly spherical drops. Within limits, the higher the molecular weight of the oil added to produce an oil-in-water microemulsion, the less oil is needed to formulate single phases with polymer for mobility control (Hirasaki et al., 2008). During screening tests, a clear surfactant-polymer solution with oil added does not mean the corresponding aqueous solution without oil will be clear. Therefore, aqueous stability tests with polymer added in the surfactant solution are necessary and important. 

Polymer can be placed in a mixed SP slug or in a polymer-only slug for mobility control. Table 9.1 compares the results from different schemes. In SIM 1, 0.25 PV 0.07 wt.% polymer is injected after the surfactant slug (0.1 PV 2% S). In SIM 2, 0.1 PV X 0.07% polymer is moved to the surfactant slug. In SIM 3, all the polymer in 0.25 PV, 0.07 % polymer slug (0.25 x 0.0007 = 0.000175 PV) is placed in the 0.1 PV surfactant slug. Then the polymer concentration in the 0.1 PV surfactant slug is 0.175%. The recovery factors and incremental recovery factors are almost the same in these three simulation cases. From these simulation cases, it seems that it does not matter where polymer is placed. 

Linear and radial core flood tests were conducted to determine the polymer concentration for mobility control requirement. Figure 13.39 shows Brookfield (UL adapter) viscosity properties for the Alcoflood 1275A polymer in injection water and in an alkaline-surfactant solution. Note that the AS dramatically decreased the viscosity, and a higher polymer concentration was required to provide the same viscosity. 

For those interested in foams for mobility control, I also recommend reading Surfactant-Based Mobility Control, Smith, D. H., Ed. ACS Symposium Series No. 373, American Chemical Society Washington, DC, 1988. 

In MOST APPLICATIONS IN THE PETROLEUM INDUSTRY, such as in steam-foam flooding, aqueous foams are used to control flow resistance, and this capability makes them attractive for mobility control for improving oil recovery. When the surfactant solution comes in contact with oil in the porous medium, oil is emulsified, and Figure 1 shows the presence of emulsified oil droplets inside of a capillary network. The oil has... 

Of course, the final test by which to assess the requirement that the surfactant should be capable of maintaining lamellae for mobility control in C02 floods is the measurement of foam mobility in cores. Because such tests are time-consuming and expensive, the foam durability test has been used as a useful screening tool for more rapid appraisal of surfactants at reservoir conditions. 

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