Volumetric sweep efficiency

There are two principal mechanisms of enhanced oil recovery increasing volumetric sweep efficiency of the injected fluid and increasing oil displacement efficiency by the injected fluid. In both, chemicals are used to modify the properties of an injected fluid whether water, steam, a miscible gas such as CO2 or natural gas, or an immiscible gas, usually nitrogen. Poor reservoir volumetric sweep efficiency is the greatest obstacle to increasing oil recovery (9). 

Polymer Flooding. Even in the absence of fractures and thief 2ones, the volumetric sweep efficiency of injected fluids can be quite low. The poor volumetric sweep efficiency exhibited in waterfloods is related to the mobiUty ratio, Af, the mobiUty of the injected water in the highly flooded (low oil saturation) rock, divided by the mobiUty of the oil in oil-bearing portions of the reservoir, (72,73). The mobiUty ratio is related to the rock permeabihty to oil, and injected water, and to the viscosity of these fluids by the following equation ... 

The WAG process has been used extensively in the field, particularly in supercritical CO2 injection, with considerable success (22,157,158). However, a method to further reduce the viscosity of injected gas or supercritical fluid is desired. One means of increasing the viscosity of CO2 is through the use of supercritical C02-soluble polymers and other additives (159). The use of surfactants to form low mobihty foams or supercritical CO2 dispersions within the formation has received more attention (160—162). Foam has also been used to reduce mobihty of hydrocarbon gases and nitrogen. The behavior of foam in porous media has been the subject of extensive study (4). X-ray computerized tomographic analysis of core floods indicate that addition of 500 ppm of an alcohol ethoxyglycerylsulfonate increased volumetric sweep efficiency substantially over that obtained in a WAG process (156). 

Gravity override of low density steam leads to poor volumetric sweep efficiency and low oil recovery in steam floods. Nonchemical methods of improving steam volumetric sweep efficiency include completing the injection well so steam is only injected in the lower part of the oil-bearing zone (181), alternating the injection of water and steam (182), and horizontal steam injection wells (183,184). Surfactants frequently are used as steam mobihty control agents to reduce gravity override (185). Field-proven surfactants include alpha-olefin sulfonates (AOS), alkyltoluene sulfonates, and neutralized... 

Surfactants evaluated in surfactant-enhanced alkaline flooding include internal olefin sulfonates (259,261), linear alkyl xylene sulfonates (262), petroleum sulfonates (262), alcohol ethoxysulfates (258,261,263), and alcohol ethoxylates/anionic surfactants (257). Water-thickening polymers, either xanthan or polyacrylamide, can reduce injected fluid mobiHty in alkaline flooding (264) and surfactant-enhanced alkaline flooding (259,263). The combined use of alkah, surfactant, and water-thickening polymer has been termed the alkaH—surfactant—polymer (ASP) process. Cross-linked polymers have been used to increase volumetric sweep efficiency of surfactant—polymer—alkaline agent formulations (265). 

Microbial-enhanced oil recovery involves injection of carefully chosen microbes. Subsequent injection of a nutrient is sometimes employed to promote bacterial growth. Molasses is the nutrient of choice owing to its low (ca 100/t) cost. The main nutrient source for the microbes is often the cmde oil in the reservoir. A rapidly growing microbe population can reduce the permeabiHty of thief zones improving volumetric sweep efficiency. Microbes, particularly species of Clostridium and Bacillus, have also been used to produce surfactants, alcohols, solvents, and gases in situ (270). These chemicals improve waterflood oil displacement efficiency (see also Bioremediation (Supplement)). 

The amount of oil recovery promoted by an injected fluid is related to its ability to displace the oil it contacts in the reservoir, termed the oil displacement efficiency (ODE), and to the relative amount of the reservoir invaded by the injected fluid, termed the volumetric sweep efficiency (VSE). Total oil recovery may be expressed as ... 

Volumetric sweep efficiency is determined by the permeability and wettability distribution in the reservoir and by the properties of injected fluids. Waterflooding characteristically exhibits poor volumetric sweep efficiency. The more expensive the injection fluid, the more important it is to have a high volumetric sweep efficiency so that the injected fluid contacts and thus mobilizes a larger volume of oil. High permeability streaks or layers (thief zones) and natural or induced rock fractures can channel the injected fluid through a small portion of the reservoir resulting in a low volumetric sweep efficiency. 

The displacing fluid may be steam, supercritical carbon dioxide, hydrocarbon miscible gases, nitrogen or solutions of surfactants or polymers instead of water. The VSE increases with lower mobility ratio values (253). A mobility ratio of 1.0 is considered optimum. The mobility of water is usually high relative to that of oil. Steam and oil-miscible gases such as supercritical carbon dioxide also exhibit even higher mobility ratios and consequent low volumetric sweep efficiencies. 

Gravity override is the migration of the steam to the upper portion of the formation and is caused by the low steam density. This results in channeling of the steam through the upper portion of the reservoir and a low volumetric sweep efficiency. 

Surfactants have been used as steam mobility control agents in both laboratory and field tests to prevent this gravity override thereby increasing volumetric sweep efficiency. Surfactants that have been... 

Both nonionic and anionic surfactants have been evaluated in this application (488,489) including internal olefin sulfonates (487, 490), linear alkylxylene sulfonates (490), petroleum sulfonates (491), alcohol ethoxysulfates (487,489,492). Ethoxylated alcohols have been added to some anionic surfactant formulations to improve interfacial properties (486). The use of water thickening polymers, either xanthan or polyacrylamide to reduce injected fluid mobility mobility has been proposed for both alkaline flooding (493) and surfactant enhanced alkaline flooding (492). Crosslinked polymers have been used to increase volumetric sweep efficiency of surfactant - polymer - alkaline agent formulations (493). 

Cross-linking reactions, acid-catalyzed polymer, 15 168-169 Cross-linking resists, 15 172 Cross-linking systems, covalent dye fixation using, 9 489-491 Cross-finking treatments, in volumetric sweep efficiency, 18 620 Cross-finks... 

Rock salt semiconductors, 22 141 dating, 21 317-318 selenium occurrence in, 22 78 Rock surface chemistry, in volumetric sweep efficiency, 18 621 Rock varnish, photocatalytic origin of, 19 100-101... 

Surfactant-polymer flooding, 13 628 Surfactant precipitation, in volumetric sweep efficiency, 13 621 Surfactant propagation, in enhanced oil recovery, 13 629... 

Petroleum production from subterranean reservoirs can be increased by injecting water as liquid or steam. Various chemicals have been added to the water or steam to increase volumetric sweep efficiency. One alternative is the use of emulsions which serve as diverting agents to correct the override and channeling problems that occur during fluid injection. Laboratory results show that it may be possible to control channeling and steam override with an emulsion blocking technique. The emulsion can be formed with the aid of a surfactant mixture or by use of natural surfactants that exist in some crude oils. Core-... 

After ASPF flooding, the chloride ion concentration of the effluent fluid at all the producers increased significantly, as shown in Figure 13.49. We can see that the volumetric sweep efficiency was increased because the formation water salinity was much higher. 

It may happen that a foam that is desirable in one part of the oil production process may be undesirable at the next stage. For example, in the oil fields, an in situ foam that is purposely created in a reservoir to increase viscosity (and thereby improve volumetric sweep efficiency as part of an oil recovery process) may present a handling problem when produced. 

Foam has been used in field applications involving both cyclic and steam-drive processes. Many of the steam-foam tests have been performed in Kern County, California, where most of the U.S. heavy oil is produced. In many situations, foam has successfully increased both volumetric sweep efficiency and oil recovery rates (34). Generally, the application of foam has been considered to be a technical success but economically suspect. 

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