Volumetric sweep efficiency improvement

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... 

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)). 

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). 

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. 

Consequently, such an idea led to the development of a new EOR process called the "first version of Polymer Augmented Alkaline Flooding" (PAAF). In the application of this process an alkaline solution slug is injected to mobilize the residual oil, which is the oil mechanically entrapped and/or sticking on the pore walls. The alkaline solution slug is chased by a fresh polymer slug, which provides the improved mobility control and volumetric sweep efficiency to enhance oil recovery ... 

The application of crosslinked-polymer systems for permeability modification of petroleum reservoirs has received increasing attention in recent years. A crosslinked-polymer treatment, in general, involves an injection of a polymer solution into high-permeability zones or fractures which have been previously swept by the displacing fluid. The polymer solution reacts, either before or after injection, to form a three-dimensional gel network which reduces the effective permeability of the invaded portions of the reservoir. Fluid subsequently injected is diverted to other, tighter regions of the formation, thereby improving overall volumetric sweep efficiency. 

At the present time the improvement of areal and vertical (volumetric) sweep efficiency takes a great deal of room in secondary and tertiary oil recovery. One of the widely used and perspective methods is mobility control by diluted aqueous solutions of different polyacrylamides (1,2). In the middle of the sixties some authors (3,4) proposed that the viscosity enhancement and the non-Newtonian flow behavior of the solutions were responsible for the reduction of phase mobility. Mungan (5,6), Gogarty (7), Dauben and Menzie (8) have pointed out, however, that the sorption phenomenon plays a decisive role in the flow characteristics of the polymer solutions and carrier phases. In the papers devoted to... 

Mobility control is a generic term describing any process where an attempt is made to alter the relative rates at which injected and displaced fluids move through a reservoir. The objective of mobility control is to improve the volumetric sweep efficiency of a displacement process. In some processes, there is also an improvement in microscopic displacement efficiency at a specified volume of fluid injected. Mobility control is usually discussed in terms of the mobility ratio, M, and a displacement process is considered to have mobility control if 1.0. Volumetric sweep efficiency generally increases as M is reduced, and it is sometimes advantageous to operate at a mobility ratio considerably less than unity, especially in reservoirs with substantial variation in the vertical or areal permeability. 

Polymers also are used to improve volumetric sweep efficiency in reservoirs that are fractured or have considerable permeability variation. In this application, commonly called in-situ permeability modification, the polymer is crosslinked in situ to form a gel network. In-situ gelation reduces the permeability in the region contacted by the polymer. By selection of chemical systems, treatments... 

Various methods of permeability modification have been applied. These include crosslinked polymer (gelled polymer), microbial-based, and precipitation processes. This chapter describes only the application of crosslink polymers. In-situ permeability modification is not strictly a mobility-control process in that volumetric sweep efficiency is improved through modification of rock perme-abiiity rather than through mobility adjustment of an injected fluid. The processes are related, however, because of the use of polymer systems that are similar and because the result—i.e., improved volumetric sweep—is similar. Thus, permeability modification is described in this chapter. 

Volumetric Sweep Efficiency. As discussed in Chap. 4, reduction of the mobility ratio in a displacement process results in an improvement of volumetric sweep efficiency. However, calculation of sweep from the empirical correlations presented in Chap. 4 is probably not justified in a WAG process because of the complex nature of the flow in the region behind the oil bank. In application, the process usually is modeled with computer-based mathematical models, 165,166 Limited results from properly scaled physical models have also been reported. 

The objective of polymer flooding would be to improve the displacement efficiency by lowering the mobility of the displacing fluid. No significant increase in oil recovery was expected as a result of an increase in volumetric sweep efficiency. At Frannie, the water-flood has an unfavorable mobility ratio (A/< 1 is favorable), and the viscosity ratio of the oil (15 cp [15 mPa-s]) to injected water (0.77 cp [0.77 mPa- s]) is about 19. If the viscosity of the injected fluid were increased to equal the oil viscosity (/ n// <, l)> "lO bility ratio would improve dramatically. The change in mobility ratio could produce an estimated 5 million STB [795 X10 m ] of oil (2% of the OOIP) according to the fractional flow prediction. 

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