Gas-flood mobility control

Gelled and Cross-Linked Polymers. By themselves, water-soluble polymers are unlikely to prove suitable for improving gas-flood mobility control since these agents viscosify the aqueous phase, making the gas-to-aqueous phase mobility ratio even more adverse. 

In most applications of CO2 as an oil recovery agent, the CO2 exists as a supercritical fluid above its critical pressure (7.4 MPa) and temperature (32°C), while its solutions in oil are liquids (5). Hence, the dispersion types of most direct interest are supercritical-fluid-in-a-liquid (for which no specific name yet exists) and emulsions of oleic-in-aqueous liquids (which may be encountered at low CO2 saturations). However, for historical reasons (described below), all dispersions used in research on gas-flood mobility control are sometimes called "foams," even when they are known to be of another type. 

Dispersion and Phase Behavior. The selection of surfactants for high-pressure gas-flood mobility control effectively began in 1978 when Bernard and Holm received a patent on the use of alkyl polyethoxy sulfates SO4M as mobility control agents for... 

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) ... 

T. Zhu, A. Strycker, C. J. Raible, and K. Vineyard. Foams for mobility control and improved sweep efficiency in gas flooding. In Proceedings Volume, volume 2, pages 277-286.11th SPE/DOE Impr Oil Recovery Symp (Tulsa, OK, 4/19-4/22), 1998. 

By improving "sweep" and "mobility control," surfactant-based methods offer the most promising ways to alleviate these problems. This use of surfactants appears to be just on the verge of commercialization for steam flooding. Because miscible CO2 flooding has been commercialized more recently, the use of surfactants to improve gas-flood EOR has not yet been commercialized. Conceivably, however, the long-term viability of gas flooding could prove to be dependent on the success of current research efforts in the use of surfactants to alleviate "bypass" problems. 

It can be anticipated that all gas-flood projects, as they are presently being carried out, will leave a large fraction of the reservoir oil uncontacted by the injected fluids. This bypassed oil will remain in place, undisplaced by the injected fluid. Thus, in each current field project, the amount of incremental oil produced by gas flooding could be substantially increased if the uncontacted oil could be reached. The improvement of the vertical and areal distribution of injected fluids throughout the reservoir, so that they contact substantially more oil, will require much better methods of sweep and mobility control. 

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 ... 

Of the processes that might be used to improve sweep and mobility control in gas flooding, processes that exploit the ability of surfactants to form dispersions are by far the most promising. 

The beginnings of dispersion-based mobility control can be traced back to 30 years ago. But application of these ideas to gas flooding began only about 10 years ago, and extensive research began only with the commercialization of miscible CO2 flooding in the early 1980 s. 

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. 

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. 

Instead of relying completely on theory for the determination of mobility, most researchers also performed experimental measurements of the quantities of interest. Many of the first experiments on foam were performed with water and gas with the outlet at ambient pressure, and many were simply gas floods of packs or cores saturated with surfactant solution. Although for many such transient experiments, the published data were insufficient for the estimation of the steady-state mobilities required for the estimation of mobility-control effectiveness, this was not true for some of them. Calculated values of mobility and relative mobility were derived by Heller et al. (22), from the data published in six different papers (23—28). The values they found, given in terms of relative mobilities, ranged from 0.001 to 0.6 cP-1, or in terms of effective viscosities from 1000 down to 1.6 cP (1 to 0.0016 Pa-s). Not enough information was available to trace all of the relevant parameters that may have caused these differences. 

This chapter reports adsorption data for a number of surfactants suitable for mobility control foams in gas-flooding enhanced oil recovery. Surfactants suitable for foam-flooding in reservoirs containing high salinity and hardness brines are identified. The results of adsorption measurements performed with these surfactants are presented surfactant adsorption mechanisms are reviewed and the dependence of surfactant adsorption on temperature, brine salinity and hardness, surfactant type, rock type, wettability and the presence of an oil phase is discussed. The importance of surfactant adsorption to foam propagation in porous media is pointed out, and methods of minimizing surfactant adsorption are discussed. 

The reason for the large number of hydrocarbon-miscible flooding projects in Canada is the preponderance of reasonably priced gas throughout the province of Alberta. Also, a large number of gas plants separates intermediate components, such as ethane, propane, and butane, and allows custom design of individual solvents tailored to specific reservoir conditions. Because of the large number of hydrocarbon-miscible projects in Canada, the application of mobility-control foams seems an attractive means to significantly increase oil production. 

A study of the effect of pore geometry on foam formation mechanisms shows that snap-off" bubble formation is dominant in highly heterogeneous pore systems. The morphology of the foams formed by the two mechanisms are quite different. A comparison of two foam injection schemes, simultaneous gas/surfactant solution injection (SI) and alternate gas/surfactant solution injection (GDS), shows that the SI scheme is more efficient at controlling gas mobility on a micro-scale during a foam flood. 

The Vernon Polymer flood of the Brazos Oil Gas Co., located in Sec. 2, 24 S., 16 E., Woodson County, Kans., was begun in late Oct., 1963. Injection of a 500 ppm solution of Pusher polymer was initiated in a 15-acre pilot to determine feasibility of improving oil recovery by mobility ratio control. Performance to date and the anticipated recovery of 115,000 bbl from the 329 acre-ft pilot indicate efficient displacement of the viscous reservoir oil. 

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