Gas flooding

Enhanced oil-recovery processes include chemical and gas floods, steam, combustion, and electric heating. Gas floods, including immiscible and miscible processes, are usually defined by injected fluids (carbon dioxide, flue gas, nitrogen, or hydrocarbon). Steam projects involve cyclic steam (huff and puff) or steam drive. Combustion technologies can be subdivided into those that autoignite and those that require a heat source at injectors [521]. 

C. Raible. Improvement in oil recovery using cosolvents with CO2 gas floods. US DOE Fossil Energy Rep NIPER-559, NIPER, January 1992. 

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. 

For air and water, in a one inch tube, the superficial gas velocity limits for the film suspension region were about 31 ft./sec. to 41 ft./sec. (see Fig. 10 also). The value of 31 ft./sec. agrees with the value to be expected for the critical gas flooding velocity at zero liquid flow from an extrapolation of the results of Nicklin and Davidson. Hence, the approach of the latter workers in their analysis of unstable slug flow would seem to be valid for net liquid flows down to zero. 

Below a critical power input the gas bubbles are not affected laterally but move upward with their natural buoyancy. This condition is called gas flooding of the impeller. At higher power inputs the gas is dispersed radially, bubbles impinge on the walls and are broken up, consequently with improvement of mass transfer. A correlation of the critical power input is shown as Figure 10.10. 

Early RPB researchers discovered that this flooding correlation for packed towers applied equally well to RPBs when the gravity term (g) was replaced by centrifugal acceleration (ra>2). As acceleration increases, the gas flooding velocity (Uq) increases in order to maintain the same value of the first term. Since the ratio of liquid (L) to gas (G) flow remains constant, liquid flow increases commensur-ately with gas flow. Most researchers observed higher gas velocities before the onset of flooding than predicted by the Sherwood correlation (17,26,27). 

Schramm, L.L. Mannhardt, K. Novosad, J.J. Selection of Oil-Tolerant Foams for Hydrocarbon Miscible Gas Flooding in Proceedings, 14th. International Workshop and Symposium,... 

Most of these are merely for the disposal of the acid gas, but not all. For example, in 2002 Dominion Energy Canada Ltd commissioned an acid-gas flood for its West Stoddart field near Ft. St. John, BC, Canada. In this flood, 2.5 MMSCFD of acid gas that is a 75% H2S and 25% C02 mixture is injected into a producing reservoir. The acid-gas mixture is delivered from a multistage compressor to an injection well via a 2.25-km long pipeline. 

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. 

These amounts, although approximate, are important guides to the economic value of gas flooding with current technology, to the potential value of research investments that might produce even... 

These numbers represent only the consensus of current expectations, and very large revisions of many predictions may be required after a significant number of actual full-scale production data become available. In short, the American oil industry has invested in a very costly commitment to gas-flood FOR. No one yet knows how successful this experiment will be, or, with complete certainty, what technological pitfalls may be the greatest threat to success. 

For at least three decades, long before the recent widespread initiation of field projects, adverse mobility ratio has been recognized as a major technological problem -- perhaps "the major problem" -- of gas-flood EOR (7-9). Adverse mobility ratios produce "viscous fingering," a displacement phenomenon that has been known and studied both experimentally and theoretically for many years (9-14). This phenomenon was a principal reason for the failure of many of the liquified petroleum gas (LPG) floods of the 1950 s and early 1960 s (15). 

Basically, all of these closely related problems occur because gas-flood injection fluids have very small viscosities at the temperatures and pressures at which they are used. For example, the viscosity of CO2 at 13.8 MPa (2,000 psi) and 38°C (100°F) is about 0.066 cp, whereas the viscosities of reservoir oils are at least an order of magnitude greater (16). This produces a ratio of the mobility of the CO2 to the mobility of the oil that is much greater than one. (The mobility of a fluid is defined as its relative permeability divided by its viscosity for the definition of relative permeability, see equations below.)... 

Figure 1.) The channeling can occur with either miscible or immiscible floods and results in much lower production of the displaced fluid for any given throughput of the injection fluid once the latter reaches the production well (10,12,15). The problem, which is common to water flooding and to all EOR processes, is most severe for gas flooding simply because it is in gas flooding that the injected fluids have the lowest viscosities (most unfavorable mobility ratios).

Figure 1 shows schematically viscous fingering, which occurs because the injection fluid is less viscous, and gravity override, which occurs because the injection fluid is less dense. Chapter 2 of this book, by Chang and Slattery, describes a linear instability analysis for gas flooding and also introduces a nonlinear treatment of the flow instabilities. 

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. 

Surfactants. Surfactants and other amphiphilic compounds are very versatile materials when mixed with water and/or nonpolar compounds. For this reason, many different ways have been proposed to use surfactants to modify underground flow patterns in various types of EOR. One of these proposals, coprecipitation of a cationic and an anionic surfactant to clog cracks or pores in highly permeable rock zones, might prove useful for gas flooding (K. L. Stellner, J. C. Amente,... 

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. 

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 ... 

Determination of Important Parameters in Surfactant Design. Recent work (Chapters 8 and 9) demonstrates the utility of correlating test results with surfactant structures. But as the complexities of pore level mechanisms, dispersion properties, and fluid behavior become better understood, it is also becoming increasingly clear that a variety of physical property measurements will be required for advanced surfactant design. Many of these measurements will be needed at pressures (ca. 10 MPa) that are characteristic of gas-flood conditions. 

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... 

We have seen that a very large amount of oil is believed recoverable by gas flooding (more than 15 billion barrels in the United States alone) and that large financial commitments already have been made to produce that target oil. Over 60 field projects are planned and many of these are already in operation and beginning to produce incremental oil. 

However, we have also considered that the low viscosities of gas injection fluids, combined with density differences, cause these fluids to finger through and override the target oil, leaving a substantial fraction of it uncontacted by the injected fluid and, thus, unproduced. These detrimental sweep effects present the greatest single target for improvement of gas-flood EOR, and their alleviation is currently the object of a wide spectrum of applied research. 

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