Mobility-control fluid

Intermixing of the polymer mobility control fluid with the surfactant slug can result in surfactant - polymer interactions which have a significant effect on oil recovery (476). Of course, oil - surfactant interactions have a major effect on interfacial behavior and oil displacement efficiency. The effect of petroleum composition on oil solubilization by surfactants has been the subject of extensive study (477). 

The primary factor controlling how much gas is in the form of discontinuous bubbles is the lamellae stability. As lamellae rupture, the bubble size or texture increases. Indeed, if bubble coalescence is very rapid, then most all of the gas phase will be continuous and the effectiveness of foam as a mobility-control fluid will be lost. This paper addresses the fundamental mechanisms underlying foam stability in oil-free porous media. 

Foams have been tested as diverting agents in waterfloods, 39 as steam-diverting agents to reduce gravity override or steam channeling in steamfloods, and as mobility-control fluids in CO2 displacements. 4 ... 

Water-in-oil macroemulsions have been proposed as a method for producing viscous drive fluids that can maintain effective mobility control while displacing moderately viscous oils. For example, the use of water-in-oil and oil-in-water macroemulsions have been evaluated as drive fluids to improve oil recovery of viscous oils. Such emulsions have been created by addition of sodium hydroxide to acidic crude oils from Canada and Venezuela. In this study, the emulsions were stabilized by soap films created by saponification of acidic hydrocarbon components in the crude oil by sodium hydroxide. These soap films reduced the oil/water interfacial tension, acting as surfactants to stabilize the water-in-oil emulsion. It is well known, therefore, that the stability of such emulsions substantially depends on the use of sodium hydroxide (i.e., caustic) for producing a soap film to reduce the oil/water interfacial tension. 

Petroleum recovery typically deals with conjugate fluid phases, that is, with two or more fluids that are in thermodynamic equilibrium. Conjugate phases are also encountered when amphiphiles fe.g.. surfactants or alcohols) are used in enhanced oil recovery, whether the amphiphiles are added to lower interfacial tensions, or to create dispersions to improve mobility control in miscible flooding 11.21. 

Foam is a promising fluid for achieving mobility control in underground enhanced oil recovery (1-3). Widespread application of this technology to, for example, steam, CO, enriched... 

Polymers increase the viscosity of the soil washing fiuids. Increased viscosity provides mobility control, which reduces the fingering of the displacing fiuid past the displaced fluid. It also helps ensure that the contaminated area is efficiently contacted by the soil washing solution. 

Acid-in-oil emulsion can extend the propagation of acid considerable distances into a reservoir because the continuous (oil) phase prevents or minimizes contact between the acid and the rock [4,678,689]. Emulsification also increases viscosity and will improve the distribution of the acid in layered and heterogeneous reservoirs. Acidizing foams are aqueous, in which the continuous phase is usually hydrochloric acid (carbonate reservoirs) or hydrofluoric acid (sandstone reservoirs), or a blend, together with suitable surfactants and other stabilizers [345,659]. Foaming an acidizing fluid increases its effective viscosity, providing mobility control when it is injected [678]. 

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 great differences are due to "sweep," "adverse mobility ratios," "conformance," and other fluid mechanical effects (see next section) that are exacerbated by the natural heterogeneities in porous rocks and whose alleviation is the purpose of sweep and mobility control technology. 

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. 

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). 

The fluid-fluid interfacial tension also appears as the controllable design parameter in the capillary pressure and its effects on dispersion-based mobility control. As described by Equation 9, the capillary pressure, P, is directly proportional to the fluid-fluid tension, y ... 

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). 

Enhanced oil recovery always deals with two or more fluids. By implication these fluids are conjugate phases in equilibrium with each other, although Chapter 6 shows that nonequilibrium mixing can sometimes be important when surfactants are used. When one considers the role of the critical micelle concentration (CMC) in CO2 mobility control, it is the CMC of the aqueous phase saturated with CO2 that is important. As illustrated in Figure 10, this CMC may be much lower than the CMC of C02 free surfactant solutions (R. S. Schechter, University of Texas, personal communication, October 26, 1987). 

Foam exhibits higher apparent viscosity and lower mobility within permeable media than do its separate constituents.(1-3) This lower mobility can be attained by the presence of less than 0.1% surfactant in the aqueous fluid being injected.(4) The foaming properties of surfactants and other properties relevant to surfactant performance in enhanced oil recovery (EOR) processes are dependent upon surfactant chemical structure. Alcohol ethoxylates and alcohol ethoxylate derivatives were chosen to study techniques of relating surfactant performance parameters to chemical structure. These classes of surfactants have been evaluated as mobility control agents in laboratory studies (see references 5 and 6 and references therein). One member of this class of surfactants has been used in three field trials.(7-9) These particular surfactants have well defined structures and chemical structure variables can be assigned numerical values. Commercial products can be manufactured in relatively high purity. 

Nearly all of the treatment processes in which fluids are injected into oil wells to increase or restore the levels of production make use of surface-active agents (surfactant) in some of their various applications, e.g., surface tension reduction, formation and stabilization of foam, anti-sludging, prevention of emulsification, and mobility control for gases or steam injection. The question that sometimes arises is whether the level of surfactant added to the injection fluids is sufficient to ensure that enough surfactant reaches the region of treatment. Some of the mechanisms which may reduce the surfactant concentration in the fluid are precipitation with other components of the fluid, thermally induced partition into the various coexisting phases in an oil-well treatment, and adsorption onto the reservoir walls or mineral... 

In recent years there has been considerable interest in the use of foams in chemical steam flood, CO2, and low tension processes. To date, principal applications have been as diverting agents where the foam has been used to block high permeability, low oil saturation zones and hence force drive fluids through lower permeability, higher oil saturation zones. The utility of foams in more general mobility control roles has not been extensively... 

Final laboratory testing of CO2 foam was performed in Shell s CT facility (11-12L Tertiary miscible and immiscible CO2 corefloods, with and without foam mobility control, were scanned during flow at reservoir conditions. The cores were horizontally mounted continuous cylinders of Berea sandstone. Table I lists pertinent core and fluid data. 

Tertiary oil was increased up to 41% over conventional CO2 recovery by means of mobility control where a carefully selected surfactant structure was used to form an in situ foam. Linear flow oil displacement tests were performed for both miscible and immiscible floods. Mobility control was achieved without detracting from the C02-oil interaction that enhances recovery. Surfactant selection is critical in maximizing performance. Several tests were combined for surfactant screening, included were foam tests, dynamic flow tests through a porous bed pack and oil displacement tests. Ethoxylated aliphatic alcohols, their sulfate derivatives and ethylene oxide - propylene oxide copolymers were the best performers in oil reservoir brines. One sulfonate surfactant also proved to be effective especially in low salinity injection fluid. 

Mobility control is one of the most important concepts in any enhanced oil recovery process. It can be achieved throngh injection of chemicals to change displacing fluid viscosity or to preferentially rednce specific flnid relative permeability through injection of foams, or even through injection of chemicals, to modify wettability. This chapter does not address a specific mobility control process. Instead, it discusses the general concept of the mobility control requirement in enhanced oil recovery (EOR). 

The existing concept of mobility control is that the displacing fluid mobility should be equal to or less than the (minimum) total mobility of displaced multiphase fluids. This chapter first uses a simulation approach to demonstrate that the existing concept is invalid the simulation results suggest that the displacing fluid mobility should be related to the displaced oil phase mobility, rather than the total mobility of the displaced fluids. From a stability point of view, a new criterion regarding the mobility control requirement is derived when one fluid displaces two mobile oil and water fluids. The chapter presents numerical verification and analyzes some published experimental data to justify the proposed criterion. 

Our first task is to evaluate the validity of the conventional concept about the mobility control requirement using a simulation approach. This model uses the UTCHEM-9.0 simulator (2000). The dimensions of the two-dimensional XZ cross-section model are 300 ft x 1 ft x 10 ft. One injection well and one production well are at the two extreme ends in the X direction, and they are fully penetrated. The injection velocity is 1 ft/day the initial water saturation and oil saturation are 0.5. The displacing fluid is a polymer solution. The purpose of using the polymer solutuion in the model is to change the viscosity of the displacing fluid. Therefore, polymer adsorption, shear dilution effect, and so on are not included in the model. To simplify the problem, it is assumed that the oil and water densities are the same that the capillary pressure is not included that the relative permeabilities of water and oil are straight lines with the connate water saturation and residual oil saturation equal to 0 and that the water and oil viscosity is 1 mPa s. Under these assumptions and conditions, we can know the fluid mobilities at any saturation. The model uses an isotropic permeability of 10 mD. 

In Case visc03, even though the oil viscosity is 100 mPa-s, the same as that in visc02, when the injection fluid mobility is adjusted to be the same as the oil mobility only (not the total mobility) based on Eq. 4.9, the recovery factor is 98.34%, almost the same as that in Case viscOl (only 1% different). Based on these results, we can see that to satisfy the mobiUty control requirement for a high oil recovery factor (favorable displacement condition), the injection mobility should be equal to or less than the oil mobility corrected by the initial oil saturation by Eq. 4.9, not the total mobility of fluids ahead of the displacing front. 

The conventional mobility ratio in multiphase flow is defined as the displacing fluid mobility divided by the total mobility of displaced water and oil phases. From the previous section, we can see that the unit mobility ratio based on the conventional definition is not a valid criterion to distinguish favorable and unfavorable mobility control conditions. We have found that a better criterion should be the unit mobility ratio, which is defined as the displacing fluid mobility divided by the oil mobility multiplied by the oil saturation (Eq. 4.9). In this section, we attempt to justify the proposed idea from the stability of displacement front. 

Wang et al. tried to define a viscosity ratio as a criterion for the mobility control requirement. However, we have to point ont that the viscosity ratio J, / J,o required for the mobility control should depend on relative permeabilities and fluid saturations. 

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