Mobility ratio

When water is displacing oil in the reservoir, the mobility ratio determines which of the fluids moves preferentially through the pore space. The mobility ratio or water displacing oil is defined as ... 

If the mobility ratio is greater than 1.0, then there will be a tendency for the water to move preferentially through the reservoir, and give rise to an unfavourable displacement front which is described as viscous fingering. If the mobility ratio is less than unity, then one would expect stable displacement, as shown in Figure 8.16. The mobility ratio may be influenced by altering the fluid viscosities, and this is further discussed in Section 8.8, when enhanced oil recovery is introduced. 

As a guideline, the plateau rate is usually between 2 to 5% of the STOMP per year. The lower end of the range would apply to shallow dip reservoirs with an unfavourable mobility ratio, creating a rate dependent displacement process. 

Polymer flooding alms at reducing the amount of by-passed oil by increasing the viscosity of the displacing fluid, say water, and thereby improving the mobility ratio (M). 

This technique is suitable where the natural mobility ratio is greater than 1.0. Polymer chemicals such as polysaccharides are added to the injection water. -... 

FIG. 6 Comparison of protein electrophoretic mobility ratios as functions of protein molecular weight for SDS-templated gels of various compositions (data points) to Fergnson plots of reference normal gels. (Reprinted by permission of Wiley-VCH from Ref. 322, Copyright 1996, Wiley-VCH.)... 

The mobility ratio equal to the diffusion ratio in this equation would naturally follow from application of the Nemst-Einstein equation, Eq. (88), to transport gels. Since the Nemst-Einstein equation is valid for low-concentration solutes in unbounded solution, one would expect that this equation may hold for dilute gels however, it is necessary to establish the validity of this equation using a more fundamental approach [215,219]. (See a later discussion.) Morris used a linear expression to fit the experimental data for mobility [251]... 

It should be noted that when a mobility ratio is expressed in terms of the transport number, an experimental error, say, of 0.01, is not small enough at small concentrations, as seen from Eq. (8). For example, at Xi = 0.05, this error amounts to as much as 20% for m,. It is thus preferable to express the ratio in terms of ,2 rather than f. 

While water-soluble polymers can be used to achieve a favorable mobility waterflood for low to modestly viscous oils, usually toe process cannot economically be apphed to achieving a favorable mobility displacement of more viscous oils—those having viscosities of from approximately 100 cP or higher. These oils are so viscous that the amount of polymer needed to achieve a favorable mobility ratio would usually... 

Key mechanisms important for improved oil mobilization by microbial formulations have been identified, including wettability alteration, emulsification, oil solubilization, alteration in interfacial forces, lowering of mobility ratio, and permeability modification. Aggregation of the bacteria at the oil-water-rock interface may produce localized high concentrations of metabolic chemical products that result in oil mobilization. A decrease in relative permeability to water and an increase in relative permeability to oil was usually observed in microbial-flooded cores, causing an apparent curve shift toward a more water-wet condition. Cores preflushed with sodium bicarbonate showed increased oil-recovery efficiency [355]. 

Even in the absence of fractures and thief zones, the volumetric sweep efficiency of injected fluids can be quite low. The poor volumetric sweep efficiency exhibited in waterfloods is related to the mobility ratio, M. This is defined as the mobility of the injected water in the highly flooded (watered-out) low oil saturation zone, m, divided by the mobility of the oil in oil-bearing portions of the reservoir, m, (253,254). The mobility ratio is related to the rock permeability to oil and injected water and to the viscosity of these fluids by the following formula ... 

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. 

Despite its relatively high mobility, water has been used to decrease the mobility of even higher mobility gases and supercritical CO used in miscible flooding (361). While water mobility can be up to ten times that of oil, the mobility of gases can be 50 times that of oil (362). The following formula is used to calculate gas oil mobility ratios (363) ... 

For a miscible displacement at the required reservoir conditions, carbon dioxide must exist as a dense fluid (in the range 0.5 to 0.8g/cc). Unfortunately, the viscosity of even dense CO2 is in the range of 0.03 to 0.08 cp, no more than one twentieth that of crude oil. When CO2 is used directly to displace the crude, the unfavorable viscosity ratio produces inefficient oil displacement by causing fingering of the CO2, due to frontal instability. In addition, the unfavorable mobility ratio accentuates flow non-... 

These tests show that CC -foam is not equally effective in all porous media, and that the relative reduction of mobility caused by foam is much greater in the higher permeability rock. It seems that in more permeable sections of a heterogeneous rock, C02-foam acts like a more viscous liquid than it does in the less permeable sections. Also, we presume that the reduction of relative mobility is caused by an increased population of lamellae in the porous medium. The exact mechanism of the foam flow cannot be discussed further at this point due to the limitation of the current experimental set-up. Although the quantitative exploration of this effect cannot be considered complete on the basis of these tests alone, they are sufficient to raise two important, practical points. One is the hope that by this mechanism, displacement in heterogeneous rocks can be rendered even more uniform than could be expected by the decrease in mobility ratio alone. The second point is that because the effect is very non-linear, the magnitude of the ratio of relative mobility in different rocks cannot be expected to remain the same at all conditions. Further experiments of this type are therefore especially important in order to define the numerical bounds of the effect. 

The effectiveness of C02 in displacing oil from reservoirs is marred, however, by its extremely low viscosity. The viscosity of dense C02 remains low (in the range from 0.03 to 0.08 cp or 0.03 to 0.08 mpa) despite its relatively high density (above 0.45 g/cm3) under reservoir conditions. This low viscosity of C02 as compared to that of crude oil (1-10 cp) results in a high mobility ratio which degrades the macroscopic efficiency of the displacement process. Therefore, some method of mobility control is required for efficient use of C02, to increase greatly the quantity of producible oil. 

The desired low mobility ratio (approaching 1) could be achieved by the viscosification of C02. This prompted the search for compounds which can dissolve in and viscosify dense C02 and nonpolar hydrocarbons. 

Changing the wettability of reservoir rock surfaces from oil-wet to water-wet, increases the permeability of the formation to oil, decreases the permeability to water, decreases mobility ratio, increases sweep efficiency, increases the flowing fraction of oil at every saturation, and increases oil recovery at the economic limit of the waterflood. 

The selectivity of separation is mainly affected by parameters of the bulk electrolyte in the capillary. These include type of anion and cation, pH, ionic strength, concentration, addition of modifiers such as com-plexing agents, organic solvents, surfactants, etc. It is expressed in terms of mobility differences (A/i) or the mobility ratio s (a) ... 

Dinitrobenzoyl-amino acids /3-CD Linear mobility ratio 42... 

Double reciprocal x-reciprocal y-reciprocal Mobility ratio... 

J Kawaoka, FA Gomez. Use of mobility ratios to estimate binding constants of ligands to proteins in affinity capillary electrophoresis. J Chromatogr B 715 203-210, 1998. 

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