Capillary number efficiency

Gas injection can also recover oil by reducing oil viscosity and residual oil saturation, even when miscibility is not achieved. Reduction in viscosity is more significant if the oil viscosity is large, and this process is attractive in viscous or semiviscous reservoirs, especially when accompanied by some other improved recovery mechanism. Residual oil saturation in three-phase flow in water-wet rock is very low (essentially zero), even at very low capillary numbers. Two main problems in such a process are the low relative permeabilities and sweep efficiencies. This process can be implemented in a highly dipping reservoir to take... 

Figure 6.18 shows the residual oil saturation (lower six curves) and displacement efficiency (upper six curves) versus capillary number in a slightly... 

FIGURE 6.18 Residual oil saturation and displacement efficiency versus capillary number. Source Wang et al. (2007). 

Castor et al. (1981b) observed that the IFT in the alkaline flooding was on the order of 0.1 mN/m. Their capillary numbers of alkaline floods are presented in Figure 10.18. The capillary number of alkaline floods was about 100 times higher than the capillary numbers in waterfloods. The alkaline flooding results from Castor et al. show that the recovery efficiencies could be better correlated with the stability of emulsions and wettability alteration than with IFT of the systems. 

The mechanisms governing deformation and breakup of drops in Newtonian liquid systems are well understood. The viscosity ratio, X, critical capillary number, and the reduced time, t, are the controlling parameters. Within the entire range of X, it was found that elongational flow is more efficient than shear flow for breaking the drops. 

Chemical flooding involves the injection of a surfactant solution that can cause oil-aqueous interfacial tension to drop from about 30 mN m to near-zero values on the order of 10 -10 mN m , allowing spontaneous or nearly spontaneous emulsification of the oU, with an increase in the capillary number by several orders of magnitude and with greatly increased displacement and recovery of the oil [6, 10,19, 67, 75, 79-87]. The micelles present also help to solubilize the released oil droplets hence, this process is sometimes referred to as micellarflooding. Having mobilized the oil, these processes are even more efficient if the oil droplets are... 

In the previous section, it was noted that the microscopic displacement efficiency is largely a function of interfacial forces acting between the oil, rock, and displacing fluid. If the interfacial tension between the trapped oil and the displacing fluid could be lowered to 10 to 10 dyn/cm, the oil droplets could be deformed and could squeeze through the pore constrictions. A miscible process is one in which the interfacial tension is zero that is, the displacing fluid and the residual oil mix to form one phase. If the interfacial tension is zero, then the capillary number Nyc becomes infinite and the microscopic displacement efficiency is maximized. 

Fig. 1. The effect of capillary number, N, on the microscopic oil displacement efficiency in porous media of various size distribution (Ref. 1).

The microscopic mobilization efficiency (or the percentage reduction in the residual oil saturation of a secondary waterflood) of tertiary surfactant floods has been experimentally correlated to be a function of the capillary number. 

Mobilization of trapped nonwetting phase Laboratory studies show that trapped residual oil can be recovered if the pressure difference due to viscous flow is sufficient to overcome capillary forces so that trapped blobs, or at least parts of blobs, are mobilized (2-4). If the ratio of viscous to capillary forces is raised sufficiently, almost complete recovery of residual oil can be achieved. For packs of very coarse sand, Leverett (8) reported data which showed indications t)f slightly improved recovery efficiency for capillary numbers (,j.n this case, of the form yv/a( )) as low as 10". In the present work, some of the systems in which trapping was measured were later subjected to high flow rate in an attempt to measure the capillary number required for mobilization of trapped nonwetting phase so that results could be compared with capillary numbers for trapping. 

Fig. 11. Plots of microscopic displacement efficiency versus capillary number [vy/( )a].

In order to delineate the effect of surfactant mass transfer on in situ behavior of oil ganglia, we carried out several oil displacement experiments using equilibrated and nonequilibrated oil/ micellar solution systems. For equilibrated systems, the oil displacement efficiency showed an excellent correlation with IFT and capillary number. However, for unequilibrated systems, the oil displacement efficiency depended on salinity. Below optimal salinity, the oil displacement efficiency almost remained the same for both equilibrated and nonequilibrated systems, whereas at and above optimal salinity the oil displacement efficiency was higher for nonequilibrated systems as compared to equilibrated systems. This was attributed to mass transfer rate effects in these systems. 

A comparison of equilibrated and nonequilibrated systems for oil displacement efficiency Figure 2 shows the IFT and the percent oil recovery as a function of initial TRS 10-80 concentration in 1% NaCl for equilibrated and nonequilibrated systems. It was observed that for the pre-equilibrated system, 94% oil was recovered at 0.05% TRS 10-80 concentration corresponding to minimum IFT at this concentration. However, for nonequilibrated systems, the maximum oil recovery shifted from 0.05% to 0.1% TRS 10-80 concentration. The maximum oil recovery for nonequilibrated systems was much lower than that observed for equilibrated systems (Figure 2). Since the amount of surfactant injected was the same for each run (0.125 gm), the maximum oil recovery was interpreted as a result of the capillary number vs. final oil saturation correlation (21). 

For equilibrated systems, there is an excellent correlation between the capillary number and oil recovery efficiency. However, in calculating capillary number for nonequilibrated systems, care should be exercised because the IFT measured in vitro may not be achieved in situ and, in certain cases, the interfacial viscosity and not interfacial tension, may be a predominant factor influencing the oil displacement efficiency. 

In order to improve production from waterflooding operations, various water-soluble thickening polymers are incorporated into the injection fluid (Mohanty and Caneba, 2005). The idea is that there is a need for a relatively high capillary number, Ac, in order to realize more efficient oil displacement from solid surfaces. Note that the capillary number is related to the displacing fluid viscosity, fx, interstitial velocity, V, and interfacial tension (IFT), y, (Pope and Baviere, 1991)... 

Low Tension Polymer Water Flood. In oil reservoirs, where the critical capillary number is relatively low, a significant amount of waterflooded residual oil can be displaced by surfactants of high efficiency even at two-phase flood conditions. This was demonstrated by the snccessfnl second Ripley surfactant flood pilot test in the Loudon field where approximately 68% of waterflooded residual oil was recovered by injecting a 0.3 PV microemulsion bank [63]. The microemulsion bank was followed by I.O PV of higher viscosity polymer drive. The chemical formnlation consisted of a blend of two PO-EO sulfates. 

Capillary column efficiency is dependent on the carrier gas used, the length and inner diameter of the column, the retention factor of the particular solute selected for calculation of the number of theoretical plates, and the film thickness of the... 

Wenxiang, W., Demin, W., and Haifeng, J. 2007. Effect of the Visco-elasticity of Displacing Fluids on the Relationship of Capillary Number and Displacement Efficiency in Weak Dil-Wet Cores. Paper SPE 109228 presented at the Asia Pacific Pil and Gas Conference and Exhibition, Jakarta, 30 Cctober-1 November. DPI 10.2118/109228-MS. 

A chromatographic column provides a location for physically retaining the stationary phase. The column s construction also influences the amount of sample that can be handled, the efficiency of the separation, the number of analytes that can be easily separated, and the amount of time required for the separation. Both packed and capillary columns are used in gas chromatography. 

Efficiency The efficiency of capillary electrophoresis is characterized by the number of theoretical plates, N, just as it is in GC or ITPLC. In capillary electrophoresis, the number of theoretic plates is determined by... 

The idea of the effective plate number was introduced and employed by Purnell [4], Desty [5] and others in the late 1950s. Its conception was evoked as a direct result of the introduction of the capillary column or open tubular column. Even in 1960, the open tubular column could be constructed to produce efficiencies of up to a million theoretical plates [6]. However, it became immediately apparent that these high efficiencies were only obtained for solutes eluted at very low (k ) values and, consequently, very close to the column dead volume. More importantly, on the basis of the performance realized from packed columns, the high efficiencies did not... 

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