Page 307 - Fundamentals of Enhanced Oil and Gas Recovery

Microbial Enhanced Oil Recovery: Microbiology and Fundamentals 295

dimensionless number referred as the capillary number (N C ). This number is
defined as the ratio of viscous forces to the capillary forces [6,15,45 48]:
Viscous Forces νμ
N C 5 5 (10.2)
Capillary Forces σcosθ
where ν is the displacing fluid velocity, μ is the displacing fluid dynamic viscosity, σ is
the oil-water IFT, and θ represents the contact angle. This number shows the relative
importance of the viscous to the capillary forces. Higher values of this parameter
denote lower residual oil saturation in the porous media and consequently higher oil
recovery [46]. Capillary number is usually large for high-speed flows and low for low-
speed flows. Typically, for fluid flow within the pores media in an oil reservoir capil-
lary number is B10 26 and for flow in production pipelines is B1 [49]. To enhance
the microscopic oil displacement, the capillary number should be increased via either
increasing the displacing fluid viscosity or decreasing the oil 2 water IFT. Chemicals
such as surfactants decrease the IFT and polymers increase the water viscosity. Reed
and Healy [50] specified that significant oil recovery demands an increase of
100 1000 folds in the capillary number. Microbially generated surfactants can be the
suitable agent for this purpose [51 54], the detail of which will be debated in
Section 10.1. Effect of the capillary number on the residual oil saturation has been
investigated by several researchers [55 59]. The governing relationship can be sche-
matically shown via a plot known as capillary desaturation curve. In this plot, the cap-
illary number and residual oil saturation are on the x-, and y-axes, respectively.
Typically, this plot shows a residual oil saturation plateau region in very low capillary
numbers through approximately N c 5 10 26 and after that the residual oil saturation
drops with the increase in the capillary number [60]. The point at which the residual
oil saturation starts to drop is called critical capillary number (N CC ). Parameters such
as rock structure, wettability, fluid types, and also testing condition affect the N CC
magnitude [61]. For waterfloods, N c is typically equal to 10 26 [62]. This value is gen-
erally considerably less than the N CC and a moderate enhancement on N C will not
significantly decrease the residual oil saturation [63].
The other important parameter is the mobility ratio. In cases in which there are large
variations between the viscosity of the displacing fluid and oil, the volumetric sweep effi-
ciency will play the main role in the recovery process [40]. Moreover, in field implemen-
ted EOR process, the recovery efficiency is often dominated by the volumetric sweep
efficiency [17,40]. In case of large differences between the viscosities, it is likely water
moves more rapidly than oil and reaches the producing well sooner. The parameter
denoting the relative mobility of the water and oil phases is the mobility ratio (M):
k w μ
M 5 o (10.3)
k o μ
w