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Waterflood Oil Recovery
Waterflood recovery, NP, can be computed at any time in the life of a project by using the formula:
N = the oil in place in the floodable pore volume at start of water injection.
ED = displacement efficiency (fraction of oil saturation at the start of water injection which is displaced by water in the invaded zone)
EA = areal sweep efficiency (fraction of floodable pore volume area swept by the injected water)
EV = vertical sweep efficiency (fraction of the floodable pore volume in the vertical plane swept by the injected water)
Note: Floodable pore volume is highly dependent on net pay discriminators such as permeability and porosity cutoffs.
Reservoir properties that significantly affect these efficiency factors are:
- Mobile oil saturation,
- Mobility ratio, and
- Reservoir heterogeneity
Mobile Oil SaturationMobile oil saturation is the oil saturation at the start of the waterflood, So, minus the residual oil saturation to water, Sorw.
Higher values of mobile oil saturation results in higher waterflood oil recovery.
Mobility Ratio, MMobility ratio, M, is the water mobility in the water invaded portion of the reservoir divided by the oil mobility in the non-contacted portion. In turn, the mobility of the fluid is the permeability of the rock to that fluid divided by the fluid viscosity. Hence, in terms of relative permeability:
krw = water relative permeability at the average water saturation in the swept zone
µw = water viscosity
kro = oil relative permeability in the oil bank ahead of the water (normally equivalent to 1.0)
µo = oil viscosity
Figure W-1. Effect of oil viscosity and rock wettability on mobility ratio, water viscosity = 0.5 cp.
(Interstate Oil Compact Commission, 1983).
It is important to mention that the manner in which water displaces oil from reservoir rock, i.e. relative permeability effects, depends upon the preferentially wettability of that rock. Figure W-1 shows water-oil mobility ratio as a function of oil viscosity and rock wettability. Mobility ratio increases with oil viscosity and is higher for oil-wet rocks.
Displacement, areal and vertical sweep efficiencies all increase as mobility ratio decreases.
Reservoir Heterogeneity, V
Dykstra and Parsons (1950) introduced the concept of the coefficient permeability variation V, which is designed to describe the degree of heterogeneity within the reservoir. The value of this uniformity coefficient ranges between 0.0 for a completely homogeneous system and 1.0 for a completely heterogeneous system.
Areal or vertical permeability variation is computed by first arranging all relevant core permeability values from maximum to minimum, and plotting each versus the percentage of the total number of samples having higher permeability. The plot is made on log-probability coordinates and a straight-line is expected if the permeability data is log-normally distributed. Coefficient of permeability variation, V is computed as follows:
k50 = permeability at the norm or 50% probability
k84.1 = permeability (one statistical deviation away from the norm) at 84.1% probability
Vertical sweep efficiency increases as vertical reservoir heterogeneity (stratification) decreases.