Page 310 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Fluid Movement in Waterflooded Reservoirs 477

definition, and many workers defined mobility ratio as oil to water mobility; in
this case, the reciprocal of mobility ratio (as now accepted) must be used. The
oil mobility used in Equation 5-213 refers to the location in the oil bank ahead
of the flood front. For the water mobility, there are several possibilities regard-
ing the location at which the relative permeability should be chosen: at the flood
front, at residual oil saturation where only water is flowing (end point), or at
some intermediate saturation. Craig [133] found a better correlation if the water
mobility was determined at the average water saturation behind the flood front
at water breakthrough. Thus for the mobility ratio expression, the relative
permeability of water is found at the average water saturation at water break-
through as determined by the Welge graphical approach. As Craig notes, the
mobility ratio of a waterflood will remain constant before breakthrough, but it
will increase after water breakthrough corresponding to the increase in water
saturation and relative permeability to water in the water-contacted portion of
the reservoir. Unless otherwise specified, the term mobility ratio is taken to be
the value prior to water breakthrough. As will be discussed later in this section,
mobility ratio is important in determining the volume of reservoir contacted
by the waterflood.
Recovery Efficiency

Recovery efficiency is the fraction of oil in place that can be economically
recovered with a given process. The efficiency of primary recovery mechanisms
will vary widely from reservoir to reservoir, but the efficiencies are normally
greatest with water drive, intermediate with gas cap drive, and least with solution
gas drive. Results obtained with waterflooding have also varied. The waterflood
recovery can range from less than the primary recovery to as much as 2.5 times
the recovery obtained in some solution-gas drive reservoirs. A recent statistical
analysis by the API [239] provided the average primary and secondary recovery
efficiencies in Table 5-37. Generally, primary and ultimate recoveries from
carbonate reservoirs tend to be lower than from sandstones. For pattern water-
floods, the average ratio of secondary to primary recovery ranges from 0.33 in
California sandstones to greater than one in Texas carbonates. For edge water
injection, the secondary-to primary ratio ranged from an average of 0.33 in
Louisiana to 0.64 in Texas. By comparison, secondary recovery for gas injection
into a gas cap averaged only 0.23 in Texas sandstones and 0.48 in California
sandstones. Ultimate primary and secondary recovery performance for different
drive mechanisms are given in Table 5-38. Solution-gas-drive reservoirs will
generally have higher oil saturations after primary recovery, and are usually the
better candidates for waterflooding.
Displacement of oil by waterflooding is controlled by f hid viscosities, oil-water
relative permeabilities, nature of the reservoir rock, reservoir heterogeneity,
distribution of pore sizes, fluid saturations (especially the amount of oil present),
capillary pressure, and the location of the injection wells in relation to the
production wells. These factors contribute to the overall process efficiency. Oil
recovery efficiency (E,) of a waterflood is the product of displacement efficiency
(E,) and volumetric efficiency (E"), both of which can be correlated with
fluid mobilities:
E, = E,E, = E,E,E, (5-219)
where E, = overall reservoir recovery or volume of hydrocarbons recovered
divided by volume of hydrocarbons in place at start of project