Page 160 - Formation Damage during Improved Oil Recovery Fundamentals and Applications

138 Thomas Russell et al.

10 23 PVI, injectivity decreases from 2.5 to 5.1 times. The higher is the
rate, the higher is the velocity and hence the greater is the detached con-
centration. Well index impairment is highly sensitive to injection rate.
Fig. 3.23B shows impedance curves for injection rate of 100 bbl/day/
m. Arbitrary values for U mI and U mJ for low-salinity water and fresh water
are applied. After injection of 10 23 PVI, injectivity decreases from 3.8 to
7.2 times. The injectivity damage after injection of formation water is
induced by fine particles, mobilized near to the wellbore by excessive
drag force under high velocities. Well index impairment is also highly
sensitive to injection-water salinity.
Clearly from Figs. 3.23A and 3.23B, the effects of particle detachment
and straining due to velocity happens much faster than the effects due to
salinity alteration. Stabilization time for impedance in the first case hap-
pens almost instantly. This behavior can be explained by the fact that
velocity alteration detaches particles at once and only near the wellbore
where velocity is higher; whereas, salinity alteration causes particles to
detach throughout the reservoir, but only when the salinity front reaches
that point.

3.5.4 Field cases
This section presents three field cases where the effects of low-salinity
waterflooding are observed. The model outlined in Section 3.5.3 is used
to adjust the model parameters to produce good agreement between the
field data and the model. This provides a means of evaluating the appro-
priateness of the model in capturing the effects of fines migration at the
field-scale. The results are presented in Fig. 3.24.
The first example of injectivity decline due to low-salinity waterflood-
ing comes from the Ventura Oil Field, located in the north of the city of
Ventura, California, United States (Fig. 3.24A). The field was operated at
the time the data were recorded by Shell. Extensive water quality control
and monitoring to remove solids was performed to ensure that all injec-
tivity decline was due to low-salinity water. The initial injected water
salinity was equal to 0.35 M (20000 ppm); afterward, the water salinity
was decreased to 0.08 M (5000 ppm). Initial injectivity loss of 23% was
observed with total loss being equal to 50% at the end of 6 months
(Barkman et al., 1975).
A similar effect was observed in the West Delta Block 73 field, located
27 miles offshore of Grand Isle, Lousiana, United States (Fig. 3.24B).