EEOR in Carbonate Reservoirs - Electrokinetics for Petroleum and Environmental Engineers - page 168

rise to a higher permeability enhancement of 43%, which is 5-9% higher
than that with a gradient of 0.5 V/cm (41% enhancement). This is due to
the electrophoresis phenomena of EK (Haroun et al., 2009) that results in a
rigorous migration of clay colloids and micelles or fines emptying the pore
throats and, thus, potentially increasing the permeability. Therefore, there
is an optimum voltage gradient of about 1 V/cm which will be verified by
further experimental work.
In addition to this, it was also observed that 1-inch core plugs achieve
higher values of permeability enhancement by about 20-30% as compared
to the 1.5-inch core plugs with a maximum of 70% enhancement in perme-
ability. This may be due to the reduced heterogeneity of 1-inch core plugs.
The displacement efficiency demonstrates an exponential relationship
with the acid concentration. The increment in displacement efficiency
decreases with increasing acid concentration as the curve approaches an
optimum concentration beyond which the displacement efficiency factor
starts to drop. The clogging of pore throats beyond the optimum con-
centration may decrease the oil recovery due to a significant reduction in
permeability.
Another significant observation is that the effect of voltage gradient in
increasing the displacement efficiency is not very dominant at lower acid
concentrations. This indicates that an increase in acid concentration can
significantly increase the displacement efficiency factor more than increas-
ing the voltage gradient. This is potentially caused by a reduction in the
rate of movement of ions (anions and cations) between the anode and
the cathode, reducing the electromigration effect (Wittle et al., 2008 and
Pamukcu et al., 2009).
At higher concentrations above 0.5%, the differences in displacement
efficiency are observed and the maximum displacement efficiency of 89%
was recorded in the case of 2V/cm potential gradient, as it produces the
maximum EK driving force. At 1.0% acid concentration, when the voltage
gradient increases from 0.5 V/cm to 1.0 V/cm, the displacement efficiency
increases by 8%, but when the voltage gradient increases from 1 V/cm to
2 V/cm, the displacement efficiency increases by 5%. Thus, the optimum
voltage gradient needs to be determined.

4.6 Effect of EEOR and SMART EOR in Carbonate

Reservoirs at Reservoir Conditions

Following are the first laboratory results to date on EEOR and SMART
EOR at elevated temperature and pressure. The displacement efficiency was