Introduction to Electrokinetics - Electrokinetics for Petroleum and Environmental Engineers - page 20

power supply located on the surface. The acid must be injected with corro-
sion inhibitors into the formation; however, the aluminum anode will also
serve as a sacrificial anode.
By applying DC current during acidizing operations, the volumetric
rate of flow increases, which, in turn, increases the penetration distance
of the acid before it is being spent. Without application of DC current, the
penetration distance is usually very short, especially in tight rocks.
1.10 EconomicFeasibility
Many assumptions should be made in order to make a rough calculation
of economic feasibility of using DC current. The variables which will affect
the economic evaluation include: (1) thickness, depth, and resistivity of
the pay zone, (2) arrangement of the electrodes, (3) duration of the elec-
trical treatment, (4) labor cost, and (5) price of electricity at the site of
application.
In general, the rock resistivity is a function of the amount of interstitial
water present, which is determined by the porosity and its type. The flow
of electrical current in such a case is not a simple linear flow but follows
an irregular path around the individual sand grains. This flow pattern
will increase the length of the current flow lines. Inasmuch as the cur-
rent is mainly transmitted through the rock in the form of electrolytic
conduction, the resistivity of the interstitial formation water seems to be
the deciding factor in the formation resistivity as a whole. The presence
and types of clays, however, greatly affect the electrical resistivity of the
formation, especially in the case of fresh formation water (Anbah et al.,
1965).
Although the electrokinetics depend mainly on the imposed electri-
cal potential gradient, the associated electrical current is conditioned by
the type of formation under consideration and its electrolyte content.
There is no simple relation between the amount of liquid transported by
electrokinetics and the quantity of electricity consumed. The presence of
expandable colloidal matter in the porous media further complicates the
picture.
In field application, the energy consumption will depend on the dimen-
sions of the electrodes, the applied electrical potential, and the lithology. If
these factors are known, the amount of transmitted electrical current can
be estimated. Upon switching the current on, it will drop gradually because
the overall resistance will increase.