Introduction to Electrokinetics - Electrokinetics for Petroleum and Environmental Engineers - page 18

GROUND ELECTRODES (±)

OIL WELL

PRODUCING
ZONE

CURRENT FLOW LINES

WELL ELECTRODE (±)

Figure 1.6 Proposed EEOR field installation by Anbah et al. (1965).

1.9 Improving Acidizing of Carbonates

The stimulation of carbonate reservoirs is mainly achieved by acidizing
treatments (Hendrickson, 1972; Chilingarian et a1., 1992). Acids may be
injected into pores and pre-existing fractures or at hydraulic fracturing
rates depending upon the results desired. The acid dissolves the carbon-
ates (limestones/dolomites), enlarging the pores and increasing the width
of pre-existing fractures. This gives rise to an increase in permeability. The
principal acid commonly used is hydrochloric (HCl), which is pumped
through tubing.
In the case of the uniform penetration of acid, the reaction rate declines
uniformly with decreasing acid concentration. The weight of carbon-
ate dissolved per increment of distance penetrated declines uniformly
until the acid is completely spent. With stronger acid, the spending time
decreases.
In the case of matrix acidizing, with enlargement of pores or pre-
existing fractures, (1) the specific surface area decreases, (2) the velocity
decreases, (3) spending time increases, and (4) the penetration distance
increases.
The main problem in acidizing is the fact that the radial distance the
acid will penetrate until being spent is short, especially in tight carbonates.
When assuming a homogeneous formation and that the volume of acid