Recently,
self-diverting acid systems based on viscoelastic surfactants
(SDVAs) have been successfully used for carbonate reservoir treatment.
Changes in the SDVA viscosity during the interaction with carbonate
reservoirs are associated with the transformation of spherical surfactant
micelles into worm-shaped micelles as the acid concentration decreases
and the fluid salinity increases. The highly viscous fluid acts as
a temporary barrier and diverts the acid fluid into the untreated
lower permeability zones. After treatment, the SDVA barrier breaks
down when it makes contact with newly formed hydrocarbons. The objectives
of this study were to examine in detail the viscoelastic surfactant
effect on the HCl and calcite reaction and to examine the effects
of the surfactant and acid concentrations on the SDVA apparent viscosity.
Rheological measurements were conducted using rotational viscometers
at 25 °C. A proposed semi-empirical rheological model describes
the relationships between the apparent viscosity, rate, and HCl concentration.
The core flooding tests were conducted in a specialized laboratory
unit to simulate the downhole conditions. The testing unit allows
for control of the pressure, temperature, and injected volume of acid
flowing across the core sample. The results showed that the pressure
drop varies as a function of the pumped fluid pore volume. The steady
increase in the pressure as the fluid entered the core was a good
indication of the actual viscosity increase in the porous rock during
acid spending. The experiments were conducted using various flow rates.
The core flow tests indicated that the SDVA surfactant delayed the
acid breakthrough in carbonate cores. The mathematical model describing
the results, obtained from the acidification experiments, is presented.