Summary
Fracture acidizing has been a dominant practice in the industry to enhance
well productivity in low-permeability carbonate reservoirs. Many acid systems
have been developed to improve this stimulation process. The most desirable
characteristics for an acid system to be suitable for fracture acidizing are
leakoff control and retarded reaction rate. These characteristics are required
for deep acid penetration, so that when the fracture closes, long flow channels
are etched on the fracture surfaces. Leakoff control can be achieved by pumping
a pad containing a viscosifying agent or solid bridging agents to plug
wormholes generated by acid dissolution. Reaction retardation is attempted
usually by lowering the effective diffusivity of the hydrogen ion.
It is well known that during an acid-fracturing operation, the overall
reaction rate of hydrochloric acid (HCl) with limestone is
mass-transfer-limited. Designing the treatment requires knowing the effective
diffusivity of the hydrogen ion in the acid system, which, to the best of the
authors’ knowledge, has not been determined before. Because of their combined
leakoff-control and retardation capabilities, surfactant-based acids have been
used in acid-fracturing treatments. Because more carbonate reservoirs are
treated by use of this acid system, it is important to obtain the effective
diffusivity of H+.
The rotating-disk device has been used to investigate the reaction kinetics
between a reactive solution and carbonate rocks because the thickness of the
boundary layer is uniform throughout the disk surface. This paper discusses the
reaction-rate data generated recently for surfactant-based acid by use of a
rotating-disk apparatus and presents the methodology used to determine the
effective diffusivity from the measurements.
The results obtained indicated that the viscoelastic surfactant examined
(carboxybetaine-type) reduced the dissolution rate of calcite with HCl acid.
The surfactant reduced the diffusion coefficient for H+. The effect
of temperature on the diffusion coefficient did not follow the Arrhenius
law.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
6 March 2007
- Meeting paper published:
30 May 2007
- Revised manuscript received:
24 May 2008
- Manuscript approved:
11 July 2008
- Published online:
2 March 2009
- Version of record:
26 February 2009
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