Summary
In-situ-gelled acids have been used extensively in matrix acidizing and acid
fracturing for acid diversion and reducing the leakoff rate, respectively. A
few studies investigated the rate of dissolution of calcite in polymer-based
acids, yet none has addressed in detail the in-situ-gelled acids. Therefore,
the aim of this work is to examine the mass transfer and the kinetics of the
reaction of 5 wt% HCl in-situ gelled acids with calcite and determine the
effect of Fe crosslinker on the rate of calcite dissolution.
The rate of reaction of 5 wt% HCl in-situ gelled acid was measured using the
rotating-disk apparatus. Rock samples of 1.5-in. diameter and 1-in. length were
used. The effect of temperature (100 - 250°F) and disk-rotational speed (100 -
1,800 rev/min) was investigated using Pink Desert limestone rock samples.
Calcium concentration was measured in the collected samples and was used to
determine the acid-reaction rate.
Experimental results showed that the rate of calcite dissolution at 150°F
was controlled mainly by the rate of mass transfer of the acid to the surface
up to a disk rotational speed of 1,000 rev/min and by the rate of the surface
reaction above this value. On the basis of the results obtained, the diffusion
coefficient of 5 wt% HCl in in-situ-gelled acid at 150°F; the activation
energy; and the reaction rate constant at 150, 200, and 250°F were determined
for the first time. A power-law kinetic model was used to determine the
kinetics parameters. The presence of Fe3+ crosslinker had a
significant effect on the rate of dissolution in comparison with reactions with
gelled acid (no crosslinker) at the same condition. The reaction rate decreased
by a factor of 2.2 and by a factor of 1.4 when the reaction was conducted at
100 and 1,500 rev/min, respectively. A gel layer, formed on the surface, acted
as a barrier between the acid and the rock, which reduced the rate of calcite
dissolution.
© 2011. Society of Petroleum Engineers
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History
- Original manuscript received:
10 August 2010
- Meeting paper published:
9 June 2010
- Revised manuscript received:
4 March 2011
- Manuscript approved:
10 March 2011
- Published online:
28 July 2011
- Version of record:
23 December 2011
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