Summary
The success of acid fracturing depends on the conductivity created and
retained under closure stress in addition to the length of conductive fracture.
The majority of acid-fracturing treatments show a sharp decline in conductivity
with increasing closure stress, with almost no significant conductivity after a
short production time. As a result, many wells are refractured to restore them
to the original productivity after the initial fracture. However, the success
of these refracture treatments has been diverse with respect to extent of
stimulation.
A new approach to overcome productivity decline of acid-fractured wells is
to perform closed-fracture acidizing (CFA) at the appropriate time in the life
of the well to keep fractures conductive. An experimental study was conducted
to investigate the effect of CFA on already-acid-fractured cores exposed to a
certain level of closure stress. Indiana limestone cores were acidized with a
typical acid system of 15 wt% hydrochloric acid (HCl) viscosified with a
polymer under typical field conditions. After the first acidizing process,
conductivity measurements were conducted on acid-etched core faces up to a
certain closure stress. While the fracture was kept under the closure stress, a
CFA treatment was conducted under the same conditions as the initial acidizing.
The re-etched fractures were once again placed under different levels of
closure stress, and conductivity measurements were taken at each stress.
Experiments were conducted under different conditions of leakoff, polymer
concentration, and closure stress after the first acidizing in order to
determine the influence of these parameters on the refracturing
conductivity.
In all cases, CFA enhanced fracture-face etching, while it significantly
increased fracture conductivity under closure stress. However, leakoff, polymer
concentration, and closure stress did influence the degree of success of CFA.
Leakoff allowance and lower polymer concentration resulted in the most enhanced
fracture conductivity in comparison to the initial acid-fracturing process.
© 2011. Society of Petroleum Engineers
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History
- Original manuscript received:
7 July 2009
- Meeting paper published:
7 October 2009
- Revised manuscript received:
2 August 2010
- Manuscript approved:
20 September 2010
- Published online:
26 January 2011
- Version of record:
22 February 2011
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