Summary
In acid fracturing, while the near-wellbore area of the fracture receives
live, unspent acid, sections of the fracture farther down the length receive
partially spent acid because of the acid's reaction with the rock. As a result,
there is a distribution of acid concentration along the fracture length.
However, almost all experimental studies on acid-fracture conductivity have
been conducted with live acid, which is representative of what occurs near the
wellbore.
An experimental study was conducted to investigate the effect of spent acids
on the resulting fracture conductivity in a laboratory facility designed to
perform acid-fracture-conductivity characterization. Gelled HCl acid systems
with different degrees of acid spending were used to etch fractured cores under
identical treatment conditions, which were set to mimic field conditions.
Detailed etched surface characterization, fluid analysis, and conductivity
measurements were performed on acid-etched fracture faces.
Experimental results show that the amount of rock dissolved and the actual
etching pattern depend highly on acid concentration. While there was less
fracture-face etching with more spent acid systems, the etching pattern also
changed with acid spending, which sometimes offset the additional etched volume
of unspent acid. As a result, the retained conductivity was generally higher
for more spent acid, and the decline in conductivity with closure stress was
more subtle with the spent acid, probably because of less weakening of the
fracture face.
While there have been many developments in enhancing the etched length of
acid-fractured wells, a major reason for the lack of long etched fractures is
acid spending along the fracture. In order to understand the etched fracture
profile, there is a need to realize how the degree of acid spending limits
etching and affects the resulting fracture conductivity. On the basis of the
newly discovered effect of acid spending on conductivity, acid-etched length
and conductivity can be predicted more accurately and acid-fracture treatments
better designed.
© 2013. Society of Petroleum Engineers
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History
- Original manuscript received:
12 August 2010
- Meeting paper published:
27 October 2010
- Revised manuscript received:
16 November 2012
- Manuscript approved:
27 November 2012
- Published online:
7 February 2013
- Version of record:
26 February 2013
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