Summary
Fracture-height prediction and evaluation is critical in understanding the
effectiveness of a fracturing treatment. Volumetrically, fracturing must adhere
to mass-balance equations. Therefore, proppant placed in the fracture must be
accounted for in the creation of fracture height, width, and length. In many
cases, excessive fracture height generation is at the expense of fracture-width
and -length creation. As a result, in fracture treatments where excessive
height growth is believed to have occurred, premature screenouts are usually
the result of insufficient fracture width. This unfortunate circumstance
creates an operational strain and productivity underperformance for wells.
Various methods have been used to evaluate fracture height before the
fracture treatment. These methods can be as simple as height estimates based on
sensitivity studies of fracture-height growth for different fluids, pumping
rates, and other factors, using a derived rock-stress profile within a
fracturing simulator, and as complex as the most robust methods of direct
measurement, using passive seismic monitoring of fracture events during an
injection test. As the industry comes closer to what is believed to be direct
measurement of fracture height, the level of trust increases as well.
Both temperature logs and radioactive tracers have been used on a regular
basis by the fracturing industry to evaluate fracture-height containment (or
the lack thereof). However, the information from these tools may be quite
misleading when planning a fracture treatment because of the inherent
assumption made when analyzing the log data. The major assumption is of a
vertical fracture propagating from the wellbore. Nevertheless, in a
tectonically stressed environment, where the maximum principal stress is not
always an overburden, the assumption of a vertical fracture may be incorrect.
Where this is true, the indication of height control seen in these log types
may be a mirage. This paper describes, through case studies, the unique
problems of evaluating fracture height in tectonically stressed formations.
Furthermore, it shows that while both temperature and tracer logs may add value
to evaluating fracture effectiveness, their results should be validated by a
thorough pressure analysis of the injection data. If this critical step is
forgone, many assumptions of height control, as indicated by a temperature log
alone, may in fact conceal an environment of excessive height growth and lead
to premature screenouts.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
31 July 2007
- Meeting paper published:
4 December 2007
- Revised manuscript received:
27 March 2008
- Manuscript approved:
28 April 2008
- Published online:
1 June 2009
- Version of record:
1 June 2009
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