Summary
After-closure analysis (ACA) in homogeneous-matrix reservoirs provides a
method for extracting critical reservoir information from pre-frac injection
tests. This paper extends the theory and practice of ACA to identify the
presence of productive natural fractures.
Natural fractures are important to identify before conducting a stimulation
treatment because their presence may require designs that differ from
conventional matrix treatments. Literature shows that naturally fractured
reservoirs are very susceptible to formation damage and require stimulation
treatments to account for this issue. The historical problem, however, has been
to confidently characterize the reservoirs pre-frac in terms of both the
reservoir quality and the deliverability mechanism (fractures vs. matrix)
before committing to these design specifications.
This paper presents the results of a simulator used to analyze the mini-frac
after-closure period to identify the presence of natural fractures. The
simulation results are distilled into a field implementation methodology for
determining the extent of natural fracturing and the formation reservoir
quality. This methodology is also applied to a field case study to verify the
practicality of the technique. Unlike previous mini-frac-analysis methods, this
approach identifies natural fractures that are material to production and
allows the engineer to distinguish them from “fissures” that are open only
during injection and are not a production mechanism.
Introduction
Motivation for Identifying Natural Fractures. Identifying the
presence of natural fractures is important for a broad range of reasons. On a
field scale, realizing the presence of natural fractures can impact reserves
estimation, initial well rates, production declines, and planned well
locations. With respect to well completions, fractured reservoirs may
necessitate a special stimulation approach. Because fractured reservoirs tend
to produce from a relatively small reservoir volume (i.e., the fractures),
these formations can be highly susceptible to damage (Cippolla et al. 1988).
The literature shows that the use of foamed treatments (Cippolla et al. 1988),
100 mesh, and low gel loadings can be used to stimulate these reservoirs
effectively. The literature also shows the disastrous results that can arise
when damage-prevention steps are not taken (Cippolla et al. 1988). As a result,
there is a definite need to identify natural fractures before a stimulation
treatment so that the appropriate design decisions can be made.
In the past, conventional well testing, such as pressure-buildup tests, has
been used for determining the reservoir description. However, these techniques
often prove costly both in terms of additional equipment requirements and
delays in well on-line dates. In addition, conventional well testing may not be
successful in low-permeability reservoirs because these wells may not flow at
measurable rates before stimulation. These cost and reservoir limitations have
forced the engineer to seek other low-cost methods for determining reservoir
properties. One such option for acquiring these data is the use of a mini-frac
injection test conducted before a stimulation treatment. The mini-frac analysis
techniques available to provide estimates of the formation capacity (kh) and
indications of the presence of natural fractures include preclosure and
post-closure methods.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
2 December 2004
- Meeting paper published:
26 September 2004
- Revised manuscript received:
25 October 2005
- Manuscript approved:
22 November 2005
- Version of record:
20 February 2006
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