Summary
Well test analysis has been used for many years to assess well condition and
obtain reservoir parameters. Early interpretation methods (by use of straight
lines or log-log pressure plots) were limited to the estimation of well
performance. With the introduction of pressure-derivative analysis in 1983 and
the development of complex interpretation models that are able to account for
detailed geological features, well test analysis has become a very powerful
tool for reservoir characterization. A new milestone has been reached recently
with the introduction of deconvolution. Deconvolution is a process that
converts pressure data at variable rate into a single drawdown at constant
rate, thus making more data available for interpretation than in the original
data set, in which only periods at constant rate can be analyzed. Consequently,
it is possible to see boundaries in deconvolved data, a considerable advantage
compared with conventional analysis, in which boundaries often are not seen and
must be inferred. This has a significant impact on the ability to certify
reserves.
This paper reviews the evolution of well test analysis techniques during the
past half century and shows how improvements have come in a series of step
changes 20 years apart. Each one has increased the ability to discriminate
among potential interpretation models and to verify the consistency of the
analysis. This has increased drastically the amount of information that one can
extract from well test data and, more importantly, the confidence in that
information.
Introduction
Results that can be obtained from well testing are a function of the range
and the quality of the pressure and rate data available and of the approach
used for their analysis. Consequently, at any given time, the extent and
quality of an analysis (and therefore what can be expected from well test
interpretation) are limited by the state-of-the-art techniques in both data
acquisition and analysis. As data improve and better interpretation methods are
developed, more and more useful information can be extracted from well test
data.
Early well test analysis techniques were developed independently from one
another and often gave widely different results for the same tests (Ramey
1992). This has had several consequences:
• An analysis was never complete because there always was an alternative
analysis method that had not been tried.
• Interpreters had no basis on which to agree on analysis results.
• The general opinion was that well testing was useless given the wide range
of possible results.
Significant progress was achieved in the late 1970s and early 1980s with the
development of an integrated methodology on the basis of signal theory and the
subsequent introduction of derivatives. It was found that, although reservoirs
are all different in terms of depth, pressure, fluid composition, geology,
etc., their behaviors in well tests were made of a few basic components that
were always the same. Well test analysis was about finding these components,
which could be achieved in a systematic way, following a well-defined process.
The outcome was a well test interpretation model, which defined how much and
what kind of knowledge could be extracted from the data. The interpretation
model also determined which of the various published analysis methods were
applicable and when they were applicable. Importantly, the integrated
methodology made well test analysis repeatable and easy to learn. The evolution
of the state-of-the-art techniques in well test analysis throughout these years
can be followed from review papers that have appeared at regular intervals in
the petroleum literature (Ramey 1980, 1982, 1992; Gringarten 1986;
Ehlig-Economides et al. 1990).
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
13 September 2006
- Meeting paper published:
24 September 2006
- Revised manuscript received:
6 April 2007
- Manuscript approved:
2 June 2007
- Version of record:
25 February 2008
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