Summary
This paper discusses specific issues encountered when pressure tests are
analyzed in reservoirs with complex geological properties. These issues relate
to questions concerning the methodology of scaleup, the degree of aggregation,
and the reliability of conventional methods of analysis. The paper shows that
if we desire to use pressure-transient analysis to determine more complex
geological features such as connectivity and widths of channels, we need a
model that incorporates reservoir heterogeneity. This complexity can lead to
significantly more computational effort in the analysis of the pressure
transient.
The paper demonstrates that scaleup criteria, based on steady-state
procedures, are inadequate to capture transient pressure responses.
Furthermore, the number of layers needed to match the transient response may be
significantly greater than the number of layers needed for a
reservoir-simulation study. The use of models without a sufficient number of
layers may lead to interpretations that are in significant error.
The paper compares various vertical aggregation methods to coarsen the
fine-grid model. The pressure-derivative curve is used as a measure of
evaluating the adequacy of the scaleup procedure. Neither the use of
permeability at a wellbore nor the average layer permeability as criteria for
the aggregation was adequate to reduce the number of layers significantly.
Introduction
The objectives of this paper are to demonstrate the impact of the detailed
and small-scale heterogeneities of a formation on the flow characteristics that
are obtained from a pressure test and how those heterogeneities affect the
analysis of the pressure test. The literature recognizes that special scaleup
procedures are required in the vicinity of wells located in heterogeneous
fields. Our work demonstrates that these procedures apply only to rather small
changes in pressure over time and are usually inadequate to meet objectives for
history-matching well tests. Using a fine-scale geological model derived by
geological and geophysical techniques, this work systematically examines the
interpretations obtained by various aggregation and scaleup techniques. We will
demonstrate that unless care is taken, the consequences of too much aggregation
may lead to significant errors on decisions concerning the value of a
reservoir. Current scaleup techniques presume that spatial (location of
boundaries, location of faults, etc.) variables are maintained. In analyzing a
well test, however, one of our principal objectives is to determine the
relationship between the well response and geometrical variables. We show that
a limited amount of aggregation will preserve the spatial and petrophysical
relationships we wish to determine. At this time, there appears to be no method
available to determine the degree of scaleup a priori. Because the objective of
well testing is to estimate reservoir properties, the scaleup process needs to
be made a part of the history-matching procedure.
By assuming a truth case, we show that too much vertical aggregation may
lead to significant errors. Comparisons with traditional analyses based on
analytical techniques are made. Whenever an analytical model is used in the
analysis, unless otherwise stated, we use a single-layer-reservoir
solution.
© 2005. Society of Petroleum Engineers
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History
- Original manuscript received:
14 December 2002
- Revised manuscript received:
2 February 2005
- Manuscript approved:
10 February 2005
- Version of record:
15 June 2005
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