Summary
Boundary effects are often observed in buildup data—or at least that is the
conclusion frequently drawn from an observed increase in derivative on a
log-log plot or an increase in slope on a semilog plot. Furthermore, if (for
instance) it is concluded that the effects of a sealing fault are seen in a
given data set, then simple line methods or direct analytical-modeling efforts
are normally used to determine the distance to the boundary. A sealing fault is
the normal choice of boundary model if a doubling is observed in derivative or
semilog slope. If a four-fold increase in derivative is observed, then a model
with the well placed somewhere between two sealing faults forming a right angle
would be a normal choice. But what if the two faults are not sealing? If the
flow capacity on the other side of the faults is only one-third of the value on
the well side, what will be the derivative characteristics?
Problems like these are addressed in detail in this paper, with a series of
simple rules given for possible combinations that will generate buildup data of
a specific type (i.e., with specific “familiar characteristics”). The rules can
be used to list alternative interpretations without running separate analyses.
For instance, it is shown that the derivative characteristics of any sector
model bounded by sealing faults correspond to an infinite number of two-zone
sector models with an angle between the boundaries and permeability contrast
satisfying a single equation. Other pairs of models with similar
characteristics are models with partially sealing faults and specific
three-zone sector models, and either of these types of models and radial
composite models. This clearly complicates analyses.
Also addressed are problems related to possible differences in the boundary
effects observed in drawdown and buildup data for certain models. As one
example, U-shaped and sector models can have identical buildup characteristics
over a wide time range, although drawdown data from the models have distinctly
different boundary characteristics. Radial composite and composite sector
models are also of this type, with potentially significant differences between
drawdown and buildup data. The reason for bringing up such cases is to
emphasize the importance of attempting to collect high-quality drawdown data in
addition to buildup data to limit the range of possible interpretation
models.
For completeness, effects of uncertainties in basic input parameters on the
final analyses are also covered in the paper.
Introduction
It is well known that buildup data from a well near a sealing fault might
exhibit a doubling of derivatives on a log-log diagnostic plot, as illustrated
in Fig. 1. This doubling of derivatives corresponds to a doubling of slope on a
semilog plot, as shown by Horner, and refers to a change between early and late
data requiring storage effects to become negligible and radial flow to be
reached before the onset of boundary effects. For this behavior to occur, it is
also necessary for the flow period before shut-in to be long enough to be fully
or almost fully affected by the boundary effect.
© 2005. Society of Petroleum Engineers
View full textPDF
(
686 KB
)
History
- Original manuscript received:
18 November 2004
- Revised manuscript received:
5 July 2005
- Manuscript approved:
5 August 2005
- Version of record:
15 October 2005
View Cart
