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Chapter
3 – P ressure Transient Analysis (PTA)- p70/743
3.C
The ‘right’ stuff
To speak about ‘right stuff’ vs. ‘old stuff’ is deliberately provocative. Old stuff is not always
wrong. Old stuff was smartly developed to adapt to what we had before modern PC’s: slide
rules, graph paper, basic calculators, programmable calculators, etc. Old stuff has been of
great benefit to the industry and was on the critical path to get the modern tools we have
today.
What is sometimes wrong is to continue using old techniques that are less accurate than more
recent developments. What is definitely wrong is to continue using such tools today, simply
out of inertia, while everybody knows that their limitations are successfully addressed by more
modern techniques. What is near criminal is using the good old stuff to address completely
new problems that were never encountered when this good old stuff was developed.
Simpler is better when simple is right.
3.C.1
Before the Bourdet Derivative
Until 1983, PTA methodology was a manual process alternating type-curve matching and
specialized analyses. Type-curves without derivative had poor diagnostic capabilities. The
results from specialized plots would help position the data on the type-curve.
For example, drawing the IARF straight line on the Horner plot would give a permeability that
could be used to set the pressure match on the type-curve. Selecting a type-curve would give
the approximate time of start of IARF, which in turn would help define the Horner plot line. For
gas we would complement this with AOF / IPR analyses.
The shortcomings of this methodology were numerous:
Type-curves with pressure only had very poor resolution on a loglog scale.
Type-curves were generally printed for drawdown responses, and ruined by superposition.
Type-curves were set for a discrete number of parameter values.
Specialized plots required a pure flow regime that may never have been established.
Skin calculation on the Horner plot required that IARF was reached during the drawdown.
Drawing a straight line through the two last pressure points was a common practice.
The process required moving back and forth between at least two or more different plots.