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Chapte
r 4 – R ate Transient Analysis (RTA)- p161/743
4.F.5
Validity of the PTA hypothesis in Rate Transient Analysis
PTA can provide a clean snapshot of what the well and reservoir system is at a given time. RTA
covers a much wider time range, and some of the assumptions valid during a single well test
will not be true over the well producing history. The three main differences are related to the
well productivity, the drainage area and multiphase production.
PTA models account for rate-dependent skin. It is also known that the well may be cleaning up
during the initial production phase. So the well productivity may not be constant during a well
test. However this is a reasonable assumption for a single build-up, and optimization will be
possible with a single mechanical skin model. In RTA this is unlikely. Well productivity does
change over time, and no optimization process is reasonably possible over a long period of
time without considering a time-dependent skin.
In PTA, boundary effects are generally material boundaries, even though interfering wells can
produce the same effects as boundaries. In RTA we consider the well drainage area. Except
when there is only one producing well, part or all of the drainage area boundaries are
immaterial, depending on the flow equilibrium between the neighboring wells. The drainage
area will change in time when new wells are produced, or even when the flow rates change
asymmetrically. To account for these changes, a multi well model, either analytical or
numerical, may be required.
In PTA we approximately handle multiphase cases using pseudo-pressures or considering that
saturations are constant and the flow can be modelled with an equivalent single-phase fluid as
in Perrine’s method. In RTA, solutions exist that consider that individual fluids develop PSS
independently. However these solutions make a global correction for multiphase production
but they are unable to forecast the breakthroughs. There is a point where only a history match
with a numerical model can account for multiphase production.