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Dynamic Data Analysis – v5.12.01 - © KAPPA 1988-2017
Chapte
r 4 – R ate Transient Analysis (RTA)- p160/743
4.F.2
Common tools
RTA and PTA methods share the same assumptions in terms of the use of the diffusion
equation and limiting conditions. Most of the analytical and numerical models developed in PTA
may be used in RTA with minor adjustments, such as the ability to use the pressures as the
input to simulate the rates with superposition.
Modern RTA and PTA share a similar path. After loading, synchronizing and extracting data,
one first tries to run a diagnostic using specialized plots and straight lines. An analytical or
numerical model is then run, and an optimization process adjusts the parameters to minimize
the difference between the simulated model response and the observed data.
4.F.3
PSS vs. IARF
The main regime of interest in PTA is Infinite Acting Radial Flow (IARF). We look primarily for a
stabilization of the Bourdet derivative. When IARF is detected, specialized analysis will give a
value of mobility and a total equivalent skin factor. We can refine this and diagnose other well,
reservoir and boundary behaviors from various parts of the response; however the starting
point will always be IARF.
The main regime of interest in RTA is Pseudo Steady State (PSS). We look primarily for a unit
slope on the Loglog or the Blasingame plot. Specialized analysis will determine the size of the
well drainage area from the slope, and the intercept will be a function of three main factors:
the well productivity index, the mobility and a shape factor. More complex models could be
used, but there may not be enough information to determine the additional parameters.
However the pressure transient results may be used to determine these.
4.F.4
Diagnostic capabilities
One of the key steps in PTA is the diagnostics, where, based on the derivative behavior the
engineer decides which model could most appropriately be used to perform the analysis. This
is made possible by very clean data and constant production i.e. zero, during the build-up.
Production history may be so scattered that the responses will be dominated by transients. In
this case there is no way to identify pseudo steady state behavior. This may happen even
though the well is still producing and the pressure is declining globally.
Despite the lack of pure PSS behavior it will be possible with a model to history match the data
and obtain a reliable drainage area estimate and even sometimes discriminate mobility, skin
and shape factor. No specialized plot will show such a behavior. So the use of models and
optimization is likely to change the way RTA is performed completely, even more radically than
happened with PTA.