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Other type-curves were later published to address further complex configurations including
layered and fractured reservoirs. By the end of the 1970’s at this stage the methodology was
somewhat equivalent to the standard procedure in PTA in the late 1970s. The Arps plot was
the counterpart of the Horner plot, and the constant pressure type-curves were the
counterpart of the PTA constant rate type-curves.
Superposition and derivative came ten years later, shortly after the publication of the Bourdet
PTA derivative. Two formulations of a loglog plot were made available. The Blasingame plot
showed productivity index and derivative on the time scale, while the loglog plot showed rate
normalized pressure, providing a response similar to its PTA counterpart. In both cases, on the
time axis, the logarithm of elapsed shut-in time was replaced by the logarithm of the material
balance time, in order to align long term Pseudo-Steady State (PSS) responses.
Fig. 4.A.5 – Blasingame plot:
Normalized rate Pressure
Fig. 4.A.6 – The loglog plot:
Normalized pressure rate
At this stage, RTA’s theory had caught up with PTA. However, while the Bourdet derivative and
PC based software had an immediate and dramatic impact on day-to-day PTA, this did not
happen as fast for RTA, where most work continued to be done using Arps and Fetkovich
methods, generally as an annex to a production database.
The use of modern RTA methods and modern software only developed early 2000s, partly due
to the development of permanent pressure gauges. When engineers started receiving long
term continuous pressure data, the first reaction was to load this into a PTA program: “I have
rates; I have pressures, so I treat this as a well test”. However PTA methodology was not
designed for this type of data. Material balance errors and over-simplifications using Perrine’s
approach for multiphase flow property evaluation were among the most frequently
encountered errors.
Another recent factor for the development of modern RTA was the recent investments in
unconventional plays, where RTA, because of the very slow and long transient responses,
somehow replaced PTA as an analysis tool.