Equivalent time is essentially a time transformation which is used in order to apply drawdown analysis techniques to buildup data. The transformation takes shut-in time, or delta time, and converts it into an equivalent time using the following equation:

Note that this can also be defined as the flow
time (*t**c*)
divided by the Horner time as follows:

This definition is derived for radial flow
and thus works very well for analyzing wellbore
storage and radial flow regimes, but
breaks down in heterogeneous reservoirs or when boundaries come into play.
When analyzing bilinear
or linear flow regimes, corresponding
definitions of bilinear or linear
equivalent time are used. Equivalent time has little effect
on early time data but compresses late time shut-in data significantly.
The maximum value of equivalent time is flow
time (*t**c*)
as shut-in time (*D**t*) approaches infinity. Thus
no matter how long the well is shut-in, equivalent time cannot be greater
than *t**c*. Thus
the behavior of a buildup depends heavily on the flow
time.

The preceding definition of equivalent time assumes a constant rate before shut-in. When the rate has been variable, a complex superposition equivalent time function should be used.