## WHAT IS A "DERIVATIVE PLOT"

A derivative plot is a slightly different way to look at a traditional
Horner plot. The big advantage of a derivative plot over the more
traditional well-test analysis plots is that ALL the main well
and reservoir characteristics are shown clearly on a single plot.
This allows you to associate a pattern in the derivative plot
to a particular well and reservoir description. I strongly recommend
using the derivative plot; it really helps to improve a well-test
analysis! However, don't get carried away; you still need to look
at the traditional plots to do a complete analysis.

Here is an example of a Horner plot for a well with damage in
a simple "infinite-acting" (i.e. no reservoir boundaries)
system (time increases to the LEFT). Note the initial "S"
shape for the well-bore storage effects followed by a straight-line.

A Derivative plot is displayed on log-log scales and is composed
of two curves; an upper curve to plot delta-pressure verses delta-time,
and a lower curve to plot the derivative verses delta-time. Why
is this lower curve called the "derivative"? Because
it is the first derivative of the Horner plot i.e. the instantaneous
slope along the curve of the data on a Horner plot. Here is an
example of the derivative plot; note how the well-bore storage
"S" from the Horner plot becomes a "hump"
with an initial UNIT SLOPE, and the straight-line on the Horner
plot becomes a STABILISED derivative value.

And that is all there is to it. All of the other traditional plots
(e.g. a square-root plot) will have a characteristic shape on
the derivative plot (e.g. a half-slope).

__Q: Get it?__

A1: No, but I trust you...

A2: Yes!