The reservoir pressure can be estimated from the last recorded pressure as picked by the testing company if the build-up curve has reached some semblance of stabilization.
If the buildup curve has NOT achieved stabilization, a Horner plot of the shut-in pressure will have to be constructed. Most modern DST's include a Horner plot as a standard part of the reporting process.
The Horner plot is used to identify the initial or static reservoir pressure from the build-up data obtained from the DST, by extrapolating these data to "infinite time" (i.e., a Horner plot is constructed so that the time axis has a minimum at log(1)).
The Horner technique relies on a unique solution to the diffusivity equation derived by Horner. This solution states that, if homogenous, radial flow is present in the build-up data, then a plot of pressure versus the function log((T+dt)/dt) will create a straight line on this plot. As a result the interpreter can extrapolate this straight line to the log(1) value on the time axis to obtain the pressure that would be obtained if the build-up were left for an infinite length of time. The value of T is obtained by summing all of the flow times prior to the build-up, and is called the total flow time. The values of dt are obtained by subtracting the elapsed time from the initial start time to obtain the incremental delta time.