The two-cycle drill stem test, consisting of an initial flow period, an initial shut-in period, a second flow period and a second shut-in period, remains the most common type of test run in the field. Unfortunately, the most widely used method of analyzing data for these tests can result in significant errors. By far the most widely used method for DST analysis is a Horner plot prepared using Tp = t1F + t2F. In this method, an average flowrate is used in the calculation. Earlougher1  suggests that plots can be based on Tp = t2F or Tp = t1F + t2F, but is decidedly vague on when one is preferable to the other. Actually, the selection of Tp is not nearly as significant as is the selection of the flowrate to be used in the calculations.

In practice it seems that most tests are analyzed with Tp = t1F + t2F presumably because it generates a straighter line on the Horner plot. Unfortunately, the slope of this straight line can be very misleading. This is not to say that this slope is "wrong" since Horner slope in and of itself is not the parameter we are seeking. Only when combined with a flowrate does the Horner slope take on any physical significance. Thus while a Horner slope itself cannot be "wrong, " the combination of a Horner slope with an inappropriate flowrate can result in a "wrong" answer. Conversely any slope can be the right slope if it is combined with the appropriate flowrate. Thus we must remember that any discussion of Horner slopes must be rate-specific.

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