In principle, drawdown tests provide data that allow a complete characterization of a gas well. Unfortunately, rapid analysis of drawdown test data is generally regarded as possible only for the special case in which production rate is maintained strictly constant for the duration of the drawdown test. In many tests, constant production rates are either inconvenient or impossible to maintain; analysis of data from such tests has been thought to require the tedious use of superposition, which, as a practical matter, means that a computer must be used practical matter, means that a computer must be used to perform the necessary arithmetic.
Actually, a technique for analyzing variable-rate drawdown test data easily and rapidly has been in the literature since 1965. This method, proposed by Winestock and Colpitts, is virtually as rapid as conventional techniques used to analyze constant-rate drawdown tests; and it provides estimates of formation permeability-thickness product, skin factor, and permeability-thickness product, skin factor, and turbulence effects. Knowledge of these parameters allows more accurate predictions of current and future deliverability of a tested well than are usually possible with conventional back pressure tests or isochronal tests.
This paper reports the results of a study which was performed to test the validity of the analysis technique proposed by Winestock and Colpitts. The study, which was based on a numerical simulation of the pressure-production history of a gas well, showed that the analysis technique of Winestock and Colpitts is indeed valid for pressure drawdown data obtained with smoothly varying production rates.
To completely characterize a gas well, a testing program should provide an estimate of reservoir permeability-thickness product, information about the extent of well damage, information leading to estimates of the possible effect of stimulation, and deliverability. Unfortunately, conventional gas well testing methods do not provide all this information. For example, the most common method is the Back-Pressure Test. This test has two major weaknesses: First, this test, at best, provides only deliverability estimates, and gives none of the other information needed to completely characterize a gas well.