The well test analysis techniques of falloff and buildup tests usually assume that at the end of the period of injection or production, the wellbore storage effect is negligible; nevertheless, this assumption is not always valid in impulse tests due to the short duration of the flow time. This situation is important in low mobility reservoirs, where the wellbore storage effects may last hours or days.
In this work, impulse tests are studied for the cases in which the period of injection/production is dominated by the wellbore storage effect. New equations are presented and it is demonstrated that the conventional expressions, existing for decades in the technical literature, must be modified since it is not feasible to apply them under these operational conditions.
Based on the results presented in this work it is concluded that in general, the pressure response of the unit slope period observed at early shut-in times, depends not only on wellbore storage coefficient and rate, but also on the formation transmissibility, skin factor, injection/production time, and the parameters that intervene in the definition of the dimensionless wellbore storage coefficient. In addition, guidelines are proposed for the impulse tests analysis, which combine the use of the shut-in pressure change, the superposition time pressure derivative, and the second drawdown derivative response.
With the aim to illustrate the validity of the new approximations, several cases of tests with short time of injection/production are shown, in such a way that it is possible to identify situations for which the classical expressions of well test analysis are not valid and must be substituted by the equations introduced in this work.