Abstract: The invention relates to a well test method for determining the physical characteristics of a system made up of a well and a subsurface formation containing a fluid such as a hydrocarbon. A change in the flow rate of said fluid is produced for a short period (duration t.sub.P of the order of a few minutes) so as to obtain a flow pulse resembling a Dirac pulse; the variations .DELTA.P of the down-hole fluid pressure is measured during said short period and then during the subsequent period of return to the initial state of the well-formation system, and the experimental pressure curve thus obtained is compared with the curves of a double network of type curves representing, as a function of a common parameter, the pressure P.sub.D and its derivative P'.sub.D with respect to time, by matching the branch of the experimental curve corresponding to the short period with a curve P.sub.D and the branch of this curve corresponding to the subsequent period with the curve P'.sub.D of the same parameter.

Abstract: Disclosed is a method for determining the physical characteristics of a system made up of a well and an underground formation containing a fluid and communicating with the well. A change in the rate of flow of the fluid is produced and a measurement is made of a parameter characteristic of the pressure P of the fluid at successive time intervals .DELTA.t. One then compareson the one hand, the theoretical evolution of the logarithm of the derivative P'.sub.D of the dimensionless pressure as a function of the logarithm of t.sub.D /C.sub.D, the derivative P'.sub.D being with respect to t.sub.D /C.sub.D, t.sub.D representing the dimensionless time and C.sub.D the wellbore storage (compression or decompression) effect, withon the other hand, the experimental evolution of the logarithm of the derivative .DELTA.P' of the pressure as a function of the logarithm of the corresponding time intervals .DELTA.t, the derivative .DELTA.P' being with respect to time t.