Abstract: Methods and devices for determining the quality of an oil well reserve or the like delivering a given fluid derived from a productive layer, by measuring the response R of the well. The method implemented by the device comprises modulating the fluid flow rate (4) in the well (1) by a sinusoidal function with a controllable preventer (5), and measuring the variations in flow rate and pressure of the fluid respectively with a flow meter (7) and a pressure sensor (9), the response Rc of the well (1); if the productive layer comprises a damaged zone having a positive formation damage of non-null thickness, is obtained by the equation (I), and the response Rf of the well; if the productive layer comprises a fracture having a negative formation damage, is obtained by the equation (II). The invention is applicable to oil prospecting.