the result of the smoothing by an equivalent signal sampled with constant step the method adopted is a simple estimation by linear interpolation which consists in constructing the curve joining the points of the test by straight line segments the estimated points constitute sampling with constant step t of the points of this curve the results of this estimation of the value of the first derivative of the pressure at the well bottom are then filtered by applying a butterworth filter the general transfer function of which is expressed as follows equ1 in which a 1 1 u221a 2 u u 2 fc is the cut off reference of the filter chosen to be equal to 1 0 the results p 39 d of the filtering of the smoothed first derivative of the pressure at the well bottom which are obtained in this way are represented in fig4 the results make it possible to calculate the second derivative of the said pressure by applying the following formula cd represents the dimensionless capacity of the well calculated from c u03b4v u03b4p with u03b4v representing the variation in the volume of fluid resulting from a variation u03b4p in the pressure applied to the well in order to calculate this second derivative at a point m on the p 39 d curve a window centred on m and with given width is opened and a linear regression of the points contained in the window is carried out the value of the derivative is thus the.