Abstract: A method for determining desired physical characteristics of an underground formation is provided. The present invention is characterized by a determination of dimensionless pressure parameters without resort to a series of type-curves. The method includes finding a pressure match that relates a dimensionless function of pressure with experimental pressure data and finding a time match that relates a dimensionless function of time with dimensioned time. In one embodiment, this determination is made using a pressure derivative curve plotted on a computer terminal display screen. An interactive graphics software package is utilized in which the user selects certain values associated with the experimental data for which corresponding dimensionless values are known. From the determined values of pressure match and time match, a dimensionless function of pressure curve and a dimensionless pressure function derivative curve are provided on the display screen.