A method for characterizing a subsurface formation using a fluid pressure response during wellbore drilling operations includes the steps of determining a change in wellbore pressure proximate the surface, calculating a change in volumetric flow rate out of the wellbore as a function of the change in wellbore pressure proximate the surface, determining a downhole fluid pressure in the wellbore corresponding to the change in wellbore pressure proximate the surface and determining a productivity index value as a function of the change in volumetric flow rate, the downhole fluid pressure and a reservoir pressure.