Abstract: Within engineering accuracy, the time for the casing external extrusion pressure reaching a stable value isrelated to the relaxation time of the stratum, and further tothe viscosity of the stratum. An expression to estimate the viscosity ofstratum using logging information is proposed. Applying this method, theorder of magnitude of viscosity of stratum in west zone of Fuyu oilfield andQian'an oilfield is estimated as 10^17\,Pa\cdots and10^16\,Pa\cdots, respectively, as is consistent withthe range of viscosity of stratum obtained by rock test and geophysical method.