Variation of velocity with effective stress in chalk:null result from North Sea well data

Abstract

Wireline log data and pore pressure measurements from eight wells have been analysed to investigate how velocity in the Chalk of the Central Graben, North Sea depends on porosity and vertical effective stress. The main conclusion is that velocity shows no detectable dependence on vertical effective stress when porosity and vertical effective stress are treated as independent variables. This result contrasts with the behaviour of shales, which exhibit a reduction in velocity on unloading. The significance is that velocity in the Chalk cannot be used to detect the presence of any overpressure caused by unloading. It is suggested that the absence of an observable velocity reduction in unloaded Chalk is due to cementation.