Stress-path dependence of ultrasonic and seismi velocities in shale

Abstract

4D seismic time-shifts are induced by stress changes in the overburden as a result of depletion from or injection into an underlying reservoir. This represents a powerful technique for tracking unrecovered hydrocarbon resources or identifying leakages from CO2 storage sites. Here we describe how the sensitivity of seismic velocities of overburden shale to reservoir pore pressure changes can be estimated from laboratory experiments. Both ultrasonic and seismic frequency laboratory measurements show stress and stress path dependent velocities. The role of the stress path is seen to be extremely important, which means that geomechanical modeling is required in order to assess in situ stress sensitivity. The experiment presented here shows higher stress sensitivity in the seismic frequency range than for ultrasonic waves.