Sammendrag
Depletion or injection into a reservoir implies stress and strain changes in the reservoir and its
surroundings. This may lead to measurable time-shifts for seismic waves propagating in the subsurface.
We have measured multi-directional ultrasonic P-wave velocity changes for three different field shale
cores, each probed with four different stress paths (i.e. different ratios between the horizontal and the
vertical stress change), to systematically quantify the time-shifts for overburden shales with respect to
ray angle (offset). The laboratory data show that for a given offset, the time-shifts are stress path
dependent, where the isotropic stress path is associated with larger time-shifts as compared to the
constant mean stress path or the triaxial stress path. Generally, the time-shifts are largest for zero offset
(propagation normal to the bedding) and are decreasing for increasing offsets. The constant mean stress
path has the most significant decrease of time-shifts with offset. By utilizing pre-stack seismic offset
data, such controlled laboratory experiments can be used to constrain the inversion of 4D seismic data
to quantify the stress and strain changes due to production. This may have important implications for
improved recovery and safety, particularly in mature fields.
Main
surroundings. This may lead to measurable time-shifts for seismic waves propagating in the subsurface.
We have measured multi-directional ultrasonic P-wave velocity changes for three different field shale
cores, each probed with four different stress paths (i.e. different ratios between the horizontal and the
vertical stress change), to systematically quantify the time-shifts for overburden shales with respect to
ray angle (offset). The laboratory data show that for a given offset, the time-shifts are stress path
dependent, where the isotropic stress path is associated with larger time-shifts as compared to the
constant mean stress path or the triaxial stress path. Generally, the time-shifts are largest for zero offset
(propagation normal to the bedding) and are decreasing for increasing offsets. The constant mean stress
path has the most significant decrease of time-shifts with offset. By utilizing pre-stack seismic offset
data, such controlled laboratory experiments can be used to constrain the inversion of 4D seismic data
to quantify the stress and strain changes due to production. This may have important implications for
improved recovery and safety, particularly in mature fields.
Main