Stress paths and predicted time-shifts around a depleting reservoir

Time-lapse (4D) seismic is an essential tool for monitoring the subsurface in and around producing hydrocarbon or CO2 storage reservoirs. The seismic time-shifts, in the reservoir as well as in the overburden, depend on the stress changes and strains induced by the subsurface depletion or the inflation. In this study, geomechanical modeling is used to quantify the stress changes and strains in a synthetic model for the formations in and around a depleting reservoir. The estimated strains are coupled to experimentally determined strain sensitivities for P-wave velocities of shales, to predict time-shifts in the surroundings of the reservoir. The modeling shows that the stiffness contrast between the reservoir and its surroundings plays an important role in controlling the stress and strain changes in the subsurface. The strain sensitivity of the vertical P-wave velocity in the surroundings is significant and is rapidly increasing in magnitude with the proximity to the reservoir. Correspondingly, the time-shifts are increasing with depth in the overburden and decreasing with depth in the underburden. In this study, the time-shifts of the surroundings are changing most between the depths corresponding to one and two reservoir radii above and below the reservoir.