Overburden 4D seismic analysis: Influence of stress and pore-pressure changes accounting for elastic contrast between a reservoir and its anisotropic surrounding rocks

Time-lapse (4D) seismic data analysis is widely used to monitor producing hydrocarbon or CO2 storage reservoirs and their surroundings. The influence of potentially significant undrained pore-pressure changes around depleting (or injected) reservoirs appears to be neglected in most previous studies. The 4D seismic response in the overburden depends on the stress changes and strains induced by the reservoir. These alterations may challenge stable infill drilling and well integrity. We have used anisotropic static elastic moduli from laboratory measurements for two different field shales to predict stress changes and strains in the surroundings of a depleting reservoir through geomechanical modeling. These mechanical changes are used to quantify undrained pore-pressure changes using anisotropic poroelastic theory constrained with Skempton parameters. Finally, an empirical rock-physics model quantifies the impact of these poroelastic changes on vertical velocity changes and time strains. This model explicitly quantifies the individual contributions to vertical velocity changes from undrained pore pressure, mean stress, and shear stress above (and below) the depleting reservoir. We find that a stiffer surrounding rock exhibits significantly larger vertical time strains and time shifts in the overburden as compared with softer surrounding rocks because of promoted arching. Undrained overburden pore-pressure changes significantly contribute to overburden velocity changes and time shifts. Ignoring anisotropy in the Skempton parameters can lead to overestimating the overburden undrained pore pressure and time shifts after reservoir depletion. The anisotropy in static stiffnesses significantly impacts the dynamic changes in the vertical direction, and the dynamic changes are particularly pronounced in the vicinity of the reservoir. We find how a tailored rock-physics model can link 4D seismic data and geomechanics to separate stress, strain, and pore-pressure changes - all essential for optimizing drilling and production.