Knowledge of reservoir saturation variation is vital for in-fill well drilling, while information on reservoir stress variation provides a useful guide for sand production management, casing design, injector placement and production management. Interpreting time-lapse difference is enhanced by decomposing time-lapse difference into saturation, pressure effects and changes in rock properties (e.g. porosity change in chalk reservoirs).
We analyze the stress and saturation sensitivity of the reservoir and overburden shale of Forties Field, located in the UK sector of the North Sea. While pore pressure variations have not been significant in most parts of the field, a slightly high decrease in pore pressure in a region of the reservoir has had a profound effect on both reservoir and overlying caprock. We find that strain development in the field accounts, in part, for increased reservoir sand production. We use changes in the AVO intercept and gradient calibrated with laboratory measurements to invert the time-lapse (4D) difference for saturation and pressure changes.