In the high-porosity, poorly-consolidated turbidites of the Deepwater Gulf of Mexico (GoM), production induced compaction is the main production-drive mechanism when aquifer support is weak and prior to pressure support by secondary recovery water injection. Time-lapse (4D) seismic monitoring of this class of reservoirs has provided several new learnings. The time-lapse amplitude response of these fields can be complicated due to saturation changes (water replacing oil) inside the reservoir, rock compaction causing density and velocity changes inside the reservoir, stress relief and associated deformation of the rock outside the reservoir, and changes in reservoir fluid pressures due to pore pressure decrease. Methods that rely on time-lapse amplitude changes with offset to discriminate pressure and saturation changes can help separate and thus simplify the interpretation of some of these effects (Tura & Lumley, 1999; Landro, 2001).
In addition to 4D amplitudes, 4D time-shifts are also observed due to alteration of the stress state inside and outside the reservoir resulting from changes in the pore pressure from production. This causes a physical displacement and also a velocity change in the rocks (due to compaction in the reservoir and expansion outside). 4D time-shifts occur in areas of depletion and the overburden, and have the potential to act as indicators of compartmentalization in the reservoir. Compartmentalization information can help better place new production and injection wells, and new sidetracks for optimized field development. Travel time changes are mostly sensitive to reservoir compaction caused by depletion rather than fluid saturation changes, and as a result are simpler to interpret compared to amplitude changes. 4D time-shifts are modeled here by computing the stress field and displacements using geomechanics, and then matched to field data (Guilbot & Smith, 2002; and Hatchell et al., 2003).
In primary-depletion reservoirs, the above mentioned 4D amplitude and travel time changes are not large. Small to moderate amplitude changes manifest at the oil-water contact or in large pressure depletion areas. The travel time changes can be on the order of one to several time samples of the typical seismic sampling interval of 4 milliseconds (ms). In addition to this, particularly in the GoM, the overburden can be quite complex with large faulting, salt bodies, steep dips, near surface channels and bodies, diffractors, and overpressured regions. As a result, 4D monitoring of this class of reservoirs requires highly repeatable seismic data with very accurate positioning of sources and receivers between surveys.
In the GoM, it is often difficult to obtain high repeatability with 3D streamer surveys due to strong currents. A novel 2- boat 3-pass acquisition method to overcome acquisition repeatability issues is investigated by the Shell Mars/Europa 4D study team. Using seismic data with this new acquisition over the Mars and Europa fields in the GoM, we are able to obtain better repeatability. In addition, we are able to match amplitude changes computed from synthetic seismic (via history matched reservoir simulation models) and also traveltime changes computed from geomechanical models of these fields.
