The 4D Experience in TOTAL

Abstract

Born nearly thirty years ago, time lapse (4D) seismic monitoring technology has been developed in such a way that it has finally proven to monitor fluid movement and to distinguish between drained and undrained portions of a reservoir. Its ultimate aim is to quantitatively improve reservoir models, particularly their predictive capability. Indeed, the benefits of time-lapse seismic for reservoir characterisation depend on the quality of 4D acquisition and processing, but they also greatly depend on the particular 4D inversion and interpretation schemes used; finally, a decisive aspect is certainly also the capability of integrating results from different disciplines in an effective way. In fact, the 4D success comes through close interaction between Geophysicists, Geologists, Rock Physicists, Geomechanicists, Reservoir Engineers and Drillers. Timing is also crucial: results delivered in a few months can have a direct operational impact such as optimising well locations.

For the last ten years, Total has recognised the importance of time lapse seismic and has therefore conducted 4D seismic monitoring in different geological environments. Examples of 4D experiences range from monitoring of water injection and production for reservoir management and field development in the Gulf of Guinea (Angola Block 17, and Nigeria); monitoring of geomechanical effects in HPHT fields (Elgin-Franklin, UK), in compacting reservoirs in Norway (Ekofisk and Valhall) and in the Gulf of Mexico (Matterhorn, US); monitoring of steam chamber in tar sands (Surmont, Canada) and monitoring of compaction and water rise in carbonates (South-East Asia).

To illustrate capabilities of time lapse seismic monitoring two cases are presented. The first case is a deep offshore field in Angola with turbiditic stacked channel reservoirs with a good 4D response. The main 4D effect is given by fluid substitution and in particular by gas both injected and generated by a small depletion, the initial pressure being close to bubble-point; in this case the seismic quality and repeatability are outstanding and in fact the fluid change in the reservoir is very well resolved. The second example pertains to an HPHT field in the UK where the seismic quality is degraded due to the extreme depth of reservoir burial (>5000m) as well as the vicinity of the platform. It shows 4D effects due to a dramatic pressure drop; it is a significant result because it shows that appropriate tools enable achieving reliable results even in critical conditions.