Permanent ocean-bottom seismic cables were installed in 2003 at Valhall Field, offshore Norway, as part of a Life of Field Seismic (LoFS) program. Six seismic surveys have been acquired from the permanent system through 2005, in addition to two historical towed-streamer surveys acquired in 1992 and 2002. The various 3D seismic data sets generally show strong time-lapse or 4D effects resulting from primary production.
In a previous paper we described a workflow used to integrate 4D seismic data with conventional well data in a Top-Down Reservoir Modeling (TDRM) computer-assisted history match of a portion of the Valhall field.1 Two seismic attributes were used to help constrain the reservoir model - a two-way time shift from the surface-tow surveys and a sum of negative amplitude difference between two of the LoFS surveys. These two seismic attributes seemed to provide independent spatial information about the reservoir that was not contained in the conventional well measurements. History matching was also conducted on the conventional well data alone, without the 4D constraints. The results indicated that when run in prediction mode, the history matches that included the additional 4D seismic constraints provided a narrower uncertainty range on the resulting forecasts than did the models that only honored the well data.
In this paper, we examine some additional issues associated with the potential use of multiple time-lapse seismic attributes from the LoFS program in our ongoing quest to further constrain the reservoir history match. We focus in particular on the considerations for choosing an appropriate set of ‘target’ seismic attribute maps for the LoFS history matching effort, which deals with time-lapse intervals that are much smaller than those associated with typical 4D surveys.
The main ‘Tor’ reservoir at Valhall is a high-porosity chalk formation that has produced over 500 MMstb since 1982 under primary depletion conditions. Compaction of the soft chalk is a key drive mechanism that has resulted in high ntrinsic reservoir energy but has also led to seabed and latform subsidence. The compaction associated with primary roduction has also had a significant effect on the 4D seismicsignature. A waterflood program has recently begun which will extend the field production plateau and also have a significant impact on the 4D observations. Additional background material on Valhall can be found in Refs 1-4.
A conventional 3D marine surface-tow seismic survey was acquired at Valhall during 2002 and matched to a 1992 surface-tow survey, capturing 10 years of production history.2 Figure 1 shows a map of the time shift (TS), or time-lapse change in two-way travel time through the reservoir, extracted from the surface-tow surveys. The strong depletion/ compaction response seen in this map is the result of increasing bulk density and acoustic velocities in the reservoir with time.