Insights into sweep efficiency using 4D seismic at Halfdan field in the North Sea

Abstract

A repeat 4D seismic survey was acquired in 2012 over the Halfdan oil field in the Danish North Sea Central Graben. The main objectives were to understand the lateral and vertical waterflood sweep in the reservoir, to identify unswept areas, to guide future well interventions, and to improve the reservoir model. The main oil reservoir is the Cretaceous chalk Tor Formation, and the field is developed via a line-drive waterflood. The reservoir saturation changes caused by water injection lead to large changes in acoustic impedance which can be imaged with 4D seismic. There were two main 4D processing projects for the Halfdan field. The 2012–2005 4D data were processed as a stand-alone project because of the high repeatability of the surveys, and a separate three-vintage processing project for all time steps also was carried out. When the three-vintage processing was not available, the original 4D processing was used for the 2005–1993 time step. Seismic inversion was performed because production effects have a large impact on the acoustic impedance in the chalk reservoir. The rock-physics model was also used to model 4D acoustic-impedance changes based on pressure and saturation changes in the reservoir. The 4D acoustic-impedance changes from the seismic and reservoir model were analyzed in conjunction with the production and geologic data on a well-by-well basis to better understand sweep efficiency.

Enriched Galerkin Discretization for Modeling Poroelasticity and Permeability Alteration in Heterogeneous Porous Media

“…The last example is based on the configuration of a layered hydrocarbon field in the North Sea [53,54]. The reservoir is exploited by several long (2-5 kilometer) parallel horizontal wells.…”

An offshore reservoir monitoring system based on fibre-optic distributed sensing of seabed strains

“…Many of the Danish gas fields in the North Sea are produced by natural depletion with gas/fluid expulsion and related compaction as drivers (Hatchell et al 2007;Danish Energy Agency 2013;Calvert et al 2014;Schutjens et al 2019). Often, reservoir compaction is translated into seabed subsidence, which has a direct adverse effect on production facilities.…”

“…Initially, the Dan field was produced by natural depletion -with water injection initiated only after several years of production. Analysis of timeshift data from the 4D seismic surveys acquired in the Halfdan/Dan field in 2005 and 2012 (Calvert et al 2014; indicated that waterflooding might have slowed down reservoir compaction in the area. Nonetheless, albeit reduced, seabed subsidence has continued -suggesting that the compaction drive remained active.…”

“…Nonetheless, albeit reduced, seabed subsidence has continued -suggesting that the compaction drive remained active. Modern developments, such as the Halfdan Field (Danish Energy Agency 2013), have parallel injection and producer wells resulting in near-zero seabed subsidence (Calvert et al 2014;.…”

Stability Analysis of Radial Jet Drilling in Chalk Reservoirs

“…In the Upper Maastrichtian, the average porosity can have values of 25-30%, whereas a sharp decrease in porosity to about 10-15% is observed in the Lower Maastrichtian to Campanian layers (Albrechtsen at el., 2001). Data from 4D seismic survey and reservoir modelling in conjunction with the production and geologic data suggest that there might be unswept areas in the deep reservoir layers (Calvert et al, 2014). RJD technique is considered as a potential method for accessing the unswept areas and improving oil recovery.…”