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.