Observations and quantitative analyses of waterflood patterns in a chalk reservoir using seismic data, Halfdan Field, Danish North Sea

Reservoir pressure maintenance and sweep of oil with water are the main objectives of the Halfdan field water flooding. The lower mobility of water compared to oil in the Halfdan field facilitates an effective sweep of oil but means that the water injectors require larger reservoir contact area in order to replace voidage. In Halfdan this has been the focus from first development and has been obtained by controlled formation of 10,000-15,000 ft long injection fractures by injection at fracturing conditions. Modelling has indicated that conditions for propagating injection fractures along the horizontal injectors were present, but until recently a field-wide verification of the expected uniform sweep has been pending seismic identification of the resulting water flood. After five years of production, a seismic survey was acquired covering the developed area. Analysis of the seismic data has revealed a strong signature reflecting the water flooding of the reservoir. The seismic imaging of the sweep patterns is observed directly in the 3-D data without any 4-D processing and is of an outstanding quality even compared to what is normally seen in dedicated time lapse seismic studies. The seismic observations confirm the modelled behaviour of the Halfdan field water flood. This paper presents examples of the seismic observations made and discusses these in a rock physics context including fluid substitution and pressure effects on acoustic impedance. It is demonstrated how the seismic results together with simple analytical models add to the quantitative description of injection under fracturing conditions by facilitating measures of fracture height, connectivity and extent.