Seismic Observation and Verification of Line Drive Water Flood Patterns in a Chalk Reservoir, Halfdan Field, Danish North Sea

Abstract

Abstract 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. Introduction The Halfdan field is located in the Danish North Sea Central Graben, approximately 250 km west of the West Coast of Denmark (Figure 1). The Halfdan accumulation was discovered in 1999 as the latest of the larger Upper Cretaceous Chalk reservoirs present in the region1 so far. Trapping of hydrocarbons is caused by a combination of stratigraphic and fluid dynamic conditions in contrast to other fields in the region that are trapped below structural closures. An early assessment of the Halfdan field development was given by Albrechtsen et al.2. The Halfdan field is operated by Mærsk Olie og Gas AS on behalf of the DUC partnership (A.P. Møller-Mærsk AS, Shell Olie- og Gasudvinding Danmark B.V. (Holland) and Chevron Denmark Inc.). A regionally extensive but relatively thin oil column characterizes the Halfdan field. Wells are drilled in a parallel pattern with horizontal well sections of 10,000–15,000 ft length and at a well spacing of 600 ft. The Halfdan field is developed with alternating production and injection wells giving 1:1 producer-injector coverage. Injection in Halfdan targets maintenance of the reservoir pressure (PRES) at or above the initial pressure (PINIT). The conditions are that water injected has a far lower mobility than the hydrocarbons produced and that the producing drawdown (PINIT - Pp ˜ 3,000 psi) is far larger than the pressure head at the injector side (PINJ - PINIT ˜ 1,000 psi). These conditions would curtail the target for pressure maintenance in a 1:1 producer-injector scenario if injecting under fracturing conditions was avoided. Fracturing was therefore planned from early on2, 3, 4. Injection fracturing in Halfdan follows Mærsk Olie og Gas AS patented FAST concept. The technique builds on flow induced stresses, which causes injection fractures to grow along the wellbores of the horizontal injectors. An important element in FAST is patience, because production and injection for a pre-defined period must precede fracturing in order to control the direction of propagation3, 4. Recently, a seismic survey was acquired over the area. The interpretations published herein are those important for the conception of the Halfdan water flood and for water injection theory in general.