Making Sense of Water Injection Fractures in the Dan Field

Abstract

Abstract Water injection above fracture propagation pressure is used in the Dan field, a low permeability chalk oil field, to provide improved recovery. A pilot scheme began in 1990 and by 1995 the decision was taken to further develop most of the field using this concept. The pilot scheme indicated that the induced fractures have a general north-south orientation and a fracture length in some cases exceeding 4000ft. Several monitoring techniques have been applied to evaluate fracture height, length, orientation and injector/producer interaction, including: open hole and through casing saturation logging, tracer injection, producer water cut monitoring and falloff surveys in injection wells. To gain the maximum information from this data, a model of fracture growth was developed to integrate the different forms of data. Application of the model has resulted in an improved estimate of fracture height, length and near fracture permeability enhancement and has lead to a more comprehensive understanding of fracture growth mechanics in Dan. This paper presents the different monitoring techniques, the model development and various examples of the way the model has been used to enhance the understanding of field data. Introduction Water injection, as a means of enhancing recovery, has been practiced worldwide for many years m the oil industry. Frequently, small fractures are induced from injectors, sometimes as a consequence of thermal effects. Small fractures enhance injectivity but play a minor role in determining sweep patterns and reservoir management. In the case of low permeability reservoirs, it is possible to create large fractures, the size and orientation of which can have a profound effect on sweep patterns, producer placement and reservoir management. Water injection above fracture propagation pressure has been implemented in the Dan field, offshore Denmark. Following encouraging pilot schemes, most of this field is being further developed using water injection above fracturing pressure. The pilot tests indicated that the induced fractures would be large, thus creating fractures that would play a major role in determining the efficiency of the recovery process. In order to minimize the risks inherent in this process, the field development has been undertaken with continuous monitoring of fracture growth, to ensure the process was as expected. For a more general discussion of the development of the Dan Field using this concept, the reader is referred to the recent paper by Larsen et al. Background The Dan field, located in the Danish sector of the North Sea, began production in 1972, making it the oldest producing oil field in the entire North Sea (Fig. 1). The reservoir is located in Tertiary Danian and Cretaceous Maastrichtian chalk formations, which are characterized by high porosities (20-40%) but low matrix permeabilities (0.5 – 2 mD). Structurally, the field is a dome, with a south-west to northeast fault dividing the field into two blocks, A and B. In both blocks a sizeable gas cap exists. The bulk of reserves lies in the Maastrichtian units, although considerable reserves also lie in the overlying Danian. At the base of the Danian a tight, flinty hardground is present that could act as a barrier both to fluid flow and vertical fracture growth. Stylolite associated extension fractures increase effective permeability by a factor 2–3 above core measured rock permeability. Tectonic fractures are rarely observed except in the immediate vicinity of the main fault. P. 887^