Jan Age Stensen

SWAG injection on the Siri Field: An optimized injection system for less cost

Simultaneous Water And Gas (SWAG) injection has been implemented on the Siri Field on the Danish Continental Shelf and represents the first reported full field application of its kind in the North Sea. The associated produced gas is mixed with injection water at the wellhead, and injected as a two-phase mixture. The required total injection volume for voidage replacement is thus achieved with a simplified injection system, fewer wells and reduced gas recompression pressure requirements. Injection per well has typically been in the range 4,000-8,000 Sm 3 /day (25,000-50,000 bpd) water and 200,000 - 400,000 Sm 3 /day (7-14 Mscf/d) gas. Evaluation of alternative injection schemes identified SWAG as the optimum scenario for Siri. The choice reflects that: a) There is no established gas export infrastructure in the immediate area, Siri gas volumes alone are too small to warrant establishment of a system, and routine gas flaring is unacceptable. Reinjection is therefore required. b) Reservoir simulation studies indicate improved oil recovery (IOR) with combined gas and water injection as compared to pure water injection, apparently related to attic oil displacement, reduced residual oil saturation and better sweep efficiency. c) Continuous water injection from both injectors is required to maintain reservoir pressure. The SWAG concept fulfills all these requirements, representing a safe, economic and environmentally-friendly development solution.