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 Sm3/day (25,000–50,000 bpd) water and 200,000 - 400,000 Sm3/day (7–14 Mscf/d) gas.
Evaluation of alternative injection schemes identified SWAG as the optimum scenario for Siri. The choice reflects that: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.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.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.
Introduction
The Siri Field, discovered late 1995, is located in the Danish Sector of the North Sea (Figure 1). Production started in March 1999 and injection in June the same year. Plateau oil production is 8,000 Sm3/d (50,000 bpd).
The field has been developed with five producers and two SWAG injectors (one horizontal). The injectors are placed at the periphery of the reservoir in order to displace the oil to the central part of the field. Original plans called for 3 injectors, but this was reduced to 2 as the field was further delineated, making injection regularity and successful SWAG implementation even more critical.
Siri's fairly isolated location meant that connection to an existing gas export infrastructure was not feasible. At the same time, the relatively small amounts of gas produced, and the rapidly falling gas rate, made it clearly uneconomical to develop a gas export solution for Siri alone. Gas flaring, or reinjection to a disposal site, were not environmentally acceptable alternatives, despite the limited volume involved. Reinjection of the gas to provide reservoir pressure support, better sweep and hence enhanced recovery, was the best overall solution.
Reservoir Description
The reservoir is characterized by a relatively low relief structure with oil zone thickness of up to 25m. The GOR is moderate, in the region of 100 Sm3/Sm3 (562 scf/bbl) and there is no initial gas cap. An 80–100 m thick underlying water zone gives some pressure support.
Expected recoverable oil reserves have been estimated at 8.1 mill. Sm3 (51 mill. bbls), representing a recovery factor in excess of 35%.
The reservoir rocks in Siri are deposited by sediment gravity flows in a deep marine environment. Hydrocarbons are found in the Heimdal sandstone of Late Paleocene age at approximately 2,070 mMSL. The formation consists of firm, fine to very fine-grained sandstone with a high glauconite content, cross-bedded to massive, reflecting deposition by turbidity currents. It is interbedded with several types of non-reservoir facies, such as mud clast conglomerates with a muddy sandstone matrix as well as thin shale and siltstone layers.
