Dry-Gas Reinjection in a Strong Waterdrive Gas-Condensate Field Increases Condensate Recovery—Case Study: The Sleipner Ty Field, South Viking Graben, Norwegian North Sea

摘要

The Sleipner ?st Ty Field is a strong waterdrive gascondensate field, with in-place volumes of 59x109Sm3 gas and 52x106Sm3 unstabilized condensate. The reservoir consists of deep-water turbidite sandstones and associated mudstones, which act as baffles to flow. The reservoir qualities are very good, with high porosity and with permeability in the range of 100 to 1000 mD. The first production in the area started up from the Sleipner ?st Ty Field in 1993. The initial reservoir pressure of 244 bar was only one bar above the dew-point pressure. Massive dry gas reinjection started in 1996, and the reservoir pressure increased during the next two years causing an increase in the condensate to gas ratio. During the injection period, which lasted until 2005, 29x109Sm3 of dry gas was injected. The main focus during these years was to obtain good vertical and areal sweep of the dry gas in order to vaporize the dropped-out condensate. Chemical gas tracers were injected and analyzed for in the production wells to monitor the movement of the dry gas through the reservoir. This knowledge was used to identify unswept areas, and to change the drainage pattern by doing well interventions and drilling infill wells. The injection was stopped primarily due to high probability of trapping gas. Compositional reservoir simulation showed that from 10 to 20% of the injected dry gas volume could be trapped in the northern region, which has no producers. The risk of not being able to back-produce the injected dry gas was considered high, since the saddle area separating the producers in the south from the injectors in the north was partly water flooded.The gas cycling programme has increased the condensate recovery factor substantially; from the originally planned 50% by pressure depletion, to the current estimated ultimate recovery of 81%. At present, the condensate recovery factor is 76%.