Historical Performance of Gas Injection of Ekofisk

Abstract

Abstract Hydrocarbon gas has been injected into the Ekofisk field for almost twenty years. Gas injection has always been for operational reasons. This case history reviews the historical performance and benefits of the gas injection. Gas migration, gas distribution and mechanisms involved when gas is injected into a naturally fractured chalk reservoir are discussed. The findings are based on field and laboratory data. The benefits are estimated by means of reservoir modelling. The Ekofisk field is a naturally fractured chalk reservoir with low matrix permeability. The natural fracturing enhances overall permeability and has made commercial production possible. Since gas injection commenced in 1975, a total of 1.2 TSCF of gas has been injected. All the gas has been injected into the crest of the field. This paper demonstrates that the natural fracture system at Ekofisk represents a medium for the injected gas to contact and mix with the reservoir fluids and that major fault systems are important factors for gas migration and gas distribution. There is no clear oil production response to gas injection. The production response that can be observed is generally limited to elevated GOR. Gas injection still appears to have benefitted oil recovery. Reservoir modelling suggests that gas injection will increase oil recovery by 2 to 3 percent of original oil in place. Historical benefits are, in addition to pressure support, swelling of the oil and vaporization/stripping of lighter hydrocarbons.