Enhanced Oil Recovery (EOR) using CO2 as injection gas after water injection is a method to increase the oil recovery efficiency compared to only using the water injection process. In addition, it also constitutes a storage method in an oil reservoir. CO2-EOR can help to reduce CO2 emissions regionally and simultaneously improve the security of energy supplies.
The purpose of the present paper is:
To describe the base case simulations of continuous CO2 injection and CO2 Water-Alternating-Gas (CO2 WAG) injection in the North Sea "conceptual" chalk model.
To give quantitative results on EOR and CO2 storage capacity of the North Sea "generic" chalk model for the continuous CO2 injection and WAG-CO2 injection scenario.
Base case simulations of CO2 injection have been described using the North Sea anticline sector-type model as geometry. SINTEF's oil field database was used. Then conceptual reservoir properties were calculated. Porosity and permeability were computed based on the same methods as used previously for average and standard deviation to populate the conceptual model. Finally a stochastic model based on a Probability Distribution Function of porosity and permeability was used. A conceptual "chalk oil composition" was derived from the Syd-Arne oil field (DK) by an oil composition change in order to match the viscosity and density of conceptual oil. A general development strategy which consists of one producer (P1) and one injector (I1) has been applied.
Numerical simulation using a commercial compositional reservoir simulator was performed a Finite Difference Method. In addition, the dual permeability module was used in order to simulate fractures and matrix permeability flow. Molecular diffusion of CO2 (gas) into oil, oil into gas and CO2 into water was used. Gravity drainage, capillary forces and molecular diffusion are considered as the main factors to influence the oil recovery efficiency.
Results are compared for three cases:
Water flooding, continuous CO2 injection and CO2-WAG injection:
Field Oil Production Rate and Field Oil Production Total per pore volume injected
CO2 volume stored per pore volume injected
In all the reservoir model cases under investigation with CO2 continuous injection and WAG-CO2 injection, CO2 injection is found to increase the hydrocarbon recovery by 12% to 13.2% recovery after injection of 1.4 Hydrocarbon Pore Volume compared with a base case scenario which is water injection. After 90 years of injection, from 1.35 to 2.95 millions tonnes CO2 were stored for the WAG-CO2 and CO2 injection case respectively.