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Microscopic oil displacement and sweep efficiency of waterfloodiig and continuous gas injection can be improved by water alternated gas (WAG) injection.Reservoir and fluid properties are determining factors in screening WAG injection strategies. Evaluation of miscibility condition is an important step in the process design. Different correlations used for miscibility evaluation are compared on the Brent reservoir example. The effect of injection parameters on injectivity and recovery in the layers of a stratified reservoir is shown through the simulation results.Methods improving WAG efficiency like non-stationary iniection are discussed. Main parameters of cyclic injection wi.th variation of flow directions are considered in relation with non-stationary WAG. Numerical modelling with respect to three phase effects showed the advantages of non-stationary WAG in a stratified reservoir.

This paper describes the problem of disposing large amounts of CO2 into a shallow underground aquifer from an offshore location in the North Sea. The solutions presented is an alternative for CO2 emitting industries in addressing the growing concern for the environmental impact from such activities. The topside injection facilities, the well and reservoir aspects are discussed as well as the considerations made during establishing the design basis and the solutions chosen. The CO2 injection issues in this project differs from industry practice in that the CO2 is wet and contaminated with methane, and further, because of the shallow depth, the total pressure resistance in the system is not sufficient for the CO2 to naturally stay in the dense phase region. To allow for safe and cost effective handling of the CO2, it was necessary to develop an injection system that gave a constant back pressure from the well corresponding to the output pressure from the compressor, and being independent of the injection rate. This is accomplished by selecting a high injectivity sand formation, completing the well with a large bore, and regulating the dense phase CO2 temperature and thus the density of the fluid in order to account for the variations in back pressure from the well. Introduction The Sleipner fields are located in the Norwegian sector of the North Sea, approximately 250 km from the coastline. The production licenses that cover the fields are owned by Statoil, Esso, Norsk Hydro, Elf and Total and is operated by Statoil. The main reserves in the area consists of the gas/condensate in the Sleipner Ost and Vest Fields, ref. fig 1. The Sleipner Ost Field contained initially 64 GSm3 rich gas and is produced with partial reinjection of produced gas. The Sleipner Ost Field is developed with a fully integrated platform (Sleipner A) which provides drilling and process facilities as well as living accommodations. Sleipner Ost commenced contractual deliveries under the Troll Gas Sales Agreements October 1993. The Sleipner Vest Field contains 202 GSm3 rich gas. The reservoir is fairly faulted with different pressure regimes and different fluid properties in different fault blocks. The CO2 content varies between 4 - 9.5 %. The field is developed by pressure depletion with 18 production wells. The wells are drilled from a wellhead platform (Sleipner B) located on the central part of the Sleipner Vest Field itself, and from a subsea template in the northern part of the field. The Sleipner Vest Field will be produced at a plateau rate of 20.5 MSm3 sales gas per day. All produced gas will be transported untreated from the wellhead platform through a 12 km long pipeline to a process and treatment platform (Sleipner T) located next to and bridge connected to the Sleipner A platform. fig. 2. Planned production start is August 1996, and the production period is estimated to year 2022. The existing pipelines will be used for transport of petroleum products. The Sleipner Vest gas will also be delivered under the Troll Gas Sales Agreements. These agreements set a sales specification of maximum 2.5 % by volume CO2 in the sales gas delivered to the pipeline. To meet this specification the CO2 has to be removed at the field. The CO2 will be removed using an activated amine. During the early planning of the field development, increased environmental concern raised the question of an alternative to atmospheric release of the CO2. Releasing this amount of CO2, approximately 1 million metric ton per year or cumulative 20 million metric tons through the life time of the field, would represent a 3% increase in the total Norwegian CO2 emissions to the atmosphere. P. 269

Water alternate gas (WAG) injection technology is a method which may improve oil recovery efficiency by combining effects from two traditional technologies -water and gas flooding. Both microscopic oil displacement and sweep efficiency can be improved by WAG implementation. This paper describes a method of designing an effective waterlgas flooding in stratified reservoirs.The analytical approach taking into account effects of three phase flow, gravity and viscous forces in anisotropic media, allows the determination of optimum parameters of waterlgas injection. A three-phase extended black oil simulator with relative permeability and capillary pressure hysteresis, was used to validate the optimum WAG injection parameters obtained by the analytical method. In this work, the efficiency of different injection schemes was compared. Simulation results which demonstrate the influence of WAG injection parameters, as water-gas ratio, injection rates and cycle periods, on the recovery process, are discussed.