Phase Behavior of Systems Comprising North Sea Reservoir Fluids and Injection Gases
Authors: Varotsis Nikos , Stewart George , Todd A.C. , Clancy Michael 
Published: December 2007

The design and use of an experimental system to measure the phase behavior of systems comprising reservoir fluids and injection gases are described. The experiments simulate in the laboratory the mass transfer between the advancing injection gas phase and the oil phase in the reservoir. A special feature of the equipment is the direct transfer of microliter samples of the equilibrium high-pressure phases to the gas chromatograph, thus eliminating errors associated with conventional blowdown techniques.

Then, the phase behavior simulator that has been developed is described briefly and its main features are discussed. The use of the Schmidt-Wenzel equation of state (EOS) has markedly improved the accuracy of the predicted liquid-phase densities at reservoir temperature, and corresponding state correlations are recommended for stock-tank oil volumes. Generalized correlations for the interaction parameters of CO2-hydrocarbons, N2-hydrocarbons, and CH4-hydrocarbons have been developed from binary experimental data to be used with the Peng-Robinson equation. Temperature- and pressure-dependent coefficients were found to match saturation data with good accuracy. The phase behavior simulator, tuned on laboratory experimental data, was used to predict miscibility conditions for a North Sea stock-tank oil and a North Sea reservoir oil for CO2 and N2 injection schemes.

Gas Miscible Displacement as a Potential EOR Process Applied to North Potential EOR Process Applied to North Sea Reservoirs

The majority of oil reservoirs in the North Sea are highly undersaturated and are being produced with pressure maintenance by seawater injection. Waterflooding is favored because of the low oil viscosities and high permeabilities of the formations. Permeability contrasts have permeabilities of the formations. Permeability contrasts have been a problem, however, and early water breakthrough as a result of high-permeability layers has been experienced in many fields. The expected recovery efficiency is about 45 %, with higher and lower figures quoted for specific fields.

Because of the high formation permeabilities and low oil viscosities, it has been possible to develop the fields with relatively few wells. In the consideration of potential EOR applications, the large well spacing typical of potential EOR applications, the large well spacing typical of offshore development favors gas-drive processes. Some hydrocarbon-gas-injection schemes have already been initiated-e.g. in the Statfjord reservoir of the Brent field and in the Beryl and Ninian fields. The reservoir engineering aspects of EOR by miscible gas displacement in North Sea reservoirs have been discussed by Bath et al., who emphasize the role of gas drive in the offshore context. The conditions of high permeability (150 to 1,500 md), low oil viscosity (0.2 to 1.0 cp [0.2 to 1 mPa s]), and moderate dip (7 [0. 12 rad]) in the Brent field favor a gravity-stabilized process, The injection of upper Brent reservoir gas into the lower Statfjord reservoir is first-contact miscible, and this scheme was one of the first EOR applications in the North Sea. Updip gas injection in the Beryl field has been described by Steele and Adams. The upper Beryl reservoir is permeable (400 md) and highly dipping (10 to 25 [0.17 to 0. 44 rad]), which again favors a gravity-stabilized displacement. Significant mass transfer effects have been observed in the Beryl scheme, but the pressure is not high enough for full miscibility to occur.

In other cases, conditions allow injected gas to be miscible with the reservoir fluid. The availability of natural gas is limited, however, and the construction of a gas-gathering system means that most natural gas production is committed to sales contracts. Attention has been focused on other possible injection gases, notably CO2 and N2. Although CO2 is attractive because of its low minimum miscibility pressure (MMP), there are, unfortunately, no CO2 reservoirs in the North Sea; the only possible offshore source is the Sleipner gas-condensate field in the Norwegian sector, which contains a considerable amount of CO2. The technical feasibility and economics of the separation of CO2 from power-station stack gas and transportation offshore by pipeline have been examined, but the cost is prohibitively high.

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