Nitrogen injection is an attractive recovery process for gas condensate reservoirs. It maintains the reservoir pressure and thus prevents condensate drop-out as a result of pressure depletion. A disadvantage is that liquid drop-out occurs in the mixing zone between the injected nitrogen and the gas condensate. This occurs not only at the displacement front, but, due to bypassing, also at the boundary between layers of different permeability. This paper presents the results of a detailed, high resolution simulation study of nitrogen flooding of a stratified reservoir. The reservoir simulator used is a fully compositional simulator, based on the Peng Robinson EOS. The gas condensate is a 3 component hydrocarbon system that is representative of a North Sea reservoir. The main result of the study is that in stratified systems severe drop-out (up to 50 percent) may occur at the layer boundaries, in particular just downstream of the trailing displacement front. The latter can be explained by crossflow caused by an unfavourable mobility ratio. It is shown how this mechanism is affected by permeability contrasts, dispersion, aspect ratio and gravity. Introduction Retrograde condensation occurs in gas condensate reservoirs when the pressure falls below the dewpoint pressure. This leads to low recoveries as the liquid dropout is capillary trapped or left behind due to the low relative permeability. Retrograde condensation can be prevented by maintaining the reservoir pressure above the dewpoint pressure by injection of gas. The physical properties of dry hydrocarbon gases make it very suitable for injection gas. Hydrocarbon gas, however, is not always available for (re)injection. Injection of nitrogen gas is an attractive alternative. Nitrogen is cheap, safe, non-corrosive, non-polluting and available everywhere. The disadvantage of nitrogen is that liquid drop-out occurs in the mixing zone between the injected nitrogen and the gas condensate. In a homogeneous reservoir this occurs only at the displacement front. In a stratified reservoir however, additional mixing, and hence drop-out, occurs at the boundary between layers of different permeability. The objectives of this study are to identify the drop-out mechanisms and to quantify the recovery losses due to condensate drop-out. To achieve this objective we have used a fully compositional reservoir simulator. As a prototype reservoir we have used a vertical cross-section of a two-layer reservoir. As a gas-condensate fluid we have taken a simple three component hydrocarbon system. with a phase behaviour similar to gas-condensates encountered in North Sea reservoirs [1]. The simulations have been carried out at 360 bar and 100°C, representative conditions for North Sea reservoirs. In order to study the explicit effect of stratification we have used a two-dimensional reservoir model that consists of two parallel layers of equal thickness. We have investigated the sensitivity towards permeability contrast, aspect ratio and dispersivity, and the effect of gravity.

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