Nitrogen Injection Into Stratified Gas-Condensate Reservoirs

Abstract

Abstract 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.