Methane injection or gas cycling is the recovery process of choice for gas-condensate reservoirs. For economic reasons. however, this process cart often not be implemented. An alternative injection gas may be nitrogen, which is relatively cheap and available every where. Nitrogen, however, causes stronger liquid drop-out in the mixing zone between the gas-condensate and the injected gas, which could reduce recovery. The paper presents the results of slim-tube experiments and numerical simulations of the displacement of a model gas-condensate at reservoir conditions by both nitrogen and methane. The gas-condensate used is a three-component hydrocarbon system representative of a North Sea reservoir. The phase behaviour and physical properties of mixtures of the gas-condensate and injection gas were calculated with an equation of state tuned by some selected PVT experiments. The main conclusion of this study is that the displacement of gas-condensate by both nitrogen and methane is a developed miscible process, which results in high recoveries of over 90 per cent. The recovery is adversely affected by dispersion; nitrogen is more sensitive to this than methane. Introduction When the pressure in a gas-condensate reservoir falls below the dewpoint pressure, retrograde condensation occurs. The liquid that is formed during the condensation is trapped by capillary forces or is left behind due to the low liquid relative permeability. The recovery of condensate, which contains most of the heavier, valuable, components is therefore at most 30 to 40 per cent of condensate-initially-in-place. The reservoir pressure can be maintained above the dew-point pressure through injection of gas. Gas-cycling or (re-)injection of lean gas is frequently applied, as dry, methane rich, hydrocarbon gas has suitable physical properties. In areas, however, where a well developed market for dry gas exists, like for example the North Sea, re-injection of methane gas is economically not attractive. Nitrogen is a potential alternative injection gas. It is available every where, as it can be produced from air at low costs, using cryogenic or membrane separation. Economic evaluations show that nitrogen injection is realistic, provided that the gas-condensate is sufficiently rich. Nitrogen injection has been applied, often in combination with gas-cycling, in several gas-condensate reservoirs. Nitrogen has, however, also disadvantages. Addition of some nitrogen to a gas-condensate causes a strong increase of the dewpoint pressure of the mixture. This dewpoint eventually becomes much higher than the reservoir pressure. Depending on the level of mixing and dispersion. liquid drop-out occurs, thus reducing the efficiency of the process. The objective of this study is to assess the feasibility of the flooding of gas-condensate by nitrogen vis-it-vis flooding by methane. We have used the following approach to meet this objective. First we have defined a three-component synthetical gas-condensate. We have performed an experimental study of the phase behaviour of this system and used the results to tune an equation of state. To investigate the effect of mixing on the development of miscibility, we have performed nitrogen and methane flooding experiments in a slim tube. In these experiments, we have measured compositional changes and changes of the mass density in the displacement front, as well as the recovery efficiency of the process. Finally, we have interpreted the experiments by numerical simulations, using a fully compositional. one-dimensional, simulator. The results of this study may find application in the evaluation of recovery processes for gas-condensates.

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.