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.
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.