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Nitrogen Injection: An Alternative to Gas Cycling in Gas Condensate Reservoirs?
Gas cycling, which is reinjection of produced dry gas into gas condensate reservoirs, has been a common practice to increase liquid hydrocarbon recovery. Since dry gas has an increasing economic value as a commodity and, moreover, is not always available, an alternative could be nitrogen, which is not only cheaper, but also is available everywhere, safe, non-corrosive and non-polluting.
However, relative to dry gas, the uncontrolled use of nitrogen may cause excessive liquid drop-out upon introduction to gas condensate in the reservoir.
The objective of the study reported in this paper is to investigate whether nitrogen injection can be applied as an alternative to gas cycling, with the emphasis on the condensate drop-out problem.
In this study, a model condensate, consisting of methane, n-butane, n-tetradecane and nitrogen, has been formulated with the aid of an interactive PVT simulator. The model condensate exhibits characteristics which are common in the field (Gas Liquid Ratio = 2300 V/V, API gravity = 58.6).
Upon pressure depletion, the model condensate shows a maximum liquid drop-out of 30%, which is a rich condensate value.
Liquid drop-out as a result of mixing is much higher for nitrogen (max. 25 %) than for processed dry gas (max. 2 %).
Previously published results on nitrogen injection into volatile oil reservoirs compared to those on methane injection have shown that the oil recovery of nitrogen injection will approach that of methane injection, if certain field operational conditions and physical parameters are met.
One-dimensional compositional simulations run in this study suggest that in the case of condensate reservoirs, recovery of condensate after nitrogen injection is comparable to that after methane injection.
The conclusion of the paper is that, under field conditions, nitrogen injection could be an alternative to dry gas cycling.
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