Nitrogen Injection vs. Gas Cycling in Rich Retrograde Condensate-Gas Reservoirs

Abstract

Abstract The common method to prevent condensate from dropping out in gas-condensate reservoirs that are produced by pressure depletion is gas cycling: the reinjection of produced dry gas. However, dry gas is expensive and is not always available. An alternative could be nitrogen, which is available everywhere, relatively cheap and has favourable injection characteristics (safe, non-corrosive, environment-friendly). The problem with nitrogen injection, however, is that it may lead to significant in-situ condensate drop-out due to the mixing of nitrogen and gas-condensate in the reservoir. The objective of the study reported in the paper is to investigate whether nitrogen injection can be applied as an alternative for gas cycling, with emphasis on the condensate drop-out problem. The investigation comprised three phases: Formulation of a rich gas-condensate, consisting of methane, n-butane, n-tetradecane and nitrogen, using an interactive PVT simulator. Calculation of liquid drop-out for both dry gas and nitrogen injection as a result of mixing, by varying injection gas concentration. Evaluation of displacement efficiency by using a compositional one-dimensional simulator for various Peclet numbers. The chosen gas has common field characteristics (Gas Liquid Ratio = 2300 V/V, API gravity = 58.6) and is rich (shown by the maximum liquid drop-out of 30%). Liquid drop-out as a result of mixing is much higher for nitrogen (max. 25 %) than for dry gas process (max. 2%). Displacement calculations show that, at low Peclet numbers, nitrogen injection results in much poorer displacement efficiencies than dry gas injection. However, the difference decreases significantly at increasing Peclet numbers. The main conclusion of the paper is that, under field conditions, nitrogen injection could be a viable alternative to dry gas cycling. Nitrogen injection may find application in the production of condensate reservoirs in areas with a well developed gas market where dry gas is not available for injection. The main contribution of the paper is that it highlights and provides information on a relatively new process for the exploitation of condensate fields.