Nitrogen Injection for Simultaneous Exploitation of Gas Cap

Abstract

Abstract A compositional study has been carried out for evaluation of nitrogen injection in the large gas cap of a light oil reservoir (GS-4) as a low cost replacement gas. GS-4 is one of the major hydrocarbon bearing reservoirs in Gandhar field with oil in place of 6.37 MMt overlain by a large gas cap of 8.682 BCM. The gas reserves therefore represent a significant portion of the reservoir’s total economic value. The timing of production, from both the oil zone and the gas zone, is an important factor in optimum plan of exploitation for maximising the economic returns in addition to the goal of optimising oil recovery from this reservoir. The existing exploitation scheme for this reservoir focuses mainly on the optimum exploitation of the oil reserves. This scheme restricts the exploitation of the large gas cap. The gas cap can be blown down only after the exploitation of oil zone. This paper discusses the strategy of early exploitation of rich gas cap gas and associated natural gas liquids by replacing it with low cost gas like nitrogen. Cryogenic air separation method can generate nitrogen in large volumes at a fraction of the cost of natural gas and thus provide large economic benefits. This strategy can advance the gas cap gas sales by 15–20 years ahead of the currently planned blowdown stage. The paper presents the results of sectional study in GS-4 to evaluate the feasibility of nitrogen vis-à-vis lean gas injection. A detailed compositional study was carried out using a seven component equation of state (EOS) model. The paper discusses the key issues in fluid characterisation of GS-4 and its phase behaviour effects in contact with nitrogen. GS-4 gas cap presents production potential of around 1.5 MMm3/d for 10 years. The study indicates very attractive economic benefits for nitrogen injection as a replacement of gas cap gas.