Gas Cycling in Retrograde Gas Reservoirs

A significant portion of current hydrocarbon reserves exists in retrograde gas reservoirs. In these reservoirs, as pressure falls below the upper dew point of reservoir fluid, a liquid phase develops within the reservoir. This phenomenon is known as retrograde condensation. The retrograde condensate liquid is trapped as an immobile liquid phase within the pore space of reservoir. Therefore a considerable amount of intermediate hydrocarbons, which are very valuable, may be left in the reservoir. Moreover, presence of condensate liquid in a gas reservoir seriously reduces the effective gas permeability and consequently the productivity of the reservoir.To remediate this recovery reduction, gas cycling / gas injection is performed on retrograde gas reservoirs. The common method is recycling of produced dry gas, which is known as gas cycling. However, dry gas is expensive and is not always available. Therefore non-hydrocarbon gases are sometimes injected into gas condensate reservoirs. Nitrogen, Air and Carbon dioxide are the most important non-hydrocarbon gases used as injection gas in gas condensate reservoirs.The main objective of this paper is to investigate the feasibility study and economic evaluation of gas cycling / gas injection process in retrograde gas reservoirs for improving the recovery factor. Different scenarios will be discussed in details and their feasibility will be compared in terms of field conditions and economic considerations.

Keywords:

Retrograde Gas , Gas Cycling , Nitrogen , Air , Carbon Dioxide , Economics