Petroleum Conference on Production and Reservoir Engineering, 20–21 March, Tulsa, Oklahoma
Non-associated condensate-bearing gases in constant volume reservoirs are discussed. The discussion is predominantly directed to evaluation methods, sampling, determination of products originally in place, natural depletion performance, pressure maintenance, producing rate, and similar information basic to condensate reservoir evaluation.
Empirical correlations are presented on natural depletion recovery as well as benefits in pressure maintenance, and cover the range of gas-liquid ratios up to 50,000 cu ft/bbl. Displacement efficiencies observed in pressure maintenance operations in condensate reservoirs are useful in other miscible-phase displacement systems.
Condensate-bearing gas reservoirs are unique because the surface produced stock tank condensate was contained in the reservoir in a gaseous phase. Also, condensate differs from oils in that the stock-tank gravity is high, usually50°API or higher and generally is light in color. More important, however, to achieve a barrel of stock-tank condensate requires a greater reservoir voidage than a barrel of stock-tank oil will require from a liquid oil phase reservoir. In addition, a condensate-bearing gas will contain its maximum condensate content at the original [usually dew-point] pressure. Finally, the condensate content of a reservoir gas will consistently decline with pressure decline and will reach a minimum content at reservoir pressures approximating 1000–2000 psia; this content vs pressure is referred to as retrograde condensation range. Continued reduction in pressure, below the retrograde condensation range, will result in a continued enrichment of the gas due to vaporization of the previously accumulated retrograde liquid. It is the retrograde phenomenon that is the crux of the condensate problem and distinguishes it from oil and dry gas reservoirs.
It is the improved recovery methods, i.e., surface installations to remove more effectively liquifiable hydrocarbons or the pressure maintenance methods within the reservoir which prevent retrograde condensation, that provide the economic challenge in operating condensate reservoirs. Many variations or combinations in improvements have been incorporated in the past to achieve greatest economic recovery.
Fundamentally, the basic principles of condensate-bearing gases have been understood for many years and recent writings have permitted refinements in analytical methods and operating techniques. Condensate reservoirs have been recognized since the early 1930's; they may or may not be associated with down structure reservoir oil. Condensate-bearing gas may also be identified as released solution gas associated with high-volatile reservoir oils. When condensate-bearing gas is associated with a reservoir oil, the gas is accepted as saturated; however, when non-associated it could be undersaturated at the discovery reservoir pressure.