Optimizing Production From A Rich Gas Condensate Reservoir
Authors: Thomas F.B. , Anraku T. , Bennion D.B. , Bennion D.W. 
Published: December 2007

Increased interest in gas condensate reservoir which exhibit complexities due to the production of gas, condensates and many times an in-situ oil phase has been observed. As expected, there are definite production problems and exploitation concerns which are at issue with gas condensate reservoirs, or reservoirs which exhibit a combination of gas condensate and oil characteristics.

In studying reservoirs of this type, there are three main areas which must be adequately addressed in order to develop an appropriate exploitation strategy. These areas are adequately summarized as:The characterization of the gas condensate fluidsThe coupling of the inherent phase behaviour and fluid flow in the porous mediaImplementation into a simulator for forecasting capability This work seeks to provide a summary of important characteristics which must be considered in each of these three areas. The knowledge gained from studying a number of these reservoirs over the years will then be implemented by the authors to provide an example of a possible approach which should be followed for designing production from such a reservoir.

Based upon this work, it has been found that the characterization of the gas condensate fluids is strongly influenced by two main factors which include any degree of contamination by a free liquid phase in situ and hold-up of retrograde condensate in the formation resulting in excessive producing GORs.

The coupling of the fluid phase behaviour and fluid flow in the rock appears to be governed in the areas of the degree of retrograde condensate accumulation, interfacial tension effects, and mobility effects. Part of this coupling also indicates the difference between critical condensate saturation and residual condensate saturations.

Finally, an analysis of techniques to improve retrograde condensate behaviour is described and the necessary components of a reservoir simulator are discussed including results from such a simulation.

Characterization of Gas Condensate Systems

Condensate reservoirs are inherently more difficult to characterize correctly. The literature shows many differences between gas condensate reservoirs and dry gas reservoirs. Figure 1 provides a fairly typical GOR versus total flow rate response from a gas condensate reservoir. At very low flow rates, one has a high producing GOR and, beyond the certain minimum value in GOR, the trend is again upwards. It is easy to identify why this occurs, but sometimes, when faced with the possibility of having extra sampling runs and spending more time in the field some operators believe that the cost outweighs the benefit.

In the same plot the response normally seen for an oil reservoir is also shown. With the oil reservoir, the sampling technique is fairly easy to specify; one must produce the well in the domain low enough so that a constant GOR is produced. Such is not the case with gas condensate reservoirs however. At low flow rates, the liquid hold-up will increase and slugging may result. The increased GOR to the left of the vertical line is due to the low flow rate not providing enough lift to transport the liquids in the wellbore.

By contrast, one may be inducing liquid dropout in the reservoir at high flow rates to the right of the vertical line in Figure 1. In this case, as the pressure drops below the dewpoint, the liquid will begin to collect in the near wellbore region. In so doing, produced hydrocarbons will contain less liquid than they should and therefore the GOR will be high. Thus, "At what producing rate should a well be sampled?", the only way that one can adequately respond is if one already knows the character of the fluid and the dynamics of the production well. Since this information is not available, sampling gas condensate reservoirs in an optimal manner can sometimes be non-linear and include some trial and error.

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