Abstract

In the last year the authors have fielded many questions from companies, both international and domestic, concerning gas condensate reservoirs. It appears that gas condensates are becoming more important throughout the world. Many international petroleum societies are beginning to have conferences specifically oriented to gas condensate reservoirs and discussing all parameters germane to such systems. In light of this increased interest, the authors have made a short list of questions which are most often asked. Indeed, these questions point to two specific areas which govern the production and future exploitation plans for gas condensate systems. These two areas are characterization and retrograde condensate influences on relative permeability.

It has been found that the characterization of the gas condensate fluids can be strongly influenced by two main factors:

  1. Any degree of contamination by a free liquid phase insitu;

  2. Hold-up of the retrograde condensate in the formation resulting in excessive producing GOR's.

Care must be taken when sampling gas condensate wells n order to produce representative recombined fluids. In order to gain an appropriate evaluation of the gas condensate reservoir one must be able to adequately characterize the fluids insitu. Experimental and theoretical work performed on evaluating retrograde condensate effects has pointed to the fact that the influence of retrograde condensate is much more deleterious in tighter formations and higher interfacial fluids. The ability to identify the influence of retrograde liquid on gas phase production rates is a difficult task and data are provided herein which compare the retrograde condensate effects at two levels of interfacial tension and as a function of rock permeability.

It has been found that in a review of four gas condensate reservoirs, one of which included a fractured system, there was a coupling of a multiplicity of factors including:

  1. Interfacial tension effects

  2. Viscosity ratio

  3. The healing of fractures with its concomitant effect on absolute permeability

In order to adequately forecast such systems, a simulator must in corporate these effects.

Sampling condensate reservoirs

Condensate reservoirs are inherently more difficult to characterize correctly. The literature shows many differences between gas condensate reservoirs and dry gas reservoirs(l-6). One question often asked is during and after sampling. Figure 1 provides a fairly typical GOR versus total flow rate response from a gas condensate reservoir. One sees that, 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, the generation of a plot such as Figure 1 is not easy.

In the same plot one compares the response which would normally be seen for an oil reservoir. With the oil reservoir, the sampling technique is fairly easy to specify. All one must do is try to produce the well in the domain low enough so that a constant GOR is produced. Since the behavior is asymptotic as a function of decreasing total flow rate from the well, it is easy to identify what production level one needs to apply for taking the gas and liquid samples.

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