Gas condensate well test analysis using a single-phase analogy
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1999 SPE Western Regional Meeting Proceedings. All rights reserved. This paper shows that a simple, single-phase analogy using a standard dry gas pseudopressure transformation in interpreting transient buildup pressure data from a gas condensate well gives a good estimation of permeability. Under certain conditions, this method gives good estimates of damage skin, too. These results are not affected by average reservoir pressure. We used a commercial fluid PVT package and an EOSbased compositional simulator to characterize the phase behavior of retrograde gases during reservoir depletion, to compare gas and condensate PVT properties in situ and at lab conditions, and to model a vertical well under varying conditions. Then we analyzed the simulated buildup transient pressure by using a single-phase analogy incorporating a radially composite model and compared the results with reservoir simulator inputs and outputs. The numerical experiments using three actual fluid samples show that, during reservoir depletion, the vapor properties in situ are a function of pressure only, and correlate very well with z factor, gas molar density and viscosity obtained in lab constant composition expansion (CCE). The condensate PVT properties in situ depend on pressure, production mode and reservoir properties. But the effects of variables other than pressure are usually small and are constrained within a certain pressure range. As a rule, the leaner the retrograde gas, the smaller the deviations. As a practical matter, the correlations between condensate oil PVT properties obtained from lab CCE and the values in situ are also quite acceptable. Our reservoir simulation showed that the pressure buildup responses can be classified into two types. In the first type, the shape of the pressure derivative suggests a well in a radially composite two-zone reservoir. In the second type, the pressure derivative suggests a well in a radial three-zone reservoir. Our interpretation of 30 simulated cases using a radially composite two-zone model shows that a single-phase analogy gives a good estimation of permeability but underestimates condensate bank size. We cannot estimate damage skin from the first type of pressure response, but the estimation from the second type is acceptable.
