Gas Condensate Well Test Analysis


Bruno Roussennac







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Multiphase flow and mixture composition change in the reservoir make the interpretation of well test in gas condensate reservoirs a real challenge. In this report, the different techniques to analyze gas condensate well tests using single-phase pseudopressure and two-phase pseudopressure are reviewed. The ěsteady-state îand more recent ěthree-zone îmethod to compute the two-phase pseudopressure are presented. The calculation of the two-phase pseudopressure requires the knowledge a priori a pressure-saturaton relationship during the test. The steady-state method assumes the same pressure-saturation relationship during the test as during a hypothetical steady-state flow, which ignore any composition change in the reservoir. The three-zone method accounts for the composition change in the reservoir and is based on modeling the depletion by three main flow regions:

* A near wellbore region (Region 1) where the oil saturation i important allowing both phase, vapor and liquid to be mobile.

* Region 2 where condensate and gas are present but only the gas is mobile. In Region 2, the condensate builds up and the composition of the flowing mixture is changing.

* An outer Region 3 exists when the reservoir pressure is greater than the initial gas dew point and contains only gas.

Sensitivities of the ěsteady-stateî and ěthree-zone" methods to various parameter (skin, relative permeability curves, flow rate, initial pressure, initial fluid richness) are studied using compositional flow simulations for drawdown tests. The three-zone method gives accurate results for the estimated permeability and the estimated skin for all the tested cases. The steady-state method considerably underestimates the skin, the permeabilty being slightly overestimated.

Uncertainty due to errors (measurements or nonrepresentativity) in the relative permeability curves, the gas-oil ratio and the fluid characterization (sampling) is also considered here. Both the steady-state and the three-zone method were found to be very sensitive even to small errors in the relative permeability curves,the gas-oil ratio and the fluid ampling. The methods are so sensitive to the relative permeability curves, which are typically not known accurately that their use for well test interpretation seems diffcult if we want to determine precisely parameters as kh and skin.
In addition, the gas-oil ratio and the fluid sampling are also uncertain. However, the proposed method may be used for well deliverability and in flow performance calculation with more success for sensitivity studies.

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Copyright 2001, Bruno Roussennac: Please note that the reports and theses are copyright to their original authors. Authors have given written permission for their work to be made available here. Readers who download reports from this site should honor the copyright of the original authors and may not copy or distribute the work further without the permission of the author, Bruno Roussennac.

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