The evaluation of petroleum resources requires a good understanding of the reservoir fluids and their behavior. Specifically, PVT (pressure-volume-temperature) data are required at the earliest possible stage to develop economic evaluations, interpret well test data, design production equipment, and predict future reservoir performance.

A fluid sample obtained with an openhole wireline formation tester tool provides the engineer with the first (and potentially the only) chance to analyze a representative reservoir fluid sample before significant decisions are made. The fluids must be characterized by direct measurement of certain properties and the quality of the PVT laboratory results depends almost entirely on the validity of the sample used for the analysis.

This paper compares the fluid PVT data obtained using samples from wireline formation tester tools and from surface production test equipment. The PVT laboratory procedures and the accuracy of the analysis methods are discussed. The compositional analyses of the hydrocarbon samples are compared, from methane to heptanes-plus chemical structure, along with the key volumetric parameters such as density, bubblepoint pressure, viscosity, formation volume factor, and gas-oil ratio. Different types of reservoirs (with fluids ranging from low gas-oil ratio oils to dry gas) were examined to determine the effects of mud filtrate invasion in the openhole environment, type of fluid, and reservoir characteristics.

In general, the results indicate that samples obtained with wireline formation tester tools can provide representative reservoir fluid parameters from compositional analyses to basic volumetric behavior as a function of pressure and temperature. Critical reservoir parameters such as the fluid density, bubblepoint pressure, formation volume factor, viscosity, and gas-oil ratio are consistent within about 15 percent for most oil reservoir fluids.

Gas condensate reservoir fluids present the most difficult sampling conditions for both the downhole and surface sampling operations. Careful sampling techniques and detailed monitoring of the downhole and surface fluid conditions are required to reconstruct useful results for the reservoir fluid properties.

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