Influence of the quality of downhole samples on the results Of flow assurance laboratory studies of reservoir oil

The paper illustrates the influence of the quality of downhole samples on the results of studies of the phase behavior of paraffins and asphaltenes in reservoir oil. The results of studies of a conditionally representative sample of reservoir oil and deep samples taken under multiphase flow conditions (with different volumes of free gas, formation water and solid phase of asphaltenes, resins and paraffins in the sampler) were compared. For each sample, a set of PVT studies was carried out according to all required standards – PVT test, flash separation, determination of viscosity and density. Phase behavior of paraffins and asphaltenes was studied by methods of acoustic resonance (registration of the moment of phase transition), high pressure microscopy (determination of the number and geometric dimensions of solid phase particles), gravimetric and filtration methods (determination of the group composition of the solid phase). To prepare samples of oil with free gas in the chambers, a procedure was carried out to release free gas to the current bubble-point pressure. The results of the analysis revealed that the procedure of free gas release has little effect on the PVT-characteristics of reservoir fluid samples. However, it is not possible to study the phase behavior of paraffins and asphaltenes, since the deviation of different parameters reaches 70% (the content of asphaltenes, resins and paraffins; paraffin saturation temperature; asphaltene saturation pressure, etc.). As a result, the sample of fluids with multiphase flow conditions, if it is impossible to use other methods, may be acceptable for a standard PVT-analysis. But, it is not possible to study the phase behavior of paraffins and asphaltenes for such samples, since these studies are highly dependent on the quality of sampling.

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