API RP 85:2003 (R2013)

SCOPE SUMMARY Until a better alternative is found, liquid hydrocarbon measurement will be accomplished by utilizing whatever sampling information is available to determine the well's water volume fraction and GOR. Dependent on the degree of difficulty in obtaining the sample and on the importance of the liquid hydrocarbon production, repeating this activity to obtain new information on the liuid properties may be done infrequently. Although an operator will certainly have a production sample acquired from each well at its startup (i.e., from a wireline sample-taking tool, or from the flow back to the completion rig) unless the system falls out of balance, there is no requirement to take further samples. Another problem that must be addressed is the fact that the conditions at the subsea meter will be quite different from those at the reference measurement point at the host processing facility. PVT analyses must be applied to account for phase changes incurred due to the tieback fowline length and differential water depth, as well as any other changes in pressure and temperature that might alter the phase state of the fuid. This will affect both the liquid and gas measurements, and will increase the difficulty of the task. This whole subject of mass transfer between phases and its effect on measurement uncertainty is addressed in Appendix A