The Role of the Y-function for Checking the Reliability of PVT-data

PVT experiments are not free of systematic and random errors, therefore checking the validity of the results is a must. Textbooks use the Y-function as a consistency checks for constant compositions experiment (CCE) of black oil (BO). This paper extends the application of the Y-function. As it turns out, the Y-function can not only be applied for the CCE of BO, but also for the same experiment with a gas-condensate (GC). Furthermore, it will be demonstrated that the Y-function is a useful tool for evaluation of the differential liberation experiment (DLE, black oil) and the constant volume depletion (CVD, gas-condensate) as well. So far this aspect has not been used in the consistency checks of laboratory measurements. Textbooks claim that the Y-function is a straight line. Modeling the fluid system (BO or GC) with an equation of state (EOS) shows that the shape of this function is only close to a straight line. Having set up the Y-function properly, it is possible to estimate the quality of the CCE and the DLE for the black oil case and CCE and CVD for the gas-condensate by comparing the results of the experiments with the EOS calculations.