A K-value method, which is designed for fast computation, is presented. The method relates the K-value of each component with its boiling temperature, critical temperature and pressure, the mixture's pressure and convergence pressure, and overall compositional pressure, and overall compositional changes. Experimental data, from routine PVT tests of the mixture, are used to PVT tests of the mixture, are used to calibrate the K-value correlation.
A PVT simulator was developed to model the routine PVT laboratory tests. This simulator generates the K-value correlation for the mixture and employs a least squares-linear programming optimization routine to fine tune the correlation such that the simulated PVT behavior matches the actual one.
Nine reservoir fluid samples, four of them included carbon dioxide miscibility, were used to demonstrate and test the applicability of the new K-value method. The method has proven successful for predicting the phase behavior of predicting the phase behavior of hydrocarbon systems as well as the carbon dioxide-hydrocarbon mixtures.
The new method was compared with Peng-Robinson, Soave-Redlich-Kwong, and Peng-Robinson, Soave-Redlich-Kwong, and Schmidt-Wenzel equations of state which were embodied as options in the PVT model. The comparisons of the fluid samples have shown that the new method is reliable and is faster than the equations of state by a factor of seven to twenty. The method requires no iterations, less computer memory, less input data, and fewer tuning parameters than equation of state methods. parameters than equation of state methods
There is no doubt that phase behavior has an essential role in the oil recovery processes, ranging from the production of processes, ranging from the production of gas condensate and volatile oil reservoirs to the injection of CO2 and N2 for enhanced oil recovery processes. As more volatile oil and condensate reservoirs are found, the use of phase behavior techniques to predict their performance is increasing in predict their performance is increasing in importance, particularly when using compositional simulators. In the phase behavior methods, equilibrium ratios are used to predict compositional changes in the reservoir fluids.
Equilibrium ratios or K-values of any system are functions of pressure, temperature and composition of that system. However, at constant temperature, it was found that the K-values of simple mixtures could be expressed as functions of pressure only. K-values can be evaluated by three basic methods:
Raoult's and Dalton's laws, assuming ideal solutions,
using fugacities of pure components calculated from equations of state, and
by direct analysis of vapor and liquid in equilibrium at any pressure and temperature, based on the knowledge of some experimental data.
In spite of the development of more sophisticated theoretically based predictive techniques, notably the predictive techniques, notably the equations of state methods, K-value correlations are still implemented into compositional reservoir simulators and other applications where efficiency and ease of adjustment to experimental data are essential.