Reservoir-fluids Phase Behavior and Volumetric Properties

Reservoir-fluids phase behavior and volumetric properties. Reservoir fluids are a complex mixture of thousands of components that exhibit very complex phase behavior. It is, however, surprising that a simple two-constant equation of state such as the PR-EOS can do an excellent job for vapor-liquid equilibria calculations away from the critical region. A discussion of the manner in which the calculations are performed is presented. 

As was stated earlier, one needs Tp. and or cu for every component in a mixture. However, critical property data beyond a certain carbon number are not yet available. The measurement of critical properties of heavy hydrocarbons is difficult and is subject to uncertainty because of thermal decomposition and chemical reactions at high temperatures. Thermal decomposition increases with increasing carbon number and temperature. When a heavy hydrocarbon is heated, it reaches a temperature at which it starts to decompose. The product of decomposition increases with temperature and time. Therefore, the measured critical point corresponds to the critical point of a mixture. In order to determine the critical point of the original substance, certain assumptions have to be made. For mildly unstable substances, the critical locus of the mixture (original substance and the products of decomposition) is assumed to be a linear function of 

Popov (1997) measured critical pressure and temperatures of several normal alkanes including nCsg. 

Both the usefulness and promise of the cubic equations to describe gas and liquid phases and the problems of the description in the critical region and density deviation have forced some users to adjust several parameters of the EOS to match measured and calculated phase and volumetric behavior. These adjusted parameters, in essence, include critical properties and interaction coefficients. 

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