Page 57 - Handbook Of Multiphase Flow Assurance

52 3. PVT and rheology investigation

components as a function of energy of interaction between components (A) and of molecular
size (B).
Peng Robinson or PR EOS and its updated versions PR78, PR78 Peneloux work more ac-
curately with liquid-dominated systems and are equally as good for gas systems. Most hy-
drocarbon reservoir fluids are at present characterized using the PR78 EOS with the density
correction developed by Peneloux and coauthors in Peneloux et al. (1982).
Soave Redlich Kwong or SRK EOS works well for gas dominated fluids because the
Soave variant of the Redlich Kwong EOS can underpredict liquid compressibility by 10–20%
whereas the PR EOS predicts liquid compressibility a little better.
Polynomial form of P-R E.O.S. is

Z +( B 1) Z +( A 3 B − ) +( B + B − AB) = 0
−
−
2
3
2
3
2
2
B Z
A = . P / T r 2
0 45724α
r
0 26992 ) − (
α = ( 1+( 0 3 . 77464 1 54226+ . ω − . ω 2 1 )) 2
T
0 5
.
r
bP P
B = = . r
0 07780
RT T r
P
=
P =
r
P
c
T
T =
r
T
c
Values of acentric factor w, critical pressure and temperature T c and P c are known for each
component. EOS calculates compressibilities of vapor and liquid for a given pressure and
temperature.
Fluids with high CO 2 or other polar or associating components content may be modeled
with specialty variants of the equations of state.
Recent equations of state CPA (cubic plus association) and SAFT (self associating fluid the-
ory) include an additional parameter of association between molecules. These methods are
useful for polar components such as water, CO 2 , aromatics and asphaltenes.
Other more complex equations of state are seldom used because a fully compositional
model requires fast calculation of fluid properties, and the main demand for fluid properties
is in reservoir simulation.
Fluid characterization used for reservoir simulation is then usually transferred for further
fluid analysis in flow assurance and in production chemistry in order to assure flow in the
multiphase production system and surface process facilities.
Fluid characterization
Sometimes the composition analysis data are limited and reported in a form of true boiling
point analysis of volume % hydrocarbon vs temperature, with density data for the fractions
unavailable.
In order to model such fluid with EOS, it has to be converted from volume % to mass %
distribution for hydrocarbon fractions.