Page 21 - PVT Property Correlations

2 PVT Property Correlations

effects of errors in PVT data on well test analysis interpretation results. They
showed that errors in fluid compressibility affect the interpreted distance to
boundaries. Distance to boundaries affects calculation of reserves from well
tests (as in the case of reservoir limit tests). Errors in fluid viscosity affect
estimates of permeability from well test analysis. Errors in fluid compress-
ibility and formation volume factor have minor effects on estimated skin fac-
tor. In dual porosity reservoirs, errors in formation volume factor affect the
interporosity flow coefficient (which determines how much fluid is trans-
ferred from the matrix to the fracture system with pressure drop between the
two systems). Formation volume factor also affects fracture conductivity
estimates for hydraulically fractured wells.
Ambastha and van Kruysdijk (1993) performed an error analysis study to
quantify the effect of errors in the material balance equation for volumetric
gas reservoirs. They concluded that errors in gas PVT data (z-factor and two-
phase z-factor) in addition to errors in reservoir pressure can produce signifi-
cant errors in the calculated original gas-in-place (OGIP) volumes. They
used Monte Carlo simulation techniques to generate many cases for investi-
gation of the upper and lower bounds for OGIP estimation errors expected
from the errors in input data. The reported errors in OGIP reached 80% or
more. The authors also investigated the effect of depletion level on the sever-
ity of errors. From the reported data, it was concluded that at any level of
depletion, errors in gas PVT properties can lead to significant errors in OGIP
estimation.
Baker et al. (2003) studied the effect of PVT data errors on material bal-
ance equation results for oil reservoirs. They studied the use of PVT correla-
tions to derive PVT data for material balance analysis. They concluded that
deriving PVT data from correlations without tuning the correlation to match
the solution gas oil ratio above the bubble point pressure could lead to sig-
nificant errors in results of material balance analyses. They also studied the
effect of introduction of systematic and random errors into the PVT data for
material balance analysis. They concluded that the impact of PVT errors on
material balance results could be significant in two cases: (1) if the decrease
in reservoir pressure over the production history of the reservoir is small or
(2) if the oil is highly volatile.
Hutchinson (1951) reported that many parameters including oil and water
compressibility and formation volume factors affect the analysis results in
material balance and pressure build-up test analysis calculations. The effect
of isothermal oil compressibility in undersaturated oil reservoirs is signifi-
cant; hence, the importance of using accurate estimates.
Trengove et al. (1991) reported the effects of changing PVT data on sim-
ulation results in the case of a gas condensate reservoir. They used an equa-
tion of state (EOS) program to match the laboratory-measured PVT data and
to produce modified black-oil (MBO) PVT functions for further use in reser-
voir simulation. They observed that variation in PVT data resulted in