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Question 1
The R sand is a volumetric oil reservoir whose PVT properties are shown in the following
figure. When the reservoir pressure dropped from an initial pressure of 2500 psia to an
average pressure of 1600 psia, a total of 26 MMSTB of oil had been produced. The
cumulative GOR at 1600 psia was 954 SCF/STB, and the current GOR is 2250 SCF/STB.
The average porosity for the field is 18% and average connate water is 18%. No
appreciable amount of water was produced, and standard conditions were 14.7 psia and
60oF.
a) Calculate the initial oil in place.
b) Calculate the SCF of evolved gas remaining in the reservoir at 1600 psia.
c) Calculate the average gas saturation in the reservoir at 1600 psia.
d) Calculate the barrels of oil that would have been recovered at 1600 psia if all the
produced gas had been returned to the reservoir.
e) Calculate the two phase formation volume factor at 1600 psia.
f) Assuming no free gas flow, calculate the recovery expected by depletion drive
performance down to 2000 psia.
g) Calculate the initial SCF of free gas in the reservoir at 2000 psia.
Question 2
An oil reservoir initially contains 4 MM STB of oil at its bubble-point pressure of 3150
psia with 600 SCF/STB of gas in solution. When the average reservoir pressure has
dropped to 2900 psia, the solution gas oil ratio is 550 SCF/STB. Boi was 1.34 bbl/STB
and Bo at a pressure of 2900 psia is 1.32 bbl/STB.
Other data:
Rp = 600 SCF/STB at 2900 psia
Swi = 0.25
Bg = 0.0011 bbl/SCF at 2900 psia
Volumetric reservoir no original gas cap
a) How many STB of oil will be produced when the pressure has decreased to 2900
psia?
b) Calculate the free gas saturation that exists at 2900 psia.
Question 3
The following data is given for an oil reservoir:
Pressure
(psia)
Np
(MMSTB)
Gp
(MMMSCF)
Bg
(bbl/SCF)
Bo
(bbl/STB)
Rs
(SCF/STB)
2742 0 0 0.000726 1.315
2639 4.38 2.628 0.000757
2506 13.16 7.896 0.000796 1.311 578
The bubble point of the reservoir is 2639 psia. The connate water saturation has been
determined to be 15%. The compressibility of the undersaturated oil is 5 x 10-5 psi-1
.
a. Perform material balance at 2639 psia and 2506 psia to determine the initial oil in
place in the reservoir. What do the results indicate?
b. How does the free gas saturation change with pressure? From a combination of
seismic survey and well logs, the bulk volume of the reservoir is estimated to be
71,000 ac-ft and the porosity is estimated to be 0.25.

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