Sample Questions & Worked Out Examples For

MINISTRY OF SCIENCE AND TECHNOLOGY
DEPARTMENT OF
TECHNICAL AND VOCATIONAL EDUCATION
Sample Questions & Worked Out Examples
For
PE-04025
RESERVOIR ENGINEERING (I)
B.Tech(Second Year)
Petroleum Engineering
30
Ministry of Science and Technology
Department of Technical and Vocational Education
Petroleum Engineering
Sample Questions for
PE 04025 RESERVOIR ENGINEERING I
Chapter 1
1.*** The following experimental data were taken in determining the gas constant. An evacuated
glass flask weighs 50g. Filled with pure nitrogen at 14.40 psia and 60°F, it weighs 51.16g. Filled
with distilled water at 60° F, it weights 1050g. Calculate the gas constant from these data.
(5 marks)
2.*
Calculate the volume of one pound mole of ideal gas will occupy at (a) 14.7 psia and 60°,
(b) 14.7 psia and 32°F, (c) 14.7 psia plus 10 oz and 80°,and (d) 15.025 psia and 60°F.
(10 marks)
3.*
A 1000 cubic-foot tank is filled with air to a pressure of 25 psia at 140°F. Calculate and
place in tabular form for comparison, the molecular weight, specific gravity, pound moles in the
tank, pound in the tank, molecules in the tank, SCF at 14.7 psia and 60°F, SCF at 14.7 psia and
32°, SCF at 14.7 psia +10oz and 80°F, and SCF at 15.025 psia and 60°F, for air, carbon dioxide,
methane, and propane.
(10 marks )
4.** A 500 cubic-foot tank contains 10 lb of ethane at 90°F. (a) How many moles are in the
tank? (b) What is the pressure of tank in psia ? (c) What is the molecular weight of mixture. (d)
What is the specific gravity of mixture.
(10 marks)
5.*** Calculate the molecular weight of air if it contains 78.06 per cent nitrogen, 21.0 per cent
oxygen, and 0.94 per cent argon by volume. ( 5 marks )
6.*** What are the molecular weight and specific gravity of a gas which contains one third each
of methane, ethane, and propane by volume.
(5 marks)
7.** A ten-pound block of Dry Ice (solid CO2) is placed in a 50 cubic-foot tank which contains
air at atmospheric pressure 14.7 psia and 75°F. What will be the final pressure of the sealed tank
when all the Dry Ice has evaporated and cooled the gas to 45°F? (5 marks )
8.*
(a) A 55,000 bbl (nominal) pipe line tank has a diameter of 110 ft and a height of 35 ft. It
contains 25 ft of oil at the time suction is taken on the oil with pumps which handle 20,000 bbl per
day. The breather and safety valves have become clogged so that a vacuum is drawn on the tank.
If the roof is rated to withstand ¾ oz per sq in. pressure ,how long will it be before the roof
collapses? Barometric pressure is 29.1 inches. Neglect the fact that the roof is peaked and that
there may be some leak. (b) Calculate the total force on the roof at the time of collapse. (c) If the
tank had contained more oil, would the collapse time have been greater or less? Explain.
(20 marks)
9.*
What is the molecular weight of a gas which contains 50 per cent C1, 30 per cent C2, and
20 per cent C3 by weight?
(5 marks )
10.* What volume will 100 lb of a gas of 0.75 specific gravity ( air = 1.0 ) at 100 psia and
100°F ?
(5 marks )
11.* A 10 cubic-foot tank contains ethane at 25 psia surrounding a balloon 2 ft in diameter
filled with methane at 35 psia. Neglecting the volume of the rubber in the balloon and assuming
isothermal conditions, calculate the final pressure when the balloon is burst.
(10 marks )
31
12.** (a) What per cent methane by weight does a gas of 0.65 specific gravity contain which
composed only of methane and ethane? What per cent by volume. (b) Explain why the per cent by
volume is greater than the per cent by weight?
( 5 marks )
13.* (a) A 1500 cubic-foot tank contains methane at 30 psia and 80°F. To it are added: 1.80
moles of ethane at 14.4 psia and 60°F, 25 lb of butane at 75°F, 4.65× 1026 molecules of propane at
30°F, and 500 SCF(14.7 psia and 60°F) of nitrogen. If the final temperature of the mixture is
60°F, what is the final pressure of the tank? (b) Of what significance are the temperature given in
the part (a), with the mole of ethane, the pound of butane, and the molecules of propane? Explain.
(20 marks)
14.* A 50 cubic-foot of tank contains gas at 50 psia and 50°F. It is connected to another tank
which contains gas at 25 psia and 50°F. When the valve between the two is opened, the pressure
is equalizes at 35 psia at 50°F. What is the volume of other tank ?
(5 marks )
15.** A high-pressure cell has a volume of 0.330 cu ft and contains gas at 2500 psia and 130°F,
at which conditions its deviation factor is 0.75. When 43.6 SCF measured at 14.7 psia and 60°F
are bled from the cell through the wet test meter, the pressure dropped to 1000 psia , the
temperature remaining at 130°F. What is the gas deviation factor at 1000 psia and 130°F?
(10 marks)
16.* (a) Calculate the bulk volume of the gas cap of a reservoir whose aerial extent is 940
acres, i.e., the area enclosed by the zero thickness contour. The areas enclosed by the
4,8,12,16,and 20-ft isopach lines are 752 ,526,316,142, and 57 acres, respectively. The greatest
thickness within the 20-ft isopach line is 23 ft . (b) Show that when the ratio of the area enclosed
by two successive contours is 0.50, the error introduced by using the trapezoidal formulae is 2 per
cent greater than the pyramidal formula. (c) What error is introduced by using the trapezoidal
formula instead of the pyramidal formula when the ratio of the areas is 0.333?
(20 marks)
17.** A volumetric gas field has an initial pressure of 4200 psia, a porosity of 17.2 per cent , and
connate water of 23 per cent. The gas volume factor at 4200 psia is 292 SCF/cu ft and at 750 psia
is 54 SCF/cu ft. (a) Calculate the initial in place gas in standard cubic feet on a unit basis. (b)
Calculate the initial gas reserve in standard cubic feet on a unit basis, assuming an abandonment
pressure of 750 psia .
( 10 marks )
18.* A volumetric gas field has an initial pressure of 4350 psia, a porosity of 16.75 per cent ,
and connate water of 20 per cent. The gas volume factor at 4350 psia is 285 SCF/cu ft and at 755
psia is 57.5 SCF/cu ft. (a) Calculate the initial reserve of a 640-acre unit whose average net
productive formation thickness is 34 ft, assuming an abandonment pressure of 755 psia. (b)
Calculate the recovery factor based on abandonment pressure of 755 psia. (10 marks)
19.* The “M” Sand is a small gas reservoir with an initial bottom-hole pressure of 3200 psia
and bottom-hole temperature of 220°F. It is desired to inventory the gas in placed at three
production intervals. The pressure production history and gas volume factor in cu ft/SCF at
standard condition of 14.7 psia and 60°F are as follows;
Pressure
Cumulative gas Production
Gas volume factor
psia
MM CF
cu ft / SCF
3200
0
0.0052622
2925
79
0.0057004
2525
221
0.0065311
2125
452
0.0077360
(c) Calculate the initial gas in place using production data at the end of each of the
production intervals, assuming volumetric behavior. (b) Explain why the calculations of the part
(a) indicate a water drive.
(10 marks)
32
20.** The gas cap of the “ S.J ”oil field had a bulk volume of 17000 ac-ft when the reservoir
pressure had declined to the 634 psig. Core analysis shows an average porosity of 18 per cent, and
average interstitial water of 24 per cent. It is desired to increase the recovery of oil from the field
by repressuring the gas cap to 1100 psig. Assuming that no additional gas dissolves in the oil
during repressuring, calculate the SCF (14.7psia and 60°F) required. The deviation factor for both
the reservoir gas and the injected gas are 0.86 at 634 psig and 0.78 at 1100 psig, both at 130°F.
(10 marks)
21.** (a)A well drilled in to a gas cap for recycling purposes is found to be in an isolated fault
block. After injecting 50MM SCF (14.7psia and 60°F), the pressure increased from 2500 to
3500psia. Deviation factors for the gas are 0.90 at 3500 and 0.80 at 2500 psia and the bottom hole
temperature is160°F. What the cubic feet of gas storage space in the fault block? (b) If the average
porosity is 16 per cent, average connate water is 24 per cent, and average sand thickness is 12 ft,
what is the aerial extent of the fault block?
(10 marks)
22.* A gas reservoir under partial water drive produced 12.0MMM SCF (14.7psia and 60 • F)
when the average reservoir pressure had dropped from 3000psia to 2200psia. During the same
interval an estimated 5.20MM bbl of water entered the reservoir based on the volume of the
invaded area. If the gas deviation factor at 3000psia and bottom-hole temperature of 170 • F is 0.88
and at 2200psia is 0.78, what is the initial volume of gas in place measured at 14.7psia and 60 • F?
( 10 marks )
23.** A gas-producing formation has a uniform thickness of 32 ft, a porosity of 19 per cent, and
connate water saturation of 26 percent. The gas deviation factor is 0.83 at the initial reservoir
pressure of 4450psia and reservoir temperature of 175 • F. (S.C. 14.7psia and 60 • F)
(a) Calculate the initial in-place gas per acre-foot of bulk reservoir rock.
(b) How many years will it take a well to deplete by 50 percent a 640-acre unit at the rate of 3MM
SCF/day?
(10 marks )
24.** Calculate the daily gas production including the condensate and water gas equivalents for
a reservoir with the following daily production.
Separator gas production = 6MM SCF
Condensate production = 100 STB
Stock tank gas production = 21M SCF
Fresh water production =10bbl
Initial reservoir pressure = 6000psia
Current reservoir pressure = 2000psia
Reservoir temperature = 225 • F
Water vapor content at 6000psia and 225 • F = 0.86 bbl/MM SCF
Condensate gravity = 50 ο API
(10 marks )
25.* A gas condensate reservoir initially contains 1300 MSCF of residue ( dry or sale gas ) per
ac-ft and 115 bbl of condensate. Gas recovery is calculated to be 85 per cent and condensate
recovery 58 per cent by depletion performance. Calculate the value of initial gas and condensate
reserves per ac-ft if the condensate sells for $ 2.50/bbl and the gas sells for 20 cent / MSCF.
(10 marks)
26.** A well produces 45.3 bbl of condensate and 742 MSCF of sales gas daily. The condensate
has a molecular weight of 121.2 and gravity of 52.0° API at 60F. (a) What is the gas oil ratio on
dry gas basis? (b) What is the liquid content expressed in bbl/MMSCF on a dry gas basis?
(c) What is the liquid content expressed in GPM on a dry gas basis?
(10 marks)
27.* The initial daily production from a gas-condensate reservoir is 186 bbl of condensate,
3750 MSCF of high pressure gas, and 95 MSCF 0f stock tank gas. The tank oil has a gravity of
51.2° API at 60° F. The specific gravity of the separator is 0.712 and of the stock tank gas, 1.30.
33
The initial reservoir pressure is 3480 psia and reservoir temperature is 220°F. Average
hydrocarbon porosity is 17.2 per cent . Assume standard condition of 14.7 psia and 60°F.
(a) What is the average gravity of the produced gases?
(b) What is the initial gas oil ratio?
(c) Estimate the molecular weight of the condensate?
(10 marks)
28.** In a PVT study of a gas-condensate fluid 17.5 cu cm of wet gas (vapor) measured at cell
pressure of 2500 psia and temperature of 195°F was displaced into an evacuated low-pressure
receiver of 5000 cu cm volume which was maintained at 250°F to insure that on liquid phase
developed in the expansion. If the pressure of the receiver rose to 620mm Hg, what is the
deviation factor of the gas in the cell at 2500 psia and 195°F, assuming the gas in the receiver
behaved ideally?
(10 marks )
29.** Calculate the contents of a tank of ethane in moles, pounds, molecules, and SCF. Data are
as a 500 cu ft tank of ethane at 100 psia and 100°F.
(5 marks )
30.* Calculate the gas deviation factor of the Bell Field gas from its specific gravity. Use the
Fig.1.2 and Fig.1.3 of textbook. The required data are as follows:
Specific gravity = 0,665 (air =1.0) , CO2 content
= 0.10 mole per cent
N2 content
= 2.07 mole per cent, Reservoir temperature = 213°F
Reservoir pressure = 3250 psia
(10 marks )
31.**
Calculate the net volume of an idealized reservoir from the isopachous map data
(planimetered areas):
Area (acres)
A0
450
-
A1
375
5
A2
303
5
A3
231
5
A4
154
5
A5
74
5
A6
0
Interval (ft)
4
( 5 marks )
32.* Calculate the initial gas reserve of a 160 acre unit of the Bell Gas Field by volumetric
depletion and under partial and complete water drive. The required data are given as follows:
Average porosity = 22 %,
Connate water = 23 % ,
Areas = 160 acres ,
Net productive thickness = 40 ft,
Bgi =188.0 SCF/ cu ft at pi =3250 psia
Bg =150.0 SCF/ cu ft at 2500 psia and 27. 6 SCF/ cu ft at 500 psia,
Residual gas saturation after water displacement = 34 %
(10 marks )
33.* Calculate the initial gas in place and the initial reserve of a gas reservoir from pressure
production data for a volumetric reservoir. Required data are as follows:
Initial pr. = 3250 psia ,
Reservoir temp. = 213°F ,
Standard pr. =15.025 psia ,
Standard temp. = 60°F ,
9
Avg. res. pr. = 2864 psia ,
Cumulative production = 1.0 × 10 SCF,
Gas deviation factor at 3250 psia =0.910 , Gas deviation factor at 2864 psia
=0.888 , Gas deviation factor at 500 psia = 0.951 .
(10 marks)
34.** Calculate the water influx and residual gas saturation in water drive reservoir. Required
data are as follows:
Average porosity = 0.172 ,
Average connate water = 0.25 ,
Initial pressure = 3200 psia, Final pressure = 2925 psia, Bw = 1.03 bbl/STB ,
Gp = 935.4 MMSCF at 14.7 psia and 60°F ,
Bgi = 0.005262 cu ft/SCF,14.7 psia and 60°F ,
Bgf = 0.005700 cu ft/SCF, 14.7 psia and 60°F,
Cumulative water production = 15200 bbl (surface) ,
Bulk volume invaded by water at 2925 psia = 13.04 MMCF.
34
Bulk reservoir volume, initial = 415.3 MMCF,
(10 marks)
35.* Calculate the total daily gas production including the gas equivalents of water and
condensate. Required data are as follows:
Initial reservoir pressure = 4000 psia ,
Reservoir temperature = 220°F ,
Current reservoir pressure = 1500 psia, Condensate gravity = 55°API (0.759 sp. gr)
Daily gas saperator production = 3.25 MMSCF, Daily stock tank gas =10 MSCF ,
Daily stock tank condensate = 53.2 STB , Daily fresh water production = 5.5 bbl
(10 marks)
36.** The “M” Sand is a small gas reservoir with an initial bottom-hole pressure of 3200 psia
and bottom-hole temperature of 220°F. It is desired to inventory the gas in placed at three
production intervals. The pressure production history and gas volume factor in cu ft/SCF at
standard condition of 14.7 psia and 60°F are as follows;
Pressure
Cumulative gas Production
Gas volume factor
psia
MM CF
cu ft / SCF
3200
0
0.0052622
2925
79
0.0057004
2525
221
0.0065311
2125
452
0.0077360
(a) Show that a water drive exists by plotting the cumulative production versus p/z. (b) Based on
electric log and core data , volumetric calculations on the “ M “ Sand showed that initial volume
of gas in place is 1018 MMSCF . If the sand is under the partial water drive, what is the volume of
water encroached at the end of each of the periods? There was no appreciable water production.
( 20 marks )
37.** The gas cap of the “ S.J ”oil field had a bulk volume of 17000 ac-ft when the reservoir
pressure had declined to the 634 psig. Core analysis shows an average porosity of 18 per cent, and
average interstitial water of 24 per cent. It is desired to increase the recovery of oil from the field
by repressuring the gas cap to 1100 psig. Assuming that no additional gas dissolves in the oil
during repressuring, calculate the SCF (14.7psia and 60°F) required. The deviation factor for both
the reservoir gas and the injected gas are 0.86 at 634 psig and 0.78 at 1100 psig, both at 130°F. If
injected gas has a deviation factor 0.94 at 634 psig and 0.88 at 1100 psig, and the reservoir gas
deviation factors are as above, recalculate the injected gas required.
(20 marks)
38.** The initial daily production from a gas-condensate reservoir is 200 bbl of condensate,
3700 MSCF of high pressure gas, and 100 MSCF 0f stock tank gas. The tank oil has a gravity of
51.2° API at 60° F. The specific gravity of the separator is 0.712 and of the stock tank gas, 1.30.
The initial reservoir pressure is 3475 psia and reservoir temperature is 223°F. Average
hydrocarbon porosity is 17.5 per cent . Assume standard condition of 14.7 psia and 60°F.
(a)Calculate the specific gravity (air = 1.0) of the total well production. (b)Calculate the gas
deviation factor of the initial reservoir fluid at initial reservoir pressure. (c)Calculate the initial
moles in place per ac-ft.
(10 marks )
39.** Calculate the initial gas reserve of a 225 acre unit of the “ X “ gas field by volumetric
depletion and under partial and complete water drive. The required data are given as follows:
Average porosity = 18.75 %,
Connate water = 22.35 % , Areas = 225 acres ,
Net productive thickness = 30 ft,
Bgi =188.0 SCF/ cu ft at pi =3250 psia
Bg =150.0 SCF/ cu ft at 2500 psia and 27. 6 SCF/ cu ft at 500 psia,
Residual gas saturation after water displacement = 27.5 %
(10 marks )
40.* Calculate the initial oil and gas in place per acre-foot for a gas condensate reservoir.
35
Given:
Initial pressure 2740 psia
Reservoir temperature 215°F
Average porosity 25%
Average connate water 30%
Daily tank oil 242bbl
Oil gravity , 60°F
48°API
Daily separator gas 3100MCF
Separator gas gravity 0.650
Daily tank gas 120MCF
Tank gas gravity 1.20
(10 marks )
41.* The initial daily production from a gas-condensate reservoir is 200 bbl of condensate,
3700 MSCF of high pressure gas, and 100 MSCF 0f stock tank gas. The tank oil has a gravity of
51.2° API at 60° F. The specific gravity of the separator is 0.712 and of the stock tank gas, 1.30.
The initial reservoir pressure is 3475 psia and reservoir temperature is 223°F. Average
hydrocarbon porosity is 17.5 per cent . Assume standard condition of 14.7 psia and 60°F.(a)
Calculate the more fraction which is gas in the initial reservoir fluid. (b) Calculate the initial
(sales) gas and condensate in place per acre-foot.
(10 marks )
42.*** Gas was contracted at 5.5 cents per MCF at contract conditions of 14.4 psia and 80°F.
What is the equivalent price at a legal temperature of 60°F and pressure of 15.025 psia?
(5 marks )
43.** A gas producing formation has a uniform thickness of 32 ft, a porosity of 19%, and
connate water saturation of 26%. The gas deviation factor is 0.83 at the initial reservoir pressure
of 4450 psia and reservoir temperature of 175°F.(S.C.14.7 psia and 60°F ). (a) If the reservoir is
under an active water drive so that the decline in reservoir pressure is negligible, and during the
production of 50.4MMM SCF of gas, measured at 14.7psia and 60 • F, water invades 1280 acres,
what is the per cent recovery by water drive? (b) What is the gas saturation as per cent of total
pore space in the water invaded portion of the reservoir?
(10 marks )
44.* The initial volume of gas in place in the ' P' sand reservoir of the Holden Field is
calculated from electric log and core data to be 200MMMSCF (14.7 psia and 60°F) underlying
2250 productive acres, at the initial pressure of 3500 psia and 140°F. The pressure-production
history is
Pressure, psia
Production ( MMMSCF )
Gas deviation factor at 140°F
3500 ( initial )
0.0
0.85
2500
75.0
0.82
(a) What is the initial volume of gas in place as calculated from the pressure-production history
assuming no water influx? (b) Assuming uniform sand thickness, porosity, and connate water, if
the volume of gas in place from pressure-production data is believed to be correct, how many
acres of extension to the present limits of the " P " Sand are predicted? (c) If, on the other hand,
the gas in place calculated from the log and core data is believed to be correct, how much water
influx must have occurred during the 75MMMSCF of production to make the two figures agree?
(20 marks)
45.** Calculate the contents of a tank of ethane in moles, pounds, molecules, and SCF. Given a
(10 marks )
1500 cu-ft tank of ethane at 325 psia and 120°F.
46.** Calculate the gas deviation factor of the Bell Field gas from its specific gravity. Given
data are the following. Specific gravity 0.75, CO 2 content is 0.15 moles per cent, N 2 content is
2.2 moles per cent, reservoir temperature 200°F, reservoir pressure 3500 psia. Used the Fig.1.2
and Fig.1.3.
(10 marks )
47.* Calculate the gas deviation factor for the gas-condensate fluid whose composition at 5280
psia and 265°F is given as follows:
Component
C1
C2
C3
C4
C5
C6
C+7
Gas condensate
87.07
4.39
2.29
1.74
0.83
0.60
3.80
Mol. wt C+7
225
181
112
157
…
GOR, SCF/bbl
625
2000
18200 105000
Inf.
36
Tank gravity, API
34.3
50.1
60.8
54.7
(10 marks)
48.* Calculate the gas deviation factor for the " Z " Field gas from its composition given below.
Used the Fig.1.2 and 1.3.
Component Comp: Mole Fraction
Mol: wt
Pc
Tc
Methane
0.8612
16.04
673
343
Ethane
0.0591
30.07
708
550
Propane
0.0358
44.09
617
666
Butane
0.0172
58.12
550
766
Pentane
0.0050
72.15
490
846
CO2
0.0010
44.01
1070
548
N2
0.0207
28.02
492
227
(20 marks )
49.** Calculate the initial oil and gas in place per acre-foot for a gas condensate reservoir. Given
data are as follows:
Initial pressure ………………………………………2740psia
Reservoir temperature ……………………………… 215 ο F
Average porosity……………………………………..25 per cent
Average connate water……………………………….30 per cent
Daily tank oil…………………………………………242 bbl
Oil gravity, 60 ο F …………………………………… 48.0 ο API
Daily separator gas……………………………………3100 MCF
Separator gas gravity………………………………….0.650
Daily tank gas …………………………………………120 MCF
Tank gas gravity……………………………………….1.20
Use the Fig.1.2 and 1.3.
(10 marks)
50.*
A 725 cubic-foot tank contains 12.5-lb of methane and 21.5-lb of ethane at 95°F.
(c) How many moles are in the tank?
(d) What is the pressure of the tank in psia ? psig ?
(e) What is the molecular weight of the mixture?
(f) What is the specific gravity of the mixture?
(10 marks)
* = Must know, ** = Should know, *** = Could know
37
Ministry of Science and Technology
Department of Technical and Vocational Education
Petroleum Engineering
Worked Out Examples for
PE 04025 RESERVOIR ENGINEERING I
1. Calculate the contents of a tank of ethane in moles, pounds, molecules, and SCF. Data are as a
500 cu ft tank of ethane at 100 psia and 100°F.
Solution:
Assuming ideal gas behavior,
Moles =
100 × 500
= 8.324
10.732 × 559.7
Pounds = 8.324 × 30.07 = 250.3
Molecules = 8.324 × 2.733 × 1026 = 2.275 × 1027
At 14.7 psia and 60°F,
SCF = 8.324 ×379.4 = 3158
(or)
SCF =
100 × 500 × 519.7
= 3158 (or)
559.7 × 14.7
SCF =
nRT 8.324 × 10.73 × 519.7
=
= 3157
p
14.7
2. Calculate the gas deviation factor of the Bell Field gas from its specific gravity. Use the
Fig.1.2 and Fig.1.3 of text book. The required data are as follows:
= 0.10 mole per cent
Specific gravity = 0,665 (air =1.0) ,
CO2 content
N2 content
= 2.07 mole per cent , Reservoir temperature = 213F
Reservoir pressure = 3250 psia
Solution:
are
From the Fig .1.2 the critical pressure and temperature corrected for CO2 and N2
pc = 669 – ( 2.07 × 1.7 ) + (0.10 × 4.4 )
= 666 psia
Tc = 378 – ( 2.07 × 2.5 ) - ( 0.10 × 0.8 )
= 373°R
For 3250 psia and 213°F, pseudoreduced pressure and pseudoreduced temperature
are
38
pr =
3250
p
=
pc
666
= 4.88 ,
Tr =
460 + T
460 + 213
=
Tc
373
= 1.80
Enter Fig.1.3 with the values of pr = 4.88, and Tr = 1.80 . Then we obtain the
value of z = 0.90 .
3. Calculate the gas deviation factor for the " Z " Field gas from its composition given below.
Used the Fig.1.2 and 1.3.
Component
Methane
Ethane
Propane
Butane
Pentane
CO2
N2
Comp:
Mole Fraction
0.8612
0.0591
0.0358
0.0172
0.0050
0.0010
0.0207
Mol: wt
16.04
30.07
44.09
58.12
72.15
44.01
28.02
Pc
Tc
673
708
617
550
490
1070
492
343
550
666
766
846
548
227
Solution :
Mole Fraction × Mol: wt
Mole Fraction × Pc
Mole Fraction × Tc
13.81
579.59
295.39
1.78
41084
32.51
1.58
22.08
23.84
1.00
9.46
13.18
0.36
2.45
4.23
0.04
1.07
0.55
0.58
10.18
4.70
................
........................
....................
19.15
666.67
374.40
The specific gravity may be obtained from the sum 19.15 , which is the average molecular weight
of the gas ,
19.15
= 0.661
28.97
The sum 666.67 and 374.40 are the pseudocritical pressure and temperature, respectively. Then
at 3250 psia and 213°F, the pseudocritical pressure and temperature
SG =
39
Pr =
3250
= 4.87
666.67
673
= 1.80
374.4
The gas deviation factor using Fig. 1.3 is z = 0.91
Tr =
4. Calculate the initial gas reserve of a 160-acre unit of the Bell Gas Field by volumetric
depletion and under partial and complete water drive.
Given
Average porosity = 22 per cent
Connate Water = 23 per cent
Residual gas saturation after water displacement = 34 per cent
Bgi = 188.0 SCF / cuft at Pi = 3250 psia
Bg = 150.0 SCF / cuft at 2500 psia
= 27.6 SCF / cuft at 500 psia
Area = 160 acres
Net productive formation = 40 ft
Solution :
Pore Volume = 43560 × 0.22 × 160 × 40
= 61.33 × 106 cu ft
Initial gas in place ;
G1 = 61.33 × 106 × ( 1- 0.23 ) × 188.0
= 8878 MMSCF
Gas in place after volumetric depletion to 2500 psia ;
G2 = 61.33 × 106 × ( 1- 0.23 ) × 150.0
= 7084 MMSCF
Gas in place after volumetric depletion to 500 psia ;
G3 = 61.33 × 106 × ( 1- 0.23 ) × 27.6
= 1303 MMSCF
Gas in place after water invasion at 3250 psia;
G4 = 61.33 × 106 × 0.34 × 188.0
= 3920 MMSCF
Gas in place after water invasion at 2500 psia;
G5 = 61.33 × 106 × 0.34 × 150
= 3128 MMSCF
Initial reserve by depletion to 500 psia ;
G1 – G3 = ( 8878 – 1303 ) × 106
= 7575 MMSCF
Initial reserve by water drive at 3250 psia ;
G1 – G4 = ( 8878 – 3920 ) × 106
= 4958 MMSCF
Initial reserve by water drive at 2500 psia ;
( G1 – G2 ) + ( G2 – G5 ) = ( G1 – G5 )
= ( 8878 – 3128 ) × 106
= 5750 MMSCF
If there is one updip well, the initial reserve by water drive at 3250 psia is
40
1
1
(G–G)=
( 8878 – 3920 ) × 106
2
2
= 2479 MMSCF
5. Calculate the initial gas in place and the initial reserve of a gas reservoir form pressureproduction data for a volumetric reservoir.
Given;
Initial Pressure
= 3250 psia
Reservoir temperature
= 213°F
Standard temperature
= 60° F
Standard pressure
= 15.025 psia
Cumulative production
= 1.00 × 109 SCF
Average reservoir pressure
= 2864 psia
Gas deviation factor at 3250 psia
= 0.910
Gas deviation factor at 2864 psia
= 0.888
Gas deviation factor at 500 psia
= 0.951
Solution;
The reservoir gas pore volume Vi
3250 × Vi
2864Vi
15.025 × 1.00 × 10 9
−
=
520
0.910 × 673 0.888 × 673
Vi = 56.17 MM cu ft
The initial gas in place is
G=
PiVi Tsc 3250 × 56.17 × 10 6 × 520
×
=
= 10.32MMSCF
z i T p sc
0.910 × 673 × 15.025
The gas remaining at 500 psia abandonment pressure is
Ga =
PaVi Tsc 500 × 56.17 × 10 6 × 520
×
=
= 1.52 MMSCF
z a T p sc
0.951 × 673 × 15.025
The initial gas reserve based on a 500 psia abandonment pressure is the difference
between the initial gas in place and remaining at 500 psia or
Gr = G - Ga
= (10.32 – 1.52 ) × 109
= 8.80 MMSCF
6. Calculate the water influx and residual gas saturation in water drive reservoir. Required data
are as follows:
Average porosity
Initial pressure
Gp =
Bgi =
= 0.172 ,
Average connate water = 0.25 ,
= 3200 psia, Final pressure
= 2925 psia,
935.4 MMSCF at 14.7 psia and 60°F ,
0.005262 cu ft/SCF, 14.7 psia and 60°F ,
41
Bgf = 0.005700 cu ft/SCF, 14.7 psia and 60°F,
Bw = 1.03 bbl/STB ,
Cumulative water production
= 15200 bbl (surface) ,
Bulk volume invaded by water at 2925 psia = 13.04 MMCF.
Bulk reservoir volume, initial
= 415.3 MMCF,
Solution:
415.3 × 10 6 × 0.172 × (1 − 0.25)
0.005262
= 10180 MMSCF at 14.7 psia and 60° F
Initial gas in place = G =
We = 935.4 × 106 × 0.005700 - 10,108 × 106
= ( 0.005700 – 0.005262 ) + 15200 × 1.03 ×5.615
= 960400 cu ft
This much water has invaded 13.04 MM cu ft of bulk rock, which initially contained 25
per cent connate water. Then the final water saturation of the flooded portion of the
reservoir is
Sw =
=
Connatewater + Water inf lux
Porespace
(13.04 × 10 6 × 0.172 × 0.25) + 960400
13.04 × 10 6 × 0.172
= 0.68 or 68 percent
Then the residual gas saturation Sgr is 32 per cent.
------------------------------------7. Calculate the initial oil and gas in place per acre-foot for a gas condensate reservoir. Given
data are as follows:
Initial pressure ………………………………………2740psia
Reservoir temperature ……………………………… 215 ο F
Average porosity……………………………………..25 per cent
Average connate water……………………………….30 per cent
Daily tank oil…………………………………………242 bbl
Oil gravity, 60 ο F …………………………………… 48.0 ο API
Daily separator gas……………………………………3100 MCF
Separator gas gravity………………………………….0.650
Daily tank gas …………………………………………120 MCF
Tank gas gravity……………………………………….1.20
Use the Fig.1.2 and 1.3.
Solution:
Average gas gravity =
3100 × 0.650 + 120 × 1.20
= 0.670
3100 + 120
42
γo =
Mo =
R=
γw =
141.5
= 0.7883
48.0 + 131.5
6084
= 144.5
48.0 − 5.9
3100 + 120
× 1000 = 13300SCF / bbl
242
13300 × 0.670 + 4584 × 0.7883
= 0.893
13300 + 132800 × 0.7883 / 144.5
From Fig. 1.2 , Tc = 425°R , Pc = 652 psia using the condensate curves. Then Tr = 1.59
and Pr = 4.20 , from which, using Fig. 1.3, the gas deviation factor is 0.82 at 2740 psia
and 215°F. Then the total initial gas in place per ac-foot of bulk reservoir rock is
G=
379.4 pV 379.4 × 2740 × 43560 × 0.25 × (1 − 0.30)
=
= 1334 MCF/ac ft
0.82 × 10.73 × 675
zRT
Since the volume fraction equals the mole fraction in the gas state, the fraction of the total
which is produced on the surface as gas is
fg =
Then,
ng
n g + no
=
R / 379.4
= 0.9483
R / 379.4 + 350 × 0.7883 / 144.5
Initial gas in place = 0.9483 × 1334 = 1265 MCF/ac ft
1265 × 10 3
Initial oil in place =
= 95.1bbl / acft
13300
8. A gas condensate reservoir initially contains 1300 MSCF of residue ( dry or sale gas ) per acft and 115 bbl of condensate. Gas recovery is calculated to be 85 per cent and condensate
recovery 58 per cent by depletion performance. Calculate the value of initial gas and
condensate reserves per ac-ft if the condensate sells for $ 2.50/bbl and the gas sells for 20
cent / MSCF.
Solution:
The value of initial condensate reserve = 115 × 0.58 = 66.7 bbl
= 66.7 × 2.5
= $ 166.75
The value of initial gas reserve
= 1300 ×0.85 = 1105 MSCF
= 1105 × 0.20
= $ 221
43
9. A well produces 45.3 bbl of condensate and 742 MSCF of sales gas daily. The condensate has
a molecular weight of 121.2 and gravity of 52.0° API at 60F. (a) What is the gas oil ratio on
dry gas basis? (b) What is the liquid content expressed in bbl/MMSCF on a dry gas basis? (c)
What is the liquid content expressed in GPM on a dry gas basis?
Solution:
Gas production rate
= 742MSCF
Condensate production rate = 45.3 bbl
742 × 10 3
(c) Gas-Oil ratio on a dry gas basis =
= 16380 SCF/bbl
45.3
(d) Liquid content in barrel per million standard cu ft on a dry gas basis
1bbl
= 61.05bbl / millionSCF
16380SCF
1bbl
42 gal
(c) Liquid content in GPM =
×
× 1000 = 2.56 GPM
16380SCF 1bbl
Liquid content =
10. The initial daily production from a gas-condensate reservoir is 186 bbl of condensate, 3750
MSCF of high pressure gas, and 95 MSCF 0f stock tank gas. The tank oil has a gravity of
51.2° API at 60° F. The specific gravity of the separator is 0.712 and of the stock tank gas,
1.30. The initial reservoir pressure is 3480 psia and reservoir temperature is 220°F. Average
hydrocarbon porosity is 17.2 per cent . Assume standard condition of 14.7 psia and 60°F.
(a) What is the average gravity of the produced gases?
(b) What is the initial gas oil ratio?
(c) Estimate the molecular weight of the condensate?
Solution:
Initial daily production of condensate = 186 bbl
Stock tank gas
= 95 M SCF
High pressure gas
= 3750 M SCF
S.G of separator gas
= 0.712
S.G of tank gas
= 1.3
API = 52.1°, T = 60°F, Pi = 3480 psia , Ti = 220 °F , Φ = 17.2%
95.3 × 1.3 + 3750 × 0.712
95 + 3750
= 0.727
(c)
Average gas gravity =
(d)
Initial GOR
(c)
Molecular weight of condensate Mo =
=
=
(95 + 3750) × 10 3
186
= 20672 SCF/bbl
6084
API − 5.9
6084
= 134.3
51.2 − 5.9
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