Advanced Reservoir Engineering_Assistant Prof Dr. Madi_Lecture_0001.pdf

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1. ADVANCED RESERVOIR ENGINEERING

ul Gal) | LECTURE NO#001
The Libyan Academy

By:

Dr. Madi Abdullah Naser

Email: [email protected]

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Email:

[email protected]

3. Definition of a Reservoir

A reservoir is that portion of a trap which contains oil and/or gas as
a single hydraulically connected system.

-it is a rock layer

-porous so that stores oil

-permeable so that allows flow of oil

-has a mechanism to prevent escape of oil(trapping

mechanism)

By: Dr. Madi Abdullah Naser

4. Definitions of Reservoir Engineering

Craft “The application of scientific principles to the drainage
problems arising during the development and production of oil
and gas reservoirs‘.

Colhoun “The phase of engineering which deals with the transfer

of fluids to, from or through the reservoirs".

By: Dr. Madi Abdullah Naser

5. FUNCTIONS OF RESERVOIR ENGINEERING

To continuously monitor the reservoir and collect relevant

data and interpret it to be able to

1; Determine (present conditions))

2. Estimate ( future conditions) and

3: Control

the movement of fluids through the reservoir so
that

By: Dr. Madi Abdullah Naser

6. OBJECTIVES OF RESERVOIR ENGINEERING

we can

a) enhance ( increase recovery factor) and

b) accelerate ( increase production rate)

the oil recovery

By: Dr. Madi Abdullah Naser)

<— Drilling rig Natural gas

+ Impervious
rock
+ Petroleum

Water

«— Impervious
rock

7. PETROLEUM RESERVOIR HC FLUIDS

+ Oil Reservoirs
- Produce mainly oil and gas and in some cases also water
- Heavy oil
- Conventional oil
- Black oil
- Volatile oil

+ Gas Reservoirs

Produce mainly gas and in some cases also water

- Dry gas
- Wet gas

* Gas Condensate Reservoirs

- Produce gas which contains hydrocarbon liquid

- Gas condensate

By: Dr. Madi Abdullah Naser

8. PHASE DIAGRAM OF RESERVOIR FLUIDS

Why do we need to classify Reservoir Fluids?

o Determine fluid sampling

o Determine types and sizes of surface equipment

o Dictate depletion strategy

o Determine selection of EOR method

o Determine techniques to predict oil & gas reserves

o Determine Material Balance calculations

By: Dr. Madi Abdullah Naser

9. PHASE ENVELOPES

Cricondenbar

Bubblepoint Curve

Critical E

Point Fixed
Composition

Dew Point Curve

Pressure

Cricondentherm

Temperature

By: Dr. Madi Abdullah Naser

10. PHASE DIAGRAM OF RESERVOIR FLUIDS

PVT CELL

6 all liquid

YLEELIAT ELIA)

E =Eo—
We nn

Van ©,

Bu = bubble

By: Dr. Madi Abdullah Naser

11. Phase Diagram of a Dry Gas Reservoir

A

Initial Reservoir
Conditions

CP,

Path of Production

Pressure

Separator Conditions

KAooooooo———————
Temperature

By: Dr. Madi Abdullah Naser

12. Phase Diagram of a Wet Gas Reservoir

Initial Reservoir
Conditions

Pressure

Path of Production

Separator Conditions

Temperature

By: Dr. Madi Abdullah Naser

13. Phase Diagram of a Retrograde Gas
Reservoir

Initial Reservoir
Conditions

ty of Produftion

Pressure

ef Conditions

Tem perature

By: Dr. Madi Abdullah Naser

14. Phase Diagram of a Volatile Oil Reservoir

Initial Reservoir
Conditions

Path of Production

Pressure

Temperature

By: Dr. Madi Abdullah Naser

15. Phase Diagram of a Black Oil Reservoir

Initial Reservoir
Conditions

Path of Production

Pressure

Sgparator

Temperature

By: Dr. Madi Abdullah Naser

16. Phase envelopes of different mixtures with
different proportions of same HC components

7000

TR @ Critical Points

6000 +
Volatile I

5000 +

Condensate

>

Ss

Ss

s
T

Pressure (psia)
0
S
e
o
T

2000 | Wet Gas
Black Oil
1000 +

0 - nl
-200 -100 0 100 200 300 400 500 600 700 800

Temperature gi E
By: Dr. Madi Abdullah Naser

17. TYPICAL RESERVOIR FLUID COMPOSITIONS

Component | Black Oil | Volatile Oil [Gas Condensate | WetGas | Dry Gas
a 48.83 64.36 87.07 95.85 86.67
Q 2.75 752 4.39 2.67 7.77
Ca 1.93 4.74 2.29 0.34 2.95
Cs 1.60 4.12 1.74 0.52 1.73
Cs 1.18 3.97 0.83 0.08 0.88
Es 1.59 3.38 0.60 0.12
Cr 42.15 11.91 3.80 0.42
M,C,” 225 181 ve 157
GOR 625 2000 18,200 105,000
Tank “API 34.3 50.1 60.8 54.7
Liquid Greenish Medium Light Water
Color Black Orange Straw White

By: Dr. Madi Abdullah Naser

18. CLASIFICATION OF OIL RESERVOIRS

1. Oll reservoirs: When the reservoir temperature T is less than the critical
temperature T- of the reservoir fluid.
A. Undersaturated oll reservoir. When the initial reservoir pressure pi is

greater than the bubble-point pressure py of the reservoir fluid.

Wwoc

Water

Undersaturated Oil Reservoir —

By: Dr. Madi Abdullah Naser

19. CLASIFICATION OF OIL RESERVOIRS

B. Saturated oll reservoir. When the initial reservoir pressure is equal to
the bubble-point pressure of the reservoir fluid.

—— Saturated Oil Reservoir

C. Gas-cap reservolr. When the initial reservoir pressure is below the

bubblepoint pressure.

By: Dr. Madi Abdullah Naser

20. CLASIFICATION OF GAS RESERVOIRS

2. Gas reservoirs: When the reservoir temperature is greater than the critical

temperature of the hydrocarbon fluid.

eye VOL Datuateu

Gas reservoir

A. Retrograde gas-condensate reservoir. When the reservoir
temperature T lies between the critical temperature Tc and
cricondentherm Tct of the reservoir fluid.

B. Near-critical gas-condensate reservoir. When the reservoir
temperature is near the critical temperature.

C. Wet-gas reservoir. When the reservoir temperature is above the
cricondentherm of the hydrocarbon mixture.

By: Dr. Madi Abdullah Naser

21. Oil Reservoir Drive Mechanisms

+ Solution gas drive

* Gas-cap drive

+ Water drive

+ Gravity-drainage drive
+ Combination drive

Gas Reservoir Drive Mechanisms
+ Volumetric reservoir (gas expansion drive)
+ Water drive
Ultimate oil and gas recoveries vary depending on the drive
mechanism. For oil; water drive is most effective. Typical
primary recoveries are in the 25-40% range (maximum 75%).

For gas; water drive will lower than gas expansion drive

By: Dr. Madi Abdullah

22. DRIVING MECHANISMS OF RESERVOIR PRODUCTION

A brief description of some major natural driving mechanisms and their
performances is as follows.
1. Solution Gas Drive.

e In Saturated reservoirs with solution gas drive without gas cap and
water drives will show decreasing oil production performance and
increasing gas-oil ratio immediately after the start of production.

+ In Undersaturated reservoirs without water drive will show decreasing
rate of oil production and constant GOR when pressure is above the

saturation pressure.

Reservoir pressure drops
below bubble-point pressure

By: Dr. Madi Abdullah Naser |

23. DRIVING MECHANISMS OF RESERVOIR PRODUCTION

Oil recovery in solution gas drive reservoirs

Initial reservoir
pressure

Bubblepoint

"4 pressure

Reservoir pressure, psig

0 5 10 15
Oil recovery, % of OOIP

Dissolved gas reservoirs typically recover between 5 and 25% OIIP

By: Dr. Madi Abdullah Naser

24. SOLUTION GAS A

DRIVE HISTORY N

o

N
Pressure

P, tim

e
Rapid and continuous pressure drop, rate of decline falls at bubble point pressure.
R (producing gas oil ratio) low until p = p,, then increases to maximum and declines.
Absent or minimal water influx (watercut).

Gravity drainage is a special case in steeply dipping reservoirs where gas drives out
more oil.

Well production declines rapidly; early pumping often required.

By: Dr. Madi Abdullah Naser

25. DRIVING MECHANISMS OF RESERVOIR PRODUCTION

‘Reservoir
Pressure
M;

N
N

TS Production Rate
>.

Daily Oil Production: bb!
3
Pressure:psi

3
Gas - Oil Ratio: cu. ft per bbl

Figure 11-2. Production data of a solution-gas-drive reservoir. (After Clark, N. J.,
Elements of Petroleum Reservoirs, SPE, 1969.)

By: Dr. Madi Abdullah Naser

26. DRIVING MECHANISMS OF RESERVOIR PRODUCTION

2. Gas Cap Drive.
e The existence of a gas cap at the top of the oil zone in saturated
reservoirs will help stabilizing the rate of oil production and gas-oil

ratio for some time.

Reservoir pressure is initially
below bubble point pressure

By: Dr. Madi Abdullah Naser A. Map View

27. GAS CAP DRIVE
HISTORY

o pressure drops continuously, but slowly.
o R (producing gas oil ratio) increases continuously.
o water influx (watercut) absent or minimal

o gas cap cannot be allowed to shrink or oil encroachment will occur resulting in
reduced recovery.

o oil leg wells can eventually produce gas.

o Wells have long flowing life (depending on the size of the gas cap).

By: Dr. Madi Abdullah Naser’

28. DRIVING MECHANISMS OF RESERVOIR PRODUCTION

1300

1200
1100
E 1000 3
£ à
j 900 800 3
E
a 600 3
3 40 3
2
: 03
Bl
5
3
>
3
a

Figure 11-6. Production data for a gas-cap-drive reservoir. (After Clark, N. J. Ele-
ments of Petroleum Reservoirs, SPE, 1969. Courtesy of API.)

By: Dr. Madi Abdullah Naser)

29. DRIVING MECHANISMS OF RESERVOIR PRODUCTION

3. Water Drive.
Reservoirs with water drive mechanism might be reservoirs with a
surrounding-aquifer (called edge water) or reservoirs with a water "reservoir
underneath (called bottom water).

Pi

Pressure

Watercut (%)

Producing

: GOR (R=R,)
Reservoir pressure is maintained by

water encroachment from aquifer

Time

By: Dr. Madi Abdullah Naser)

30. Hydrocarbon

Cross-section view

Plane view

The Upper Devonian Leduc pools are driven by inflow from the Cooking Lake
Aquifer.

By: Dr. Madi Abdullah Naser

31. Different Water Drive Mechanisms

Both bottom water drive, where the water leg underlies the entire
reservoir, and edge water drive, where only part of the areal

extent is contacted by water, are recognized.

Edge Water Drive Bottom Water Drive
Oil producing well

Oil producing well

Sl
PCR

A

In

Cross Section

By: Dr. Madi Abdullah Naser

Cross Section

32. NATURAL WATER
DRIVE HISTORY

_..pressure

OIL PRODUCTON

GOR(R)
Ru

time

o Pressure remains high; small drop.

o R (producing gas oil ratio) remains low.

o Water influx starts early and increases to appreciable levels.
o Residual oil may be trapped behind the advancing water.

o Wells flow freely until water production (watercut) becomes excessive.

By: Dr. Madi Abdullah Naser

33. DRIVING MECHANISMS OF RESERVOIR PRODUCTION

ie jr | + Reservoir Pressure ug 3
2100 A
a0 > |e
ES | : -Dil Ratio + 3
OA sence es
7 = eve
&_ | Ml | | rares
3° | Zz Water „| | ES ~ | 79%
= 9 + eer = + int
E & El
360 ol 03
84 >
=
5% 7
A
E Y (ar 1932 1933 1634 1935 1936 1937 198 1939 1940 1941 1942

Figure 11-10. Production data for a water-drive reservoir. (After Clark, N. J., Ele-
ments of Petroleum Reservoirs, SPE, 1969. Courtesy of API.)

By: Dr. Madi Abdullah Naser)

34. DRIVING MECHANISMS OF RESERVOIR PRODUCTION

4. Gravity Drainage.
In dipping reservoirs with large enough vertical and horizontal
permeabilities, oil may flow downwards or upwards towards the wells

because of the gravity effects.

Gravity Drainage E

Figure 3.7 Gravity Drainage

By: Dr. Madi Abdullah Naser)

35. COMPACTION DRIVE

o In compaction drive, the energy for oil production is provided by the
collapse of the porous medium skeleton and expansion of the pore fluids

when the reservoir pressure drops.
o The increase in the "grain pressure" or effective stress causes pore

“collapse” and “compaction” (consolidation) of the reservoir.

This drive mechanism is common in highly compressible, unconsolidated
reservoirs such as those found in California, Venezuela, and the heavy oil

deposits of western Canada.

By: Dr. Madi Abdullah Naser

36. RESERVOIR PERFORMANCE DATA (1)

Pressure trends in reservoirs under various drive mechanisms are
distinctive.

100 %

WATER DRIVE 4

a o fncmemememsmemee

80

%
40

GAS CAP DRIVE

=

20 1- SOLUTION
GAS DRIVE

|
0
0 10 20 30 40 50

% ONP Produced

By: Dr. Madi Abdullah Naser

37. RESERVOIR PERFORMANCE DATA (2)

Producing GOR is also strongly diagnostic of drive mechanism.

100 E
: GAS CAP DRIVE
80 : |
GOR 60

: |
20 SOLUTION |

20 GAS DRIVE

Pr A A [_ WATER DRIVE

0 10 20 30 40

OIIP Produced

By: Dr. Madi Abdullah Naser

38. DRIVING MECHANISMS OF RESERVOIR PRODUCTION

Recovery Factors For Oil Reservoirs

Average Oil Recovery

Drive Mechanism Factors,
% of OOIP
Range Average

Solution-gas drive 5 - 30 15
Gas-cap drive 15-50 30
Water drive 30-60 40
Gravity-drainage 16 -85 50
drive
Volumetric reservoir 70-90 80
(Gas expansion drive)
Water drive 35-65 50

By: Dr. Madi Abdullah Naser

39. ESTIMATING OIL RECOVERY FACTORS

Solution-gas drive - API study

( Ss ) 0.1611 k 0.0979 0.1741
colo] ET or
Bo, Hop Pa

Water drive - API study

5,59 (#0580) u ES Ce (2) > |
Ba, Mos Pa
Water drive - Guthrie-Greenberger study

E, = 0.272 logs, k + 0.256 S,,, - 0.136 logy, 44, — 1.538 $ — 0.0003 h +0.114

By: Dr. Madi Abdullah Naser

40. SUITABLE CHARACTERISTICS FOR OIL RECOVERY

+ Solution-gas drive oil reservoirs
— Low oil density
— Low oil viscosity
— High oil bubblepoint pressure

+ Gas-cap drive oil reservoirs
- Favorable oil properties

- Relatively large ratio of gas cap
to oil zone

— High reservoir dip angle
— Thick oil column

By: Dr. Madi Abdullah Naser

+ Water drive oil reservoirs

Large aquifer

Low oil viscosity

High relative oil permeability
Little reservoir heterogeneity

and stratification

+ Gravity drainage oil reservoirs

High reservoir dip angle
Favorable permeability
distribution

Large fluid density difference
Large segregation area

Low withdrawal

41. SUITABLE CHARACTERISTICS FOR OIL RECOVERY

e Water-drive gas reservoir
e Volumetric gas reservoir (gas
— Small aquifer
expansion drive)
- Small degree of reservoir
« Low abandonment pressure
heterogeneity and

stratification

By: Dr. Madi Abdullah Naser

42. Drive Indexes

o The driving mechanism may be:
A. solution gas drive or
B. depletion drive,
C. water drive, or
D. gas cap drive.

o If there is more than one driving mechanism in a reservoir, the contribution of
each of the mechanisms can be expressed using a variable called drive
index.

o Drive index is volume fraction, in underground withdrawal, of the produced
fluid caused by the corresponding driving mechanism.

By: Dr. Madi Abdullah Naser

43. Drive Indexes

o According to Pirson, driving indexes for the three driving mechanisms are:

1. DDI: Depletion drive index

2. SDI: Segregation (gas cap) index

3. WDI : Water drive index.

o If the water and pore compressibilities are neglected, the general material

balance equation of Eq. (7.11) will reduce to

(Bo — Boi) +(Rsoi—Rso) Bg

N, [Bo + (Rp — Rio) By] = N Bai [
Boi

Bg

+m (— —1)] + (We- WpBw) (7.26)

Bei

By: Dr. Madi Abdullah Naser

44. Drive Indexes
If Eq. (7.26) is divided by Np [Bo + (Rp - Rso) Bg] which represents the

o

cumulative production of oil and gas calculated in the reservoir at p = pi -
Ap, in this case it is the same as the total volume change caused by fluid

and pore expansions and net water influx, the equation can then be written

Be
N i—-1
_ N[(Bo — Boi) + (Rsoi— Rso) Bg] 2 e Gui ?

+
Np[Bo + (Rp — Rso) Bg] Np[Bo + (Rp — Rso) Bg]

(We- Wp Bw)

ee ee E (7.27)
Np[Bo + (Rp —Rso) Bg]

By: Dr. Madi Abdullah Naser

45. Drive Indexes

Pirson defined each fraction as drive indexes as shown below.

N[(Bo — Boi) + (Rsoi— Rso) Bg]

DDI=
Np[Bo + (Rp — Rso) Bg]
B,
Nm Beil, -1)
SDI= — 8
Np[Bo + (Rp E Rso) Bg]
op — oe Wp Bw)

Np[Bo + (Rp —Rso) Be]
It is clear from Eqs. (7.27) through (7.30) that the following applies
DDI + SDI + WDI= 1.0

By: Dr. Madi Abdullah Naser

(7.28)

(7.29)

(7.30)

(7.31)

46. Drive Indexes
Problem#1:
The production and PVT data of a reservoir having combination driving

mechanisms of solution gas and gas cap are shown in Table below.

p B, B, Np R,
(psia) (rb/SCF) (rb/STB) | (MMSTB) | (SCF/STB)
2500 | 0.001048 1.498 0 0
2300 | 0.001155 1.523 3.741 716
2100 | 0.001280 1.562 6.849 966
1900 0.001440 1.620 9.173 1297
1700 | 0.001634 1.701 10.99 1623
1500 | 0.001884 1.817 12.42 1953
1300 | 0.002206 1.967 14.39 2551
1100 | 0.002654 2.251 16.14 3214
900 0.003300 2.597 17.38 3765
700 0.004315 3.209 18.50 4317
500 0.006163 4.361 19.59 4839

: Dr. Madi Abdullah Naser

47. Drive Indexes

Problem#1:

o Well test and well log data show that the gas cap volume of the reservoir is equal
to half of initial oil volume.

o Initial reservoir pressure and solution GOR are 2500 psia and 721 SCF/STB,
respectively.

o From the geological calculation using the volumetric method, the IOIP is known
as 56 MMSTB.

o However, the given data shows incompleteness. Additional information says that
a gas injection for pressure maintenance has been previously done to the
reservoir.

o Meanwhile, it is known that there is neither water drive mechanism nor water
production.

a) When (i.e. at what pressure) was the pressure maintenance program started?

b) How much gas (in SCF) was injected until the pressure reached 500 psia.

Assume the gas in reservoir and injected gas has the same compressibility factors.

By: Dr. Madi Abdullah Naser

48. Drive Indexes

Solution#1:
(a) Use drive index to know the contribution of each mechanism. For oil with
dissolved gas, use DDI and SDI formula below:

DDI = N(Bt- Bui)
NpÍB: El (Rp 1e Rsoi) Bg]

_ _56x105(B,-1.498)
Np[Bt + (Rp—72)) Bel

N 2

Bi (pe Bei)
Bgi

SDI —
NpIBt + (Rp = Rsoi) Bg]

56x 109 (0.5)(1.498)
0.001048
Np[Bt + (Rp-721) Bg]

By: Dr. Madi Abdullah Naser

(Bg —0.001049

49. Drive Indexes
Solution#1:

the calculation results in the drive indexes data as shown in the following Table. In is
clearly seen in the table that the sum of the drive indexes begins to deviate from
the value of 1.00 at pressure 1300 psia. Thus, the pressure maintenance may have

started ata pressure in between 1500 psia and 1300 psia.

(oh | NolBr+(Rp—Rsod Be] a
2500 - -
2300 5.68x10° 1.000
2100 12.85x10° 1.002
1900 22.47x10° 1.002
1700 34.89x10° 0.998
1500 51.40x10° 0.999
1300 86.40x10° 0.840
1100 143.12x10° 0.744
900 219.72x10° 0.690
700 346.43x10° 0.654
| By: Dr. Madi Abdullah Naser] 500 582.61x10° 0.626

50. Drive Indexes
Solution#2:

(b) The difference in the sum of the drive indexes with the value of 1.00 represents
the amount of the gas that has been injected up to the corresponding pressure
because the definition of drive index indicates the following.

Cumulative production = Sum of drive index x N,[B; + (Rp — Rsoi)Be]

In this case, until the pressure of 500 psia, the amount of gas that has been
injected can be represented with the value of 1.0 — 0.626 = 0.374, and therefore

The amount of injected gas = 0.374 x Np[Bt + (Rp — Rsoi) Bel

= (0.374) (582.61x10°)
= 217.9x10° bbl
Using B, = 0.006163 bbl/SCF, then
Total injected gas = (217.9x10° bbl) / (0.006163bb1/SCF)
= 35.36x10° SCF

By: Dr. Madi Abdullah Naser

51. HOMEWORK
Example -1

A combination-drive reservoir contains 10 MMSTB of oil initially in place. The
ratio of the original gas-cap volume to the original oil volume, ¡.e., m, is
estimated as 0.25. The initial reservoir pressure is 3000 psia at 150°F. The
reservoir produced 1 MMSTB of oil, 1100 MMscf of 0.8 specific gravity gas, and
50,000 STB of water by the time the reservoir pressure dropped to 2800 psi. The

following PVT is available:

3000 psi 2800 psi
B,, bbl/STB 1.58 1.48
R,, scf/STB 1040 850
B,, bbl/scf 0.00080 0.00092
B,, bbl/STB 1.58 1.655
B,,, bbl/STB 1.000 1.000

By: Dr. Madi Abdullah Naser

52. HOMEWORK

Example -1

The following data are also available:
Sy=0:20 G=15* 10° psi* cr= 1 X 1076 psi"!
Calculate:

a. Cumulative water influx

b. Net water influx
c. Primary driving indexes at 2800 psi

By: Dr. Madi Abdullah Naser