Making Sense of Producing Gas-Oil Ratio in Unconventional Oil
Reservoirs
AAPG Super Basins; Jan. 23, 2019, Sugar Land, TexasSteve Jones, Cimarex Energy Co.
Overview• Framework for understanding GOR performance in
unconventional solution gas-drive reservoirs• Linear vs radial flow• Four stages of GOR history• Factors that affect GOR• Practical applications• SPE 184397
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Pi=4000 psi Pb=3000 psiBubble Point
GAS
P=1500 psi
OIL
P=500 psi
PVT of Oil Reservoirs• Solution Gas Drive• Rsi = dissolved gas at initial conditions, scf/stb
Steve Jones, Cimarex
Gas-Oil Relative Permeability
Sgc
•Mobility = k / µ• µo is 20-100x more
than µg
• Sgc=Critical Gas Saturation• As pò, Sgñ, krgñ ,
kroò, GORñ
kro
krg
Steve Jones, Cimarex
re
Conventional Reservoir
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• “High” k (md)• Vertical well• Radial or psuedo-
radial flow• Rapid boundary-
dominated flow (BDF)
Radial Pseudo-Radial
Conventional GOR History
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• Radial flow, quick BDF• Average reservoir
pressure controls GOR• Rising GOR
indicates "̅# has dropped below "$
0
500
1,000
1,500
2,000
2,500
3,000
3,500
4,000
4,500
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
0 500 1000 1500 2000 2500 3000 3500
Aver
age
Rese
rvoi
r Pre
ssur
e, p
sia
GOR,
scf/
STB
Days
"$
%&'
Unconventional Reservoir
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Horizontal Wellbore
Transverse Fractures
Wellbore
Linear Flow
“Multi-Fractured Horizontal Well (MFHW)”
Unconventional GOR
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• Low k (nd)• Linear flow• pwf = BHFP• pwf strongly
influences GOR0 200 400 600
Linear vs. Radial Flow
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• Constant GOR in transient flow
• Constant pwf• Producing GOR is higher
for linear than radial flow
0
500
1,000
1,500
2,000
2,500
3,000
3,500
0 50 100 150 200 250 300 350 400GO
R, sc
f/ST
B
Days
Linear
RadialRsi = 980 scf/STB
Transient vs. Boundary-Dominated Flow
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Horizontal Wellbore
Transverse Fractures
• Transient time depends on k and fracture spacing
• Assumes no flow beyond frac tips
BDF Begins
Longer Transient PeriodCompound Linear Flow
Four Stages of GOR Performance in MFHW
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0 200 400 600
1. GOR = Rsi while pwf > pb
GOR = Rsi
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• pwf > pb• Important to correctly
estimate Rsi• Take PVT samples here• Can be very short, or
long
GOR = Rsi
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Example of long period of GOR=Rsi
Springer Shale, 8900-ft lateral
Four Stages
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1. GOR = Rsi
2. Rise due to pwf < pb
0 200 400 600
During transient flow, for a given reservoir and completion, GOR is controlled by pwf.
Rise due to pwf < pb
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Shape controlled by:• pwf schedule• Rel perm, especially Sgc• Finite frac conductivity• Frac length
Rise due to pwf < pb: Relative Permeability
SPE 184397 Steve Jones 17
• Corey exponents and endpoints • Sgc has largest effect• Suppressed bubble
point causes similar effect
0 200 400 600
Four Stages
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1. GOR = Rsi
2. Rise due to pwf < pb
3. Transient plateau during constant pwf
Transient GOR Plateau
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• Requires constant pwf• Result of constant
average pressure and saturations in distance of investigation (DOI)
Four Stages
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1. GOR = Rsi
2. Rise due to pwf < pb
3. Transient plateau4. Rise during BDF
0 200 400 600
Variations
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• pb very near pi
• BDF begins early • pwf > pb for a long time• BDF begins prior to pwf < pb
Springer Shale, 8900-ft lateral
Difficult to Explain
• GOR constant while Pwf is well below Pb
• Bubble point known from PVT
• High Sgc?• Low frac conductivity?• Depressed Pb due to
“nanopore proximity” effects?
• Not linear flow due to natural fractures?
Pb=4800 psi
22
GOR Rise during BDF
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Frac Spacing• Cum oil for 10,000 ft
lateral• 200 fracs for 50-ft spacing• 25 fracs for 400-ft spacing• Same pwf schedule
k=300 nd
0
50
100
150
200
250
300
350
400
450
0 730 1,460 2,190 2,920 3,650
Cum
Oil,
MST
B
Days
Frac Spacing=50 ft
Frac Spacing=200 ft
GOR Rise during BDF
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• Closest frac spacing has quickest GOR rise• Rate of GOR rise depends
on efficiency of access to drainage volume
k=300 nd0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
0 730 1,460 2,190 2,920 3,650
GOR,
scf/
STB
Days
Frac Spacing=50 ft
Frac Spacing=200 ftRsi=980 scf/stb
GOR Rise during BDF
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• Plot GOR vs. cum oil• Same OOIP
k = 300 nd0
1000
2000
3000
4000
5000
6000
7000
0 10 20 30 40
GOR,
scf/
STB
Cum Oil per ft of Lateral, STB/ft
Frac Spacing=50 ft
Frac Spacing=200 ft
Rsi=980 scf/stb
GOR Plateau 3400 scf/stb
0 100 200 300 400Cum Oil, MSTB
GOR rise in BDF correlates with OOIP
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
0
500
1000
1500
2000
2500
3000
3500
4000
0 10 20 30 40 50 60 70 80 90
GOR,
scf/
stb
Aver
age R
eser
voir
Pres
sure
, psia
Cum Oil per GPI, stb/ft
OOIP=x OOIP=2xOOIP=x/2
• Three model cases varying only OOIP
• For same PVT, Pi, relperms, and pwfschedule
• GOR is indicator of prand therefore OOIP
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Effect of Drainage Volume
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Fully
bou
nded
Half-
boun
ded
Half-
boun
ded
Unbo
unde
d
Fully Bounded
Half-Bounded
Unbounded
Conclusions• GOR depends strongly on pwf
• Four idealized stages of GOR history• Several factors cause deviation• Identify Rsi and flow regime (transient or BDF) to interpret history• GOR in BDF is function of cumulative oil production• GOR rises at cum oil proportional to drainage volume• SPE 184397
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