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

Steve Jones, Cimarex 2

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

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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

Steve Jones, Cimarex 6

• 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

Steve Jones, Cimarex 10

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

Steve Jones, Cimarex 28

Steve Jones, Cimarex 29

Questions ?

sjones@cimarex.com