© ChevronTexaco 2001

Use of Transient Simulators to Assess Gas Lift Viability for Offshore Angola

2002 North American Gas Lift WorkshopSHAUNA NOONAN / FRED BROWNLEE

2February 5, 2002

Produce 40,000 BWPD

from 23,000 ft MD (8800 ft TVD)

via Gas Lift

Fact or Fiction?

Actually a better title would be:

3February 5, 2002

Presentation Outline

Operating Parameters

Discussion on software (transient and static programs)

Work scope

Some of the results

What did we learn?

Questions

4February 5, 2002

Parameters

• 4500 psi injection pressure at wellhead

• Target production of 40,000 bwpd / well

• 0.68 gravity injection gas

• Up to 30 MMscfd source gas available / well

• 1.06 gravity water

• Wellhead pressure of 250 psi

• Reservoir Pressure of 3500 psi

• Dry Tree (offshore in 1200 ft water)

5February 5, 2002

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 200000

1000

2000

3000

4000

5000

6000

7000

8000

9000

Offset (feet)

Tru

e V

ert

Dep

th (

feet

)

Deviation Profile ( Whd @ 20.00 feet )

Deviation Profile

6February 5, 2002

From previous nodal analysis...

The target rate may be reached via conventional gas lift for a flow area equivalent to 8 5/8” tubing.

This would require gas lift rates of approximately 20 MMscfd.

BUT…..

7February 5, 2002

• Strategic importance of this well for project development

• Static nodal program cannot predict whether the well could be unloaded.

• Transient program can predict unloading conditions, but uses flow correlations that are not valid for large tubing sizes (greater than 5.5”)

• Existing transient flow program is not capable of reverse flow completions analysis.

• At the high injection pressures required, is the gas actually in a gaseous state at the orifice?

We have the following concerns:

8February 5, 2002

OLGA 2000

“OLGA 2000 is the market-leading simulator for transient multiphase flow of oil, water and gas in well and pipelines with process equipment. “

→ Over 10,000 experiments were run in eight years on the SINTEF Two-Phase Flow Laboratory near Trondheim, Norway

→ The tests were run in both steady state and transient modes

→ The resultant data / correlations reside within OLGA 2000

→ ChevronTexaco has been using this program for many years to assess flow assurance issues in pipelines and risers. It has not been used for downhole analysis applications

→ Limited to single point gas lift configurations

9February 5, 2002

The Plan

• Evaluate various completion options for high rate water wells using three different flow analysis software packages:

– Well Evaluation Manager (WEM) using OLGAs

– Dynalift

– OLGA2000

• Evaluate Dynalift when used for large tubing completions

• Develop & validate the well unloading sequence

• Confirm importance of surface controlled downhole choke

10February 5, 2002

Scope of Study

The following scenarios were evaluated:

• Case WA1: Lift gas injection down 11¾” casing with 8⅝” tubing.

• Case WA2: Lift gas injection down 11¾” casing with IPC 7” tubing

• Case WA3: Lift gas injection down 11¾” casing with IPC 8⅝” tubing

• Case WB1: Lift gas injection down 3½” tubing with 9⅝” casing

• Case WB2: Lift gas injection down 2⅞” tubing, with 9⅝” casing

11February 5, 2002

Results

12February 5, 2002

Case WA1 - Steady State

Water Well (Case WA1: 8 5/8 tubing, gas injection down annulus)

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

0 5000 10000 15000 20000 25000 30000

Injected Gas Rate (mscfd)

To

tal

Wa

ter

Pro

du

ce

d (

bw

pd

)

WEM (OLGAs) Dynalift (transient with Beggs Brill) Scandpower (OLGA2000)

13February 5, 2002

Case WA1 - Steady State

WA1: Comparison of Operating Pressures

0

500

1000

1500

2000

2500

3000

3500

0 5000 10000 15000 20000 25000 30000

Injected Gas Rate (mscfd)

Inje

ctio

n P

ress

ure

at

Wel

lhea

d (

psi

a) /

Dif

fere

nti

al P

ress

ure

ac

ross

Ori

fice

(p

si)

0

250

500

750

1000

1250

1500

1750

OLGA2000 Dynalift dP (OLGA2000) WEM (OLGAs) dP (WEM) dP (Dynalift)

14February 5, 2002

Case WA1

Notes:

• Separation between WEM & OLGA2000 not fully explained; (possibility - OLGA2000 starting with transient flow first)

• Dynalift example uses 0.5” orifice - significant at higher flow rates (problems with 1”)

• OLGA2000 & WEM assume a fixed pressure differential not a fixed choke

15February 5, 2002

Note the 180,000 bwpd slug OLGA predicted during the unloading cycle.

BBT Water Voidage Well - GLG Down Annulus - Std. Tubing

ACCUMULATED LIQUID VOLUME FLOW TUB_OUT_Q [bbl]

PRESSURE ANN_IN [ps ia]

PRESSURE ANN_OUT [ps ia]

PRESSURE TUB_IN [ps ia]

GAS MASS FLOW ANN_OUT_Q [kg/s]

TOTAL LIQUID VOLUME FLOW TUB_OUT_Q [bbl/d]

CONTROLLER SIGNAL CTL_GLV [-]

bb

l

2000

1500

1000

500

0

ps

ia

4500

4000

3500

3000

2500

2000

1500

1000

500

0

kg

/s

7

6

5

4

3

2

1

0

bb

l/d

200000

150000

100000

50000

0

-

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

Time [h]Benguela-Belize - Gas-Lift down Annulus

161050

16February 5, 2002

Case WA1 - Unloading

BBT Water Voidage Well - GLG Down Annulus - Std. Tubing

TubGIunload20: PRESSURE NAT_IN [ps ia]

TubGIunload20r: PRESSURE NAT_IN [ps ia]

TubGIunload20: TOTAL LIQUID VOLUME FLOW TUB_OUT_Q [bbl/d]

TubGIunload20r: TOTAL LIQUID VOLUME FLOW TUB_OUT_Q [bbl/d]

ps

ia

4500

4000

3500

3000

2500

2000

bb

l/d

200000

150000

100000

50000

0

Time [h]

16151050

17February 5, 2002

Case WA2

Water Well (Case WA2: 7" plastic lined tbg, gas injection down annulus)

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

0 5000 10000 15000 20000 25000 30000

Injected Gas Rate (mscfd)

To

tal

Wat

er P

rod

uce

d (

bw

pd

)

WEM (OLGAs) Scandpower (OLGA2000) Dynalift (transient with Beggs Brill)

18February 5, 2002

Case WA1 versus Case WA3

WA1 versus WA3 (conventional versus plastic lined 8 5/8" tubing.

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

0 5000 10000 15000 20000 25000 30000 35000

Injected Gas Rate (mscfd)

To

tal W

ate

r P

rod

uce

d (

bw

pd

)

WA1 WEM (OLGAs) WA1 Dynalift WA1 Scandpower (OLGA2000)

WA3 WEM (OLGAs) WA3 Dynalift WA# Scandpower (OLGA 2000)

19February 5, 2002

WA1 versus WA3: Comparison of Operating Pressures

0

500

1000

1500

2000

2500

3000

0 5000 10000 15000 20000 25000 30000

Injected Gas Rate (mscfd)

Inje

ctio

n P

ress

ure

at

Wel

lhea

d (

psi

a) /

Dif

fere

nti

al P

ress

ure

acr

oss

O

rifi

ce (

psi

)

WEM (OLGAs) dP (WEM) WA3 WEM (OLGAs) WA3 dP (WEM)

Effect of IPC

Roughness Difference

Case WA1: 0.0018

Case WA3: 0.00006

20February 5, 2002

Water Well (Case WB1: Reverse flow gas lift.. 9 5/8" csg with 3.5" tbg .... equivalent flow area of 8 5/8"))

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

0 5000 10000 15000 20000 25000 30000

Injected Gas Rate (mscfd)

To

tal

Wa

ter

Pro

du

ced

(b

wp

d)

WEM (OLGAs) Scandpower (OLGA2000) Dynalift (equiv. Flow area using Beggs Brill)

Case WB1

OLGA2000 & WEM include pressure loss from casing connections in annulus; Dynalift uses equivalent area

21February 5, 2002

WHAT DID WE LEARN?

• Have a higher degree of confidence that this well could be unloaded.

• Question of conventional versus reverse gas lift must include drilling feasibility and operational safety concerns

• WEM OLGAs works well for assessing well performance but does not answer unloading questions

• For higher rate / pressure single point gas lift systems, surface controlled downhole gas lift valves are mandatory to reduce upsets while unloading the well

22February 5, 2002

QUESTIONS?