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UNDERBALANCED DRILLING AS A RESERVOIR

ENHANCEMENT APPROACHFOR MATURE FIELDS

Patrick BrandBlade Energy Partners

Frisco, TX, USA



Presentation Outline

• Definition

• Mature Field Challenges and Technology

• Case Study Examples

• Reservoir Characterization• Damage Mitigation



RCH

What is Underbalanced Drilling

(UBD)?• Wellbore pressure is intentionally kept

below formation pressure• Well flows if permeable zones exist

• Rotating control head (RCH) and

surface equipment are the primarywell control devices

Formation

Pressure

Wellbore

Pressure



Mature Field Challenges• Low reservoir pressures

– Differential sticking, lost returns, low ROP while drilling

– less forgiving to invasion-induced damage – Less responsive to workovers, stimulation

• Water encroachment – Coning problems reduce ultimate recovery

– Water production and handling

• Marginal investment economics – Many “wasted” wellbores

– Shorter well life, lower ultimate production per wellbore

– Expensive enhanced recovery injection options

• Very few cost saving opportunities – Typical operator has already wrung the last dollar out of the cost

side



Why UBD?

• Three key drivers

– As a drilling enabler and cost reducer – To mitigate damage and reap productivity

and ult imate recovery improvement

benefits – To gather valuable reservoir information,

characterize reservoir , and fundamentally

improve reservoir management principles• Ultimately, UBD is a reservoir exploitation

technique, not a drilling method



Why does UBD work in Mature

Fields?Overbalanced drilling is more damaging and

the reservoir less forgiving –Deep fluid invasion; phase blockage

–Solids invasion; immobile damage, pore throat

blockage, absolute permeability reduction

–Stimulation ineffective in long zones with variable

permeability



Emerging Value Proposition• All mature fields are candidates for UBD

• Reservoir emphasis is becoming dominant – Asset team driven

– Production evidence can no longer be ignored

– UBD data sought for reservoir characterization• UB for life recognized as essential

– value may be lost if well ever killed

– Tools being developed to maintain UB• Screening, valuation, planning, equipment are

key to success



The key question

• What is the value of applying UBD on a

specific candidate reservoir?



0

5

10

15

20

25

F r e q u e n c y ( n )

1 5 9 13 17 21 25 29 33 37 41 45 49 53 57

PIF

Histogram of PIF History

No. of Observation 79Mean of Observation 4.10

Reciprocal of Harmonic mean 1.03Reciprocal of Arithmetic mean 0.24

ν 0.79

λ 1.27

Standard Deviation, σ 6.51

Variance 42.34

Mode 1.5

Minimun 0.094248

Maximum 49.40Sum 323.72

Total number of fields 35

Total number of UB wells 214

Total number of operators 16

Statistical Summary of PIF History

UBD Worldwide Historical Performance



Reservoir Characterization



0.00

0.50

1.00

1.50

2.00

2.50

3.00

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400

Time (min)

G r o s s

P I ( m 3 / k P a

/ d ) &

D r a w

D o w n

( k P a

E - 3 )

0.00

0.02

0.04

0.06

0.08

0.10

0.12

W a

t e r

P I ( m

3 / k P a

/ d )

Actual Draw Down

Actual Gross PI

Actual Water PI

WATER ?

Note Scale . . .

(Water PI =

25x Gross)

HUGE OIL BEARING

FRACTURE ?

EARLY

WATER ?

Murphy et. al., SPE 93695



0.000

0.200

0.400

0.600

0.800

1.000

1.200

1.400

1.600

1.800

2.000

0 60 120 180 240 300 360 420 480 540 600 660 720 780 840 900 960 1020 1080

Time (min)

P I ( m 3 / k P a / d )

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

W a t e r P I ( m 3 / k P

a / d )

Est'd Gross PI

Oil PI

Gross PI

Depth Scale

Water PI

NOTE:Expected PI of 0.0014m3/kPa/d/m based

on Well Proposal estimates of 700m3/d of

gross production over 500m of hole and a

representative draw down of 1000kPa.

1 7 0

1 7 5

1 7 7

1 8 0

1 7 2

1 8 5

1 8 7

1 9 0

1 8 2

1 9 5 0

1 9 7 5

2 0 0

1 9 2

2 0 5

2 0 7 5

2 1 0 0

2 0 2

2 1 5 0

2 1 7 5

2 2 0 0

2 1 2 5

Depth (m)

PI while Drill ing

KILLED WELL

POTENTIAL WATERPRODUCTION ?

FLUSH PRODUCTION ?(MORE WATER?)

±0.35 m3/kPa/dDROP IN PI

F L O W T

E S T

F L O W

T E S T

F L O W T

E S T

±0.20 m3/kPa/d

INCREASE IN PI

AFTER TEST

MOREFRACTURES ?

0. 0 0 1 4 m

3/ k P a/ d/ m

LARGE

FRACTURES ?

(WATER)

NOTE:

Water PI scale has been

amplified 10x



Nimr UBD Well NM498 Water Shut-off Versus No Action Case

0

10

20

30

40

50

60

70

D e c - 0 2

J an- 0 3

F e b - 0 3

M ar - 0 3

A pr - 0 3

M

a y - 0 3

J un- 0 3

J ul - 0 3

A u g- 0 3

S e p- 0 3

O c t - 0 3

N ov - 0 3

D e c - 0 3

J an- 0 4

F e b - 0 4

M ar - 0 4

A pr - 0 4

M

a y - 0 4

J un- 0 4

J ul - 0 4

O i l , m

3 / d

NM498 Actual

without Water Shutoff

NM498: Using UBD data we recognized water influx was coming primaril iy from a few small Intervals

We set two P&A Kits and succeeded in reduc ing BSW from >90% to abou t +60%The latest well test d ata (Apr-04) indicate the wel l is producing about 38 m3/d net at 84% BSW

The cumulative estimated gain from w ater shout off = +21,000 m3 or +130 mbbl s

(Ultimate Recovery is being improved)

The cumulative Net Cashflow Improvement = +$2.6 mln (@ $20 /bbl)

NM498 Actual Reconcil ed productio n

NM498 estimated production without shutoff *

* assume 94% init . BSW, 300 m3/d gross, 15% PA Decli ne, 0.83 reconci liation f actor

OILG A

IN

NM498 is our EZIP "Proof of Concept" Well

There is value in UBD

reservoir characterization!



Screening, Selection and

Damage Characterization



UBD Candidature• Adequate reserves and potential for drilling must exist in the target

reservoir

• Preliminary data should indicate that UBD is likely to be technicallyfeasible

• Clear evidence of permanent damage must be present

• The reservoir properties are such that: – UBD is likely to improve reservoir performance (based on experience)

– UBD reservoir characterization is likely to substantially improvereservoir management and per-well recovery

• The conventional drilling history is such that: – UBD is likely to improve drilling performance and reduce costs

• Contra-indicators to UBD should be absent or manageable

through proper design

All mature fields are candidates by definition …



Risk

Simulator

UBD PIE

eRes UBD

Conventional

ReservoirParameters

Historical

Analog Data

UBD

ReservoirParameters

UBD Days/Cost

Distribution

UBD Production

Distribution

NPV

Calculations

Pricing

Model

Risk Based

Economic

Comparison

Evaluate results

against full field

simulation (optional)

DECISION

Conventional

Days to Drill

ROP, Bit Life, NPT,

Lost Circulation,

Differential Sticking

Equip Req.

UBD Cost

Technical

Feasibility

Production Impact

Drilling Impact

Damage Characterization

RTA

Data Interpretation

Valuation and Tech Feasibility

Pre-Screening



Reservoir Model

assumptions (Kv,

Kh, φ, Swi, Kro, Krw,Well model, etc)

Drilling parameters

(length of hole, time

curve, overbalance,mud type)

Dynamic Damage

Assessment Micro-Simulator

Mud-cakeeffect

(reduced Kabs

at wellbore

wall)

Solids damage

(reduced Kabs in

invasion zone)

Special Core Tests:

Undamaged relative

perm to oil

Fluid loss profile with

mud cake buildup

Mud cake liftoff pressure

Retained oil permeability

after exposure to mud

Saturation and

Pressure profile

0

1000

2000

3000

4000

5000

6000

7000

8000

0 200 400 600 800 1000 1200

Time, (days)

O i l f l o w r a t e s ( S T B / d a y

UBD

OBD with formation damage

Unstructured Grid

Sector Model

Decline Curves

Biswas and Suryanarayana, SPE 86465,

Suryanarayana et. al., TBP



Invasion Effects

• What is the effect of invasion on near-

wellbore saturation and reservoirpressure?

• Case Study

– Filtrate invasion only (rel perm effects)



Example

• Berea sandstone (100 mD-150 mD) is tested core

• Assumed mud is inhibited water-based mud

• A high permeability zone (6 X), 210 ft thick is intersected• Also included as input is rock compressibility and rel-

perm curves

Horizontal permeability (md) 50.0

Vertical Permeability (md) 5.0

Porosity 0.22

Initial reservoir pressure (psi) 3000

Initial oil saturation 0.756

Number of gridblocks 101x31

Nominal length of horizontal well ( ft ) 3000

Pressure difference for mud filtrate invasion (psi) 650

Pressure difference for flowing back (psi) 500

Well radius (ft) 0.250

100

200

300

400

500

600

0 2 4 6 8 10 12 14Time, (days)

R

O

P

( f t / d

a y )

Basic Reservoir AssumptionsDrilling Time-ROP Profile



Flow-Back Results – Assume that mud cake is lifted-off

Note impact of high-perm

streak on clean-up and

flow-back!

As expected, high-perm streaks

(protected during invasion by mud

cake), clean up fastest and can

dominate flow-back, impacting

production from low-perm zones!



Long-Term Production

0

10000

20000

30000

40000

50000

60000

0 200 400 600 800 1000 1200

Time, (days)

O i l f l o w r a t e ( S T B / d a y

)

Peremability 29% reduction (mudcake permeability=0.025)

UBD

0.0001

0.001

0.01

0.1

1

10

100

1000

10000

100000

0 200 400 600 800 1000 1200

Time, (days)

W a t e r f l o w r a t

e ( S T B / d a y )

Permeabili ty 29% reduction (mud cakepermeability=0.025)

Oil

Water



Impact on Production

• Even with good mud cake, significant loss of production

• Remediation?

– Perforations beyond 4 ft? – Stimulation?

– Fracturing?

Time OBD (29% reduction in permeabi lity)(years)

Reduction in cum oil (MMSTB)

1 1.55

2 2.60

3 3.60



Why UBD?• Three key drivers

– As a drilling enabler and cost reducer – To mitigate damage and reap productivity

and ult imate recovery improvement

benefits – To gather valuable reservoir information,

characterize reservoir , and fundamentallyimprove reservoir management principles

• Ultimately, UBD is a reservoir exploitationtechnique, not a drilling method



Impact of Technology on Field Life

Phase 1: Openhole completions, nitroglycerine explosion

Phase 2: Acid stimulation, Hydraulic fracturing

Phase 3: Commodity price driven boom of drilling

Phase 4: Underbalanced Horizontal Drilling (cost driven innovation)

Will there be a Phase 5?

Source: Caldwell and Merkel, Scotia Group Newsletter, March 2004



North American UBD Success

StoriesSPE Papers and Industry Articles

• Bitterweed South (Caballos) Field: A Case Study in

Changing Economics Using Underbalanced Drillingand Open Hole Completions (SPE 81647, Presented March

2003) Settled vertical UBD Wells, Pecos County, Texasproduce at twice the rate as OBD wells with UBD IP’s 10times higher than OBD wells.

• Alberta Canada, Harmattam Elkton Pool (Paper presentedat the International Underbalanced Drilling Conference, The Hague, October

1997) On Production since 1967, vertical OBD wellstypically produce 1.5 – 2.5 MMcf/d. UBD wells drilled 30years later on in-field locations flowed 22 MMcf/d whiledrilling and produced at restricted rates of 9 MMcf/dwithout stimulation.



North American UBD Success

StoriesSPE Papers and Industry Articles

• Williston Basin, Wayne Field (Hart’s E&P Magazine January 2003, Cade,

Weatherford International, Vickers & Jennings, GeoResources, Inc.) UBD horizontalwells triple expected ultimate recovery. Comparing: Conventionalwells 80,000 Bbls, OBD Horizontal wells 175,000 Bbls, UBD Horizontalwells 275,000 Bbls.

• Illinois Basin (SPE Explorer Magazine, Shirley) In this mature basin a UBD

well drilled 200’ into structure produces 3,000 Bbls/d at 3,850’ verticaldepth. Well produced 1.8 MM Bbls in past 18 months. A world classproducer in Illinois!

• Underbalanced Horizontal Drilling Yields Significant Productivi ty

Gains in the Hugoton Field (SPE 81632, Presented March 2003) Horizontalwells drilled in mature Hugoton Field show initial production rates 3Xgreater than horizontal OBD wells. Paper states projected reservoirlife extended 45%; ultimate recovery increased by 70%.



Some long-term production

trends

0

0.005

0.01

0.015

0.02

0.025

0.03

0 12 24 36 48 60

Average Overbalanced

(4 Wells)

Average Underbalanced(9 Wells)

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

0 12 24 36 48 60



(25 Wells)

0

5

10

15

20

25

30

35

0 12 24 36 48 60 72 84 96 108 120

Average Underbalanced

(4 Well)


(25 Wells)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

0 12 24 36 48 60 72


Average Overbalanced(32 Wells)

Time (Months since spud)

R a t e



Conclusions• Underbalanced Drilling can solve many

problems associated with mature fields. – Drilling Problems

– Reservoir Characterization

– Enhanced production

• Value of UBD can be quantified through a

combination of dynamic reservoirsimulation and risked cost.

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