SPE DISTINGUISHED LECTURER SERIESis funded principally
through a grant of the
SPE FOUNDATIONThe Society gratefully acknowledges
those companies that support the programby allowing their professionals
to participate as Lecturers.
And special thanks to The American Institute of Mining, Metallurgical,and Petroleum Engineers (AIME) for their contribution to the program.
Oilfield Scale:A New Integrated Approach to Tackle an Old Foe
Dr Eric J. Mackay
Society of Petroleum EngineersDistinguished Lecturer 2007-08 Lecture Season
Flow Assurance and Scale Team (FAST)Institute of Petroleum EngineeringHeriot-Watt UniversityEdinburgh, [email protected]
Slide 3 of 40
Outline
1) The Old Foea) Definition of scaleb) Problems causedc) Common oilfield scalesd) Mechanisms of scale formation
2) The New Approacha) The new challengesb) Proactive rather than reactive scale managementc) Effect of reservoir processes
3) Conclusions
FormationWater (Ba)
Injection Water(SO4)
Ba2+ + SO42- BaSO4(s)
Slide 4 of 40
Outline
1) The Old Foea) Definition of scaleb) Problems causedc) Common oilfield scalesd) Mechanisms of scale formation
2) The New Approacha) The new challengesb) Proactive rather than reactive scale managementc) Effect of reservoir processes
3) Conclusions
FormationWater (Ba)
Injection Water(SO4)
Ba2+ + SO42- BaSO4(s)
Slide 5 of 40
1a) Definition of Scale Scale is any crystalline
deposit (salt) resulting from the precipitation of mineral compounds present in water
Oilfield scales typically consist of one or more types of inorganic deposit along with other debris (organic precipitates, sand, corrosion products, etc.)
Slide 6 of 40
1b) Problems Caused Scale deposits
z formation damage (near wellbore)z blockages in perforations or gravel packz restrict/block flow linesz safety valve & choke failurez pump wearz corrosion underneath depositsz some scales are radioactive (NORM)
Suspended particlesz plug formation & filtration equipmentz reduce oil/water separator efficiency
Slide 7 of 40
Examples - Formation Damage
quartz grainsquartz grains
scale crystals block scale crystals block pore throatspore throats
Slide 8 of 40
Examples - Flow Restrictions
Slide 9 of 40
Examples - Facilities
separator scaled up
and aftercleaning
Slide 10 of 40
1c) Common Oilfield Scales
Iron Scales: Fe2O3, FeS, FeCO3
Some Other Scales
Sand GrainsHF solubleinsoluble2.65SiO2silicon dioxide
Exotic Scales: ZnS, PbS
(insoluble in HCl)357,0002.16NaClsodium chlorideacid soluble2,4102.32CaSO4.2H2Ocalcium sulphateacid soluble2,0902.96CaSO4calcium sulphate
slightly acid soluble1133.96SrSO4strontium sulphateacid soluble142.71CaCO3calcium carbonate
60 mg/l in 3% HCl2.24.50BaSO4barium sulphateCommon Scales
(mg/l)othercold waterGravity
SolubilitySpecificFormulaName
SPE 87459
Slide 11 of 40
1d) Mechanisms of Scale Formation
Carbonate scales precipitate due to P (and/or T)z wellbore & production facilities
Sulphate scales form due to mixing of incompatible brinesz injected (SO4) & formation (Ba, Sr and/or Ca)z near wellbore area, wellbore & production facilities
Concentration of salts due to dehydrationz wellbore & production facilities
Ca2+(aq) + 2HCO-3(aq) = CaCO3(s) + CO2(aq) + H2O(l)
Ba2+(aq) (Sr2+or Ca2+) + SO42-(aq) = BaSO4(s) (SrSO4 or CaSO4)
Slide 12 of 40
Outline
1) The Old Foea) Definition of scaleb) Problems causedc) Common oilfield scalesd) Mechanisms of scale formation
2) The New Approacha) The new challengesb) Proactive rather than reactive scale managementc) Effect of reservoir processes
3) Conclusions
FormationWater (Ba)
Injection Water(SO4)
Ba2+ + SO42- BaSO4(s)
Slide 13 of 40
2a) The New Challenges
Deepwater and other harsh environmentsz Low temperature and high pressurez Long residence timesz Access to well difficultz Compatibility with other production chemicals
Inhibitor placementz Complex wells (eg deviated, multiple pay zones)
Well value & scale management costs
Slide 14 of 40
Access to Well
Subsea wellsz difficult to monitor
brine chemistryz deferred oil during
squeezesz well interventions
expensive (rig hire)z squeeze
campaigns and/or pre-emptive squeezes
Slide 15 of 40
Inhibitor Placement in Complex Wells
Where is scaling brine being produced?
Can we get inhibitor where needed?z wellbore frictionz pressure zones
(layers / fault blocks)z damaged zones
Options:z Bullheadz bullhead + divertorz Coiled Tubing from rigz Inhibitor in proppant /
gravel pack / rat hole
Ptubing head
Fault
Shale
Pcomp 1
Pcomp N
Presv 1
Presv N
Slide 16 of 40
Well Value & Scale Management Costs
Deepwater wells costing US$10-100 million (eg GOM)
Interval Control Valves (ICVs) costing US$0.51 millioneach to installz good for inhibitor placement controlz susceptible to scale damage
Rig hire for treatments US$100-400 thousand / dayz necessary if using CTz deepwater may require 1-2 weeks / treatmentz cf. other typical treatment costs of US$50-150 thousand /
treatment
Sulphate Reduction Plant (SRP), installation and operation may cost US$20-100 million
Slide 17 of 40
0
1
2
3
4
5
6
7
8
9
10
11
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
Year
N
o
o
f
S
R
P
p
l
a
n
t
s
p
e
r
y
e
a
r
0
1,000,000
2,000,000
3,000,000
4,000,000
5,000,000
C
u
m
u
l
a
t
i
v
e
C
a
p
a
c
i
t
y
(
B
W
P
D
)
No of SRP plantsCumulative Capacity (BWPD)
Number of SRP per Year and Total Capacity
Slide 18 of 40
2b) Proactive Rather Than ReactiveScale Management
Scale management considered during CAPEX Absolute must:
good quality brine samples and analysis Predict
z water production history and profiles well by wellz brine chemistry evolution during well life cyclez impact of reservoir interactions on brine chemistryz ability to perform bullhead squeezes:
flow lines from surface facilities correct placement
Monitor and review strategy during OPEX
Slide 19 of 40
2c) Effect of Reservoir Processes
EXAMPLE 1 Management of waterflood leading to extended brine mixing at producers(increased scale risk)
EXAMPLE 2 In situ mixing and BaSO4 precipitation leading to barium stripping(reduced scale risk)
EXAMPLE 3 Ion exchange and CaSO4 precipitation leading to sulphate stripping(reduced scale risk)
Slide 20 of 40
SPE 80252
Extended Brine Mixing at Producers
EXAMPLE 1
Slide 21 of 40
SPE 80252
Field M (streamline model)
This well has been treated > 220 times!
Extended Brine Mixing at Producers
EXAMPLE 1
Slide 22 of 40
Barium Stripping (Field A)
% injection water
B
a
r
i
u
m
(
m
g
/
l
)
Dilution line
SPE 60193EXAMPLE 2
Slide 23 of 40
Barium Stripping (Theory)
Injection water (containing SO4) mixes with formation water (containing Ba) leading to BaSO4 precipitation in the reservoir
Minimal impact on permeability in the reservoir
Reduces BaSO4 scaling tendency at production wells
SPE 94052EXAMPLE 2
Slide 24 of 40
Barium Stripping (Theory)
Ba2+
Rock
SO42-
1) Formation water (FW): [Ba2+] but negligible [SO42-]
FW
(hot)
EXAMPLE 2
Slide 25 of 40
Barium Stripping (Theory)
Ba2+ SO42-
2) Waterflood: SO42- rich injection water displaces Ba2+ rich FW
Rock
FWIW
(cold) (hot)
EXAMPLE 2
Slide 26 of 40
Barium Stripping (Theory)
Ba2+ SO42-
Rock
3) Reaction: In mixing zone Ba2+ + SO42- BaSO4
FWIW
(cold) (hot)
BaSO4
EXAMPLE 2
Slide 27 of 40
Barium Stripping (Theory)
0
100
200
300
400
500
600
700
800
900
0 20 40 60 80 100seawater fraction (%)
[
B
a
]
(
m
g
/
l
)
0
500
1000
1500
2000
2500
3000
[
S
O
4
]
(
m
g
/
l
)
BaBa (mixing)SO4SO4 (mixing)
Large reduction in [Ba]
Small reduction in [SO4](SO4 in excess)
Typical behaviour observed in many fields
EXAMPLE 2
Slide 28 of 40
Barium Stripping (Model & Field Data)
0
10
20
30
40
50
60
70
80
90
0 20 40 60 80 100% seawater
b
a
r
i
u
m
c
o
n
c
e
n
t
r
a
t
i
o
n
(
p
p
m
)
Field A - actualField A - dilution lineField A - modelled
EXAMPLE 2
Slide 29 of 40
Sulphate Stripping (Theory)
Injection water (with high Mg/Ca ratio) mixes with formation water (with low Mg/Ca ratio) leading to Mg and Ca exchange with rock to re-equilibrate
Increase in Ca in Injection water leads to CaSO4 precipitation in hotter zones in reservoir
Minimal impact on permeability in the reservoir
Reduces BaSO4 scaling tendency at production wells
SPE 100516EXAMPLE 3
Slide 30 of 40
Ion Exchange
Ca
Mg
Ca
Mg
CC
0.50 CC =
FW: 0.077
IW: 3.2
Rock: 0.038
Mg on rockMg
Ca on rockCa
Mg in solutionCMg
Ca in solutionCCa2,325
30,185
Gyda FW (mg/l)
1,368426
IW (mg/l)
EXAMPLE 3
Slide 31 of 40
Sulphate Stripping (Theory)
Ba2+
Rock
SO42- Ca2+ Mg2+
1) Formation water: [Ca2+] and [Mg2+] in equilibrium with rock
FW
(hot)
EXAMPLE 3
Slide 32 of 40
Sulphate Stripping (Theory)
Ba2+ SO42- Ca2+ Mg2+
2) Waterflood: [Ca2+] and [Mg2+] no longer in equilibrium
Rock
FWIW
(cold) (hot)
EXAMPLE 3
Slide 33 of 40
Sulphate Stripping (Theory)
Ba2+ SO42- Ca2+ Mg2+
3) Reaction 1: Ca2+ and Mg2+ ion exchange with rock
Rock
FWIW
(cold) (hot)
EXAMPLE 3
Slide 34 of 40
Sulphate Stripping (Theory)
Ba2+ SO42- Ca2+ Mg2+
4) Reaction 2: In hotter zones Ca2+ + SO42- CaSO4
Rock
FWIW
(cold) (hot)
CaSO4
EXAMPLE 3
Slide 35 of 40
Modelling Prediction: [Ca] and [Mg]
0
5,000
10,000
15,000
20,000
25,000
30,000
35,000
0 20 40 60 80 100seawater fraction (%)
[
C
a
]
(
m
g
/
l
)
0
500
1,000
1,500
2,000
2,500
3,000
3,500
[
M
g
]
(
m
g
/
l
)
CaCa (mixing)MgMg (mixing)
Large reduction in [Mg]
No apparent change in [Ca]
EXAMPLE 3
Slide 36 of 40
Observed Field Data: [Ca] and [Mg]
Large reduction in [Mg]
No apparent change in [Ca]
EXAMPLE 3
0
5000
10000
15000
20000
25000
30000
35000
40000
0 20 40 60 80 100
seawater fraction (%)
[
C
a
]
(
m
g
/
l
)
0
1000
2000
3000
4000
5000
6000
7000
8000
[
M
g
]
(
m
g
/
l
)
CaCa (mixing)MglMg (mixing)
Slide 37 of 40
Modelling Prediction: [Ba] and [SO4]
0
100
200
300
400
500
600
700
800
900
0 20 40 60 80 100seawater fraction (%)
[
B
a
]
(
m
g
/
l
)
0
500
1000
1500
2000
2500
3000
[
S
O
4
]
(
m
g
/
l
)
BaBa (mixing)SO4SO4 (mixing)
EXAMPLE 3
Small reduction in [Ba]
Large reduction in [SO4](No SO4 at < 40% SW)
Slide 38 of 40
Observed Field Data: [Ba] and [SO4]
Small reduction in [Ba]
Large reduction in [SO4](No SO4 at < 40% SW)
EXAMPLE 3
0
50
100
150
200
250
300
0 20 40 60 80 100
seawater fraction (%)
[
B
a
]
(
m
g
/
l
)
0
500
1000
1500
2000
2500
3000
[
S
O
4
]
(
m
g
/
l
)
BaBa (mixing)SO4lSO4 (mixing)
Slide 39 of 40
3) Conclusions
Modelling tools may assist with understanding of where scale is forming and what is best scale management optionz identify location and impact of scalingz evaluate feasibility of chemical options
thus providing input for economic model.
Particularly important in deepwater & harsh environments, where intervention may be difficult & expensive
But must be aware of uncertainties..z reservoir descriptionz numerical errorsz changes to production schedule, etc.
so monitoring essential.
Slide 40 of 40
Acknowledgements
Sponsors of Flow Assurance and Scale Team (FAST) at Heriot-Watt University:
Slide 41 of 40
Extra Slides
Barium stripping example (Field G) Placement example (Field X)
Slide 42 of 40
Barium Stripping (Field G)
a) water saturation b) mixing zone
c) BaSO4 deposition (lb/ft3)
SPE 80252
Field G (model)
EXAMPLE G
Slide 43 of 40
Barium Stripping (Field G)
0
50
100
150
200
250
0 500 1000 1500 2000 2500
time (days)
b
a
r
i
u
m
c
o
n
c
e
n
t
r
a
t
i
o
n
(
p
p
m
)
0
500
1000
1500
2000
2500
3000
s
u
l
p
h
a
t
e
c
o
n
c
e
n
t
r
a
t
i
o
n
(
p
p
m
)
Ba Ba (no precip)SO4SO4 (no precip)
[Ba] at well when noreactions in reservoir
[Ba] at well when reactions in reservoir
Field G (model)
EXAMPLE G
Slide 44 of 40
Barium Stripping (Field G)
0
50
100
150
200
250
0 20 40 60 80 100
% seawater
b
a
r
i
u
m
c
o
n
c
e
n
t
r
a
t
i
o
n
(
p
p
m
Field B - observedFiled B - dilution lineField B - modelled
deep reservoir + well/near well mixing
deep reservoir mixing
0
50
100
150
200
250
0 20 40 60 80 100
% seawater
b
a
r
i
u
m
c
o
n
c
e
n
t
r
a
t
i
o
n
(
p
p
m
Field B - observedFiled B - dilution lineField B - modelled
deep reservoir + well/near well mixingdeep reservoir + well/near well mixing
deep reservoir mixingdeep reservoir mixing
Field G (model & field data)
EXAMPLE G
Slide 45 of 40
Impact of Reservoir Pressures on Placement
Question for new subsea field under development:
Can adequate placement be achieved without using expensive rig operations?
EXAMPLE X
Slide 46 of 40
Placement (Field D)
-200
-100
0
100
200
300
400
500
0 200 400 600 800
well length (m)
f
l
o
w
r
a
t
e
(
m
3
/
d
) prior to squeezeshut-inINJ 1 bbl/mINJ 5 bbl/mINJ 10 bbl/m1 year after squeeze
production
injection(squeeze)
Good placement along length of well during treatment (> 5 bbls/min) Can squeeze this well
SPE 87459EXAMPLE X
Slide 47 of 40
Placement (Field D)production
injection(squeeze)
Cannot place into toe of well by bullhead treatment, even at 10 bbl/min Must use coiled tubing (from rig - cost), or sulphate removal
-600
-500
-400
-300
-200
-100
0
100
0 200 400 600 800
well length (m)
f
l
o
w
r
a
t
e
(
m
3
/
d
) prior to squeezeshut-inINJ 1 bbl/mINJ 5 bbl/mINJ 10 bbl/m1 year after squeeze
SPE 87459EXAMPLE X
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