Presentation describes the effects of phosphate brine on the gas permeability of a sandstone core. The results show that phosphate brine reduced gas permeability by more than 90%

1. IPTC 14285
Exposure to Phosphate-Based Completion
Brine Under HPHT Laboratory Conditions
Causes Significant Gas Permeability
Reduction in Sandstone Cores
John Downs
Cabot Specialty Fluids
2011 International Petroleum Technology Conference

2. Alkali metal phosphate brines
Potassium phosphate brine - K2HPO4/KH2PO4
Max density = 1.77 g/cm3
K Phosphate KH2PO4 K2HPO4 pH Density
(g/100 g H2O) (%) (%) (g/cm3)
25.19 100 0 4.01 1.15
34.84 75.13 24.87 5.58 1.20
48.12 58.56 41.44 6.19 1.28
73.52 43.18 56.82 6.92 1.40
124.31 28.93 71.07 8.06 1.58
201.89 15.63 84.37 9.51 1.77
173.32 3.24 96.79 10.39 1.72
165.72 0 165.72 10.98 1.72
Also cesium phosphate brine - Cs2HPO4/CsH2PO4
Max density = 2.80 g/cm3
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3. Potassium phosphate brine used as completion
fluid by Pertamina, 2008-9 (SPE 139169)
• Used in 5 exploration wells
• Fluid density up to 1.67 g/cm3
• HPHT wells up to 335oF, up to 20,000 ppm H2S and 35% CO2
• NPT due to elastomer failures, DST tool failures, tubing
connection failures, incompatibility with brines and cements
• Formed a film on downhole metal surfaces
• No mention of testing for formation damage
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4. HPHT laboratory core flooding test with
phosphate brine
The objective of the core flooding test was to find out if
potassium phosphate brine is compatible with sandstone gas
reservoirs under HPHT conditions
• To determine the effect of potassium phosphate brine invasion on
the gas permeability of sandstone under HPHT conditions
• To determine the cause/mechanism of any change in the gas
permeability of sandstone after exposure to the phosphate brine
Use cesium formate brine (a standard HPHT well completion fluid)
as a control substance, for comparison
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5. Potassium phosphate brine, 1.637 g/cm3, pH 9.32
- Analysis (by ICP and ion chromatography)
Analyte in solution Concentration
(mg/l)
K 248,742
PO4 481,196
Na 131
Cl 96
NO3 45
SO4 45
Ba, Ca, Sr, Mg <1
Pb < 23
Al, Cd, Cu, Hg, Mn, Mo, Ni, Zn <5
Fe < 0.5
Cr 6.1
B 8.1
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6. Cesium formate brine, 2.20 g/cm3, pH 10.5
- Analysis (by ICP and ion chromatography)
Analyte in solution Concentration
(mg/l)
Cs 1,261,000
K 17,850
Rb 7,621
Na 7,836
Li 1,516
Cl 938
S 201
NO3, SO4, PO4 <5
Ca 14.3
Ba, Sr, Mg <2
Zn 3.4
Al, Cd, Cu, Hg, Mn, Mo, Ni, Cr, Pb <1
Fe 0.08
P 35
B 11.5
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7. HPHT core flooding test rig – Corex, Aberdeen
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8. HPHT laboratory core flooding test for determining
effect of phosphate brine on gas permeability
Key features of methodology
• Clean core, saturate with reservoir water, then centrifuge to irreducible
• Measure permeability to gas (30 mD) under HPHT conditions
• Forward flow of 10 PV test brine, followed by 48 hour soak period
• Realistic drawdown build-up, simulating production start-up
• Flow large volume of gas under drawdown to achieve clean-up
• Measure permeabilityunder HPHT conditions with humidified gas
• Do SEM on core samples to identify source of any damage
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9. HPHT core flood test with phosphate brine
Test conditions
- 175o C
- 5,800 psi pore pressure
- Clashach sandstone core flooded with North Sea reservoir
water and then centrifuged to irreducible saturation
Programme
- Measure initial permeability to gas at Swi under HPHT conditions
- 10 PV flush with test brine at 1 ml/minute
- Soak for 48 hours at balance under HPHT conditions
- Drawdown ramped up in stages to 100 psi (5,700 psi in wellbore)
using 2,000 PV of humidified gas
- Measure return permeability to gas under HPHT conditions
- Examine core (dry/cryo SEM) for any signs of damage
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10. HPHT core flood test with potassium
phosphate brine
Core dimensions and properties
Core Coring Length Volume Pore Porosity Grain Gas
sample Depth (cm) (cc) volume (%) density permeability
(m) (cc) (g/cc) (mD)
#1 n/a 4.78 23.802 2.07 8.7 2.62 27.6
#2 n/a 3.97 19.919 1.98 10.0 2.63 34.0
Core from Clashach sandstone, quarried near Edinburgh, Scotland
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11. Appearance of core face under SEM – before
exposure to brine
High magnification
Low magnification
Fine/medium grained sand (D50=190µ), with grain-coating and pore-filling illite clay,
chlorite, quartz and calcite. Pore throat D50=6 µ (<1 -11 µ range)
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12. Ionic composition of the reservoir water*
NaCl content of 79,330 mg/l and TDS of 89,260 mg/l
Ion concentration (mg/l)
Na K Ca Mg Ba Fe Cl HCO3
31,190 300 2,300 350 1,000 10 53,500 610
Principal scaling ions : Ca, Mg and Ba
*Simulation of reservoir water from Franklin field (HPHT gas)
in UK North Sea
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13. Output of DownHole Sat scale prediction –
phosphate brine mixing with formation water
Red = definite chance of scale formation (calcium and iron products)
Phosphate brine in mix (% v/v)
Scale 0 16.67 33.33 50 66.67 83.33 100
Calcite CaCO3 0.538 0.0638 0.0223 0.00907 0.00335 < 0.001 0
Aragonite CaCO3 0.401 0.0476 0.0166 0.00676 0.0025 < 0.001 0
Witherite BaCO3 0.0409 0.0484 0.0206 0.00848 0.00285 < 0.001 0
Strontiante SrCO3 0 0 0 0 0 0 0
Magnesite MgCO3 1.16 0.00457 0.00103 < 0.001 < 0.001 < 0.001 0
Anhydrite CaSO4 0 0 0 0 0 0 0
Gypsum CaSO4*2H2O 0 0 0 0 0 0 0
Barite BaSO4 0 0 0 0 0 0 0
Celestite SrSO4 0 0 0 0 0 0 0
Tricalcium
phosphate 0 223472 111060 58608 26373 6628 0
Hydroxylapatite 0 2.80E+01 7.30E+09 2.20E+09 5.30E+08 4.90E+07 0
Fluorite CaF2 0 0 0 0 0 0 0
Silica SiO2 0 0 0 0 0 0 0
Brucite Mg(OH)2 2.12 0.00404 0.00139 < 0.001 < 0.001 < 0.001 0
Magnesium silicate 0 0 0 0 0 0 0
Ferric hydroxide Fe(OH)3 254 45.69 24.51 14.92 8.8 4.26 0
Siderite FeCO3 9.07 3.47 1.25 0.508 0.184 0.0409 0
Strengite FePO4*2H2O 0 3.20E+07 2.80E+07 2.10E+07 1.40E+07 6418781 0
Halite NaCl 0.00591 0.0109 0.0171 0.0243 0.0316 0.0334 0
Thenardite Na2SO4 0 0 0 0 0 0 0
Iron sulfide FeS 0 0 0 0 0 0 0
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14. HPHT core flood testing with potassium
phosphate brine
24-carat gold film wrapped around circumference of core to create a
barrier to gas diffusion/leakage under hydrothermal conditions
Encased with layers of PTFE tape, heat-shrink tubing and an outer
Kalrez sleeve before mounting in core holder
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15. HPHT humidifier for gas used in core flooding
Dry nitrogen gas enters base of
humidifier, passes through column filled
with high surface area spheres saturated
with water, and exits from top.
22.50"
Pressure vessel mounted vertically in
oven at test temperature/pressure.
Materials all in Hastelloy C-276
2.75"
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16. HPHT core flood test results with potassium
phosphate brine – Brine injection phase
Pressure development across core during injection of 10 PV of
phosphate brine @ 1ml/min (frontal advance rate of 80 cm/hour)
160.00
140.00
Differential pressure (psi)
120.00
100.00
80.00
60.00
40.00
20.00
0.00
0 2 4 6 8 10
Cumulative brine throughput (pore volumes)
Differential pressure did not stabilise
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17. HPHT core flood test results with potassium
phosphate brine
Drawdown pressure ramping, gas volume throughput
and stabilised flow rate
Drawdown pressure Cumulative gas Cumulative gas Stabilised flow rate
(psi) throughput throughput (ml/min)
(ml) (PV)
10 150 75.6 0.41
25 500 252 4.24
50 900 454 13.4
75 1700 857 26.0
100 4000 2017 39.7
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18. HPHT core flood test results with potassium
phosphate brine – Drawdown flow profile
Gas flow rates and cumulative throughput during the
drawdown sequence
45
40
35
30
25
Gas flow rate (ml/min)
20
15
10
DRAWDOWN PRESSURE UP TO 100psi
5
DRAWDOWN PRESSURE AT 100psi
0
0 500 1000 1500 2000 2500
Cumulative gas throughput ( pore volumes)
2,017 PV (4,000 cm3) of gas pulled through core in 566 minutes (118 mins at 50-100 psi drawdown)
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19. HPHT core flood test results with cesium
formate brine – Drawdown flow profile
Gas flow rates and cumulative throughput during the
drawdown sequence
400
350
300
Gas flow rate ( ml/min)
250
200
150
100
50 DRAWDOWN PRESSURE UP TO 100psi
DRAWDOWN AT 100psi
0
0 500 1000 1500 2000 2500
Cumulative gas throughput ( pore volumes)
1,931 PV (4,000 cm3) of gas pulled through core in 23 minutes (11 minutes at 50-100 psi drawdown)
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20. Gas flow rate profile during drawdown-Comparison of
cores flooded with phosphate and formate brines
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21. HPHT core flood test results with potassium
phosphate and cesium formate brines
Exposing the core to phosphate brine reduced its
permeability to gas by > 90%
Completion brine Test Initial Final Change in
system Temperature Permeability permeability permeability
(oC) (mD) (mD) (%)
Phosphate 175 10.2 0.86 -91.6
Formate 175 23.0 24.8 +7.8
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22. Appearance of core face under SEM – before and
after exposure to phosphate brine and gas drawdown
Before test After test
Sand grains and pore throats covered in blanket of phosphate scale after test
EDS analysis of the scale shows potassium, phosphorus, sodium and chloride
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23. Appearance of internal surface of core under SEM–
after exposure to phosphate brine and gas drawdown
Retained phosphate scale coating onto area of illite clay
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24. Changes in ion content of fluids during HPHT core
flood test with potassium phosphate brine
• Calcium and magnesium depleted, both in wellbore fluid and filtrate. PO4
levels reduced . Suggests precipitation and/or scaling onto surfaces
• Sodium and chloride also depleted (> x 40 diluted) in filtrate
• pH of filtrate dropped from 9.7 to 8.75 after passage through core
• Phosphate brine picked up 9-35 mg/l each of Cr, Fe, Ni and Mo during test
Analyte Concentration in fluid
(mg/l)
Formation Phosphate Wellbore fluid Filtrate fluid
water brine post-test post-test
PO4 3.8 481,196 449,911 438,250
Na 31,265 131.7 < 195 <195
Cl 51,341 96.3 57.9 72.6
Ca 2,050 <1.0 <2.7 < 2.4
Mg 337 0.3 2.2 0.9
2011 International Petroleum Technology Conference

25. HPHT core flood testing with potassium
phosphate brine
Conclusions
• Flooding a sandstone core with potassium phosphate brine under
HPHT conditions reduced its gas permeability by > 90% after
clean up by 2,000 PV drawdown.
• SEM/EDS analysis of core samples indicates that the main cause
of formation damage was phosphate scale formation blocking
pore throats
- Scale deposits concentrated on surfaces coated with illite clay
- Reduced levels of Ca,Mg, PO4, Na and Cl in fluids post-test
• Flooding a similar core with cesium formate brine under same
HPHT conditions resulted in a slight improvement in permeability
• Precipitation of phosphates onto mineral surfaces is a well-known
phenomenon, and is the desired result of scale inhibitor squeezes
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26. HPHT core flood testing with potassium
phosphate brine
Acknowledgement
I would like to acknowledge and thank Ian Patey, Murdo Munro and the
laboratory staff of Corex in Aberdeen who planned, managed
and executed the experimental programme described in this paper
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