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
Alkali metal phosphate brines
Potassium phosphate brine - K2HPO4/KH2PO4
Max density = 1.77 g/cm3
K Phosphate (g/100 g H2O)
KH2PO4 (%)
K2HPO4
(%)pH Density
(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
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Also cesium phosphate brine - Cs2HPO4/CsH2PO4
Max density = 2.80 g/cm3
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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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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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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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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HPHT core flooding test rig – Corex, Aberdeen
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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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HPHT core flood test with phosphate brine
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
Test conditions - 175o C - 5,800 psi pore pressure - Clashach sandstone core flooded with North Sea reservoir water and then centrifuged to irreducible saturation
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HPHT core flood test with potassium phosphate brine
Core from Clashach sandstone, quarried near Edinburgh, Scotland
Core sample
Coring Depth
(m)
Length (cm)
Volume (cc)
Pore volume
(cc)
Porosity (%)
Grain density (g/cc)
Gas permeabilit
y(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
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Core dimensions and properties
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Appearance of core face under SEM – before exposure to brine
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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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Low magnification High magnification
Ionic composition of the reservoir water*
NaCl content of 79,330 mg/l and TDS of 89,260 mg/l
Principal scaling ions : Ca, Mg and Ba
*Simulation of reservoir water from Franklin field (HPHT gas)
in UK North Sea
Ion concentration (mg/l)
Na K Ca Mg Ba Fe Cl HCO3
31,190 300 2,300 350 1,000 10 53,500 610
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Output of DownHole Sat scale prediction – phosphate brine mixing with formation water
2011 International Petroleum Technology Conference
Red = definite chance of scale formation (calcium and iron products)
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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 0Tricalcium 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
HPHT core flood testing with potassium phosphate brine
2011 International Petroleum Technology Conference
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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HPHT humidifier for gas used in core flooding
2.75"
22.50"
Dry nitrogen gas enters base of humidifier, passes through column filled with high surface area spheres saturated with water, and exits from top.
Pressure vessel mounted vertically in oven at test temperature/pressure.
Materials all in Hastelloy C-276
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HPHT core flood test results with potassium phosphate brine – Brine injection phase
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Pressure development across core during injection of 10 PV ofphosphate brine @ 1ml/min (frontal advance rate of 80 cm/hour)
Differential pressure did not stabilise
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0 1 2 3 4 5 6 7 8 9 100.00
20.00
40.00
60.00
80.00
100.00
120.00
140.00
160.00
Chart Title
Cumulative brine throughput (pore volumes)
Dif
fere
nti
al p
ress
ure
(p
si)
HPHT core flood test results with potassium phosphate brine
Drawdown pressure (psi)
Cumulative gas throughput
(ml)
Cumulative gas throughput
(PV)
Stabilised flow rate (ml/min)
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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Drawdown pressure ramping, gas volume throughput and stabilised flow rate
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HPHT core flood test results with potassium phosphate brine – Drawdown flow profile
2011 International Petroleum Technology Conference
Gas flow rates and cumulative throughput during the drawdown sequence
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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0 500 1000 1500 2000 2500
0
5
10
15
20
25
30
35
40
45
DRAWDOWN PRESSURE UP TO 100psi
DRAWDOWN PRESSURE AT 100psi
Cumulative gas throughput ( pore volumes)
Ga
s flo
w r
ate
(m
l/min
)
HPHT core flood test results with cesium formate brine – Drawdown flow profile
2011 International Petroleum Technology Conference
Gas flow rates and cumulative throughput during the drawdown sequence
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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200 400 600 800 1000 1200 1400 1600 1800 2000 2200
0
50
100
150
200
250
300
350
400
DRAWDOWN PRESSURE UP TO 100psi
DRAWDOWN AT 100psi
Cumulative gas throughput ( pore volumes)
Ga
s fl
ow
ra
te (
ml/m
in)
Gas flow rate profile during drawdown-Comparison of
cores flooded with phosphate and formate brines
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HPHT core flood test results with potassium phosphate and cesium formate brines
Completion brine system
Test Temperature
(oC)
Initial Permeability
(mD)
Final permeability
(mD)
Change in permeability
(%)
Phosphate 175 10.2 0.86 -91.6
Formate 175 23.0 24.8 +7.8
2011 International Petroleum Technology Conference
Exposing the core to phosphate brine reduced its permeability to gas by > 90%
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Appearance of core face under SEM – before and
after exposure to phosphate brine and gas
drawdown
2011 International Petroleum Technology Conference
Sand grains and pore throats covered in blanket of phosphate scale after testEDS analysis of the scale shows potassium, phosphorus, sodium and chloride
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Before test After test
Appearance of internal surface of core under SEM–
after exposure to phosphate brine and gas
drawdown
2011 International Petroleum Technology Conference
Retained phosphate scale coating onto area of illite clay
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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 water
Phosphate brine
Wellbore fluid post-test
Filtrate fluid 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
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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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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