May 20, 2014 - NACE Corpus Christi Section
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Transcript May 20, 2014 - NACE Corpus Christi Section
Oilfield Water Analysis:
What Does It all Mean?
Tom Pickthall
EnhanceCo, Inc.
NACE
CORPUS CHRISTI, TX – NACE SECTION MEETING
MAY 20, 2014
HISTORY
API RP 45 committee started 1958
Published 1968
Latest version 1998
No current NACE Standard
APPLIED WATER TECHNOLOGY
by Dr. Charles Patton, Campbell Petroleum Services
Second Printing 1991
OIL FIELD WATER TECHNOLOGY
by Michael Davies and P.J.B. Scott
2006 NACE
WHAT’S IMPORTANT
pH, CO2, H2S, O2 and alkalinity must be run
in the field for accuracy.
If possible, overfill sample container and seal
to minimize loss of gasses.
Go ahead and splurge: use a new sample
container, not an old Coke bottle.
Deliver samples to lab as fast as possible.
Note field readings.
MUST HAVE FIELD KITS
1. pH meter
2. Carbon Dioxide (CO2)
3. Hydrogen Sulfide (H2S)
4. Oxygen (O2)
5. Total Alkalinity
(available from HACH, Chemetrics, Calgon)
RESULTS AND WHAT THEY MEAN I
ppm, mg/L, me/L
ppm:
weight/weight or volume/volume
Mg/L:
weight per volume, used by API
ppm/Mg/L=
with distilled water
me/L
(milliequivalent/L): expression of chemical
combining power of the electrolyte in a fluid;
used to check balance of cations/anions
Water
Patterns: used to compare various waters
by constituents; see API form
RESULTS AND WHAT THEY MEAN II
pH
The single most important measurement
0
7
Acid
Neutral
Normal oilfield range = 6-8
Affected
14
Basic
by CO2, H2S, Fe, bicarbonate, alkalinity
Changes
in pH are logarithmic: pH 8 = 10 x pH 7
RESULTS AND WHAT THEY MEAN III
Cations +
Sodium (Na): 40 % of the Sodium Chloride (Brine)
compound
Used to force balance the sum of cations and anions
(TDS)
Calcium (Ca): part of the measure of the “hardness” of
water; can lead to the formation of Calcium Carbonate or
Calcium Sulfate scale.
Magnesium (Mg): another part of the total “hardness”
of water; can contribute to formation of scale.
RESULTS AND WHAT THEY MEAN IV
Cations +
Barium (Ba): This is the “bad boy” of scale formation.
Limited solubility in produced waters and insoluble in acid.
Iron (Fe): Either naturally occurring from formations or
as the result of corrosion of steel. Separate acidized sample
for analysis?
In sour (H2S) systems: Look for Manganese (Mn).
SCALE FORMATION
Calcium Carbonate (CaCO3):
Solubility
depends on temperature, pressure and
concentration of ions. Acid soluble.
Probability
Increase
of scale formation increases with:
in temperature
Partial Pressure of CO2 decreases
pH increases
Total pressure decreases as TDS decreases
SCALE FORMATION
Calcium Sulfate (CaSO4)
Not
acid soluble but may be “converted”
Solubility
depends on temperature, pressure,
dissolved salts
Probability
of scale formation increases with:
Increase in temperature
Increase in dissolved salts, up to 150F
Increase in pressure
pH has little effect on CaSO4
SCALE FORMATION
Barium Sulfate (BaSO4)
Least soluble of all scales.
Scale likely to occur whenever both Ba and
SO4 are present in a water.
Increases
slightly with temperature
Increases slightly with dissolved salts
Increases slightly with pressure
pH, no effect on solubility
RESULTS AND WHAT THEY MEAN V
Anions
Chloride (Cl): 60% of the Sodium Chloride
compound (NaCl, Brine). NaCl will be generally 80% of
TDS.
Sulfate (SO4): Combines with Ba & Ca to form
scales.
Carbonate (CO3): “P” Alkalinity found in water with
pH above 8.3
Bicarbonate (HCO3): “M” Alkalinity in water with pH
between 4.5 & 8.3. Buffers corrosiveness of brine.
RESULTS AND WHAT THEY MEAN VI
Specific Gravity: The weight of a test sample
compared to the weight of distilled water (1.0). TDS
increases weight.
Resistivity: Resistance to electrical charge. The
higher the resistivity the lower the salinity. Measured
in ohm-meter.
Hydrogen Sulfide (H2S): Acid gas contributes to
corrosion.
Carbon Dioxide (CO2): Acid gas contributes to
corrosion.
Oxygen (O2): Most corrosive of gasses in
produced water.
CORROSION
Corrosion in oilfield water is caused by one of the
following constituents:
Oxygen (O2):, does not affect pH; deep pits caused by
Cathodic Depolarization
Carbon Dioxide (CO2): Partial Pressure important;
Mesa type attack.
Hydrogen Sulfide (H2S): affects pH, shallow pits;
creates FeS (black water/solids).
Bacteria: Localized pitting caused by low pH (<3); not
part of API water analysis but very important to corrosion
process.
CONCLUSION
API water analysis very useful tool but you
need to understand what it is telling you.
pH: high or low i.e. scale or corrosion
Brine level: 20,000 Cl (low), 80,000 Cl (high). How will
this affect scale/corrosion?
Am I forming scale? (Ba, Ca, SO4, CO3) Is it acid
soluble? What temperature? What pressure?
Do I have corrosion? pH, Bicarbonate, FeS?
Is this water significantly different than last time? (water
pattern)
How will water analysis affect bacteria analysis? (Cl,
TDS, O2, H2S)