May 20, 2014 - NACE Corpus Christi Section

Report
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)

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