Oilfield Water Analysis:

What Does It all Mean?Tom Pickthall

EnhanceCo, Inc.

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

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.

1. pH meter 2. Carbon Dioxide (CO2)

3. Hydrogen Sulfide (H2S)

4. Oxygen (O2) 5. Total Alkalinity

(available from HACH, Chemetrics, Calgon)

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

pH The single most important measurement

Acid Neutral Basic Normal oilfield range = 6-8

Affected by CO2, H2S, Fe, bicarbonate, alkalinity

Changes in pH are logarithmic: pH 8 = 10 x pH 7

0 7 14

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.

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).

Calcium Carbonate (CaCO3):

Solubility depends on temperature, pressure and concentration of ions. Acid soluble.

Probability of scale formation increases with: Increase in temperature Partial Pressure of CO2 decreases pH increases Total pressure decreases as TDS decreases

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

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

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.

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 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.

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)