“Down and Dirty“ Field Scale Analysis
This is a procedure used when the Account Representative or the field Technician is called upon
to identify scale samples in the field. An operator may need to take immediate steps to remove
scale from production tubing, flow lines, or other pieces of equipment, and time does not permit
submitting the sample scale to a laboratory for analysis. The PCC personnel must be able to
determine where the scale is calcium carbonate, iron carbonate, calcium sulfate, barium sulfate,
strontium sulfate, or a combination of scales. The following procedure outlines various methods
that the personnel in the field may use to determine the type of scale in question. At the very
least, this method of identification will give a reasonable basis for recommending a removal
treatment.
Prior to subjecting any scale sample to an analytical procedure, the sample should be rinsed in a
solution of xylene to remove any oil coating.
1.
Check with magnet to determine if any iron is in the solids. (Strongly magnetic? Slightly
magnetic, Not magnetic) If magnetic, then you already know that it is some type of iron
compound.
2.
Put solid in water. If it dissolves, it’s salt (NaCl). If not, got to step 3.
3.
Put solid in 15% HCl. If it violently effervesces (like Alka-Seltzer) with no odor and does not
appreciably turn the acid yellowish/greenish, it’s Calcium Carbonate (CaCO3) scale. If not, go
to step 4.
4.
If it violently effervesces (like Alka-Seltzer), turns the acid yellowish/greenish, gives off
“rotten egg” odor, it’s Iron Sulfide (FeS). If it slowly efferveseces, turns slightly
yellowish/greenish, gives off slight “rotten egg” odor, turns white while effervescing, it’s Iron
Carbonate (FeCaCO3). If not, go to step 6. NOTE: IRON CARBONATE SCALE IS A
CORRROSION BY-PRODUCT OF CO2 AND WILL ALWAYS HAVE UNDER-DEPOSIT PITTING!
5.
Put solid sample in scale converter. After some time, if it turns into “cottage cheese”, add
15% HCl. If it solubilizes, it’s Calcium Sulfate (CaSO4) scale.
6.
If still no reaction, put solid into solvent. If it solubilizes, it’s hydrocarbon (paraffin,
asphaltene). (If it burns with lighter, it’s hydrocarbon.
7.
If no response from previous steps, the solid is either inert (sand, silica, clay) or Barium
Sulfate (BaSO4) or Strontium Sulfate (SrSO4) scale…get to lab.
Soluble in water = salt (NaCl)
Not soluble
in water
BUT, when exposed to acid =
Violent reaction with HCl = CaCO3
No reaction with water or HCl
Scale converter turns it into an
acid soluble sludge
HCl solubilizes acid soluble sludge,
i.e. converted scale = CaSO4
Scale
Sludge Coated Scale in
15% HCL + Micel
Scale in Heavy Sludge
Approximately 10
Seconds Later
Micel solvents, such as PCC Acid Booster, prevents acid emulsions,
removes hydrocarbons from acid soluble material, prevents sludge,
increases permeability, leaves formation rock “water wet”
QUALITATIVE ANALYSIS OF SOLIDS
Soluble in HCL
Component
Soluble in
Solvent
Magnetic
Reaction Acid Color
Smell
Water Soluble
Hydrocarbons
Yes
No
None
None
None
No
CaCO3
No
No
Violent
None
None
No
CaSO4
No
No
None
None
None
No
BaSO4, SrSO4
No
No
None
None
None
No
FeS
No
Weak
Strong
Yellow
H2S
No
Fe2O3
No
No
Weak
Yellow
None
No
Fe3O4
No
Strong
Weak
Yellow
None
No
FeCO3
No
No
V Strong
Yellow
None
No
NaCl
No
No
None
None
None
Yes
Sand, Silt, Clay
No
No
None
None
None
No
QUALITATIVE ANALYSIS OF SOLIDS
Soluble in HCL
Component
Soluble in Solvent
Magnetic
Reaction
Hydrocarbons
Yes
No
None
CaCO3
No
No
Violent
CaSO4
No
No
None
No
BaSO4, SrSO4
No
No
None
No
FeS
No
Weak
Strong
Yellow
H2S
No
Fe2O3
No
No
Weak
Yellow
None
No
Fe3O4
No
Strong
Weak
Yellow
None
No
FeCO3
No
No
V Strong
Yellow
None
No
NaCl
No
No
None
Yes
Sand, Silt, Clay
No
No
None
No
Hydrocarbons
Paraffin; Asphaltenes
CaCO3
Calcium Carbonate; Calcite
CaSO4
Calcium Sulfate; Anhydrite; Gypsum
BaSO4, SrSO4
Barium Sulfate, Barite; Strontium Sulfate, Celestite
FeS
Iron Sulfide
Fe2O3
Iron Oxide
Fe3O4
Iron Hydroxide
FeCO3
Iron Carbonate
NaCl
Salt
Sand, Silt, Clay
Inerts
Acid Color
Smell
Water Soluble
No
None
None
No
Scaling Tendency Calculations
Calcium Carbonate:
Multiply mg/liter of Bicarbonate by the mg/liter of Calcium
If the resulting product is:
Below 500,000
Above 500,000
Above 1,000,000
Tendency Remote
Tendency Possible
Tendency Probable
Calcium Sulfate (Gypsum):
Multiply mg/liter of Sulfate by the me/liter of Calcium
Below 5,000,000
5,000,000 to 10,000,000
Above 10,000,000
Tendency Remote
Tendency Possible
Tendency Probable
Cations
*Calcium (Ca)
*Magnesium (Mg)
*Sodium (Na)
*Iron (Fe)
*Barium (Ba)
Strontium (Sr)
Manganese (Mn)
Table 1
Primary Constituents of Oilfield Waters
Anions
Other Properties
*Chloride (Cl)
*pH
*Carbonate (CO3)
*Temperature
*Bicarbonate (HCO3)
*Specific Gravity
*Sulfate (SO4)
*Dissolved Carbon Dioxide
*Sulfide as H2S
Resistivity
Dissolved Oxygen
Bacterial Population
Oil Content
Turbidity
Suspended Solids amount, size,
shape, chemical composition
–
The components marked with an (*) are essential to obtaining a thorough and meaningful water analysis.
Table 2
Relative Solubilities of Mineral Scales in Water
Mineral Scale
Solubility (mg/L) in Water
Calcium Sulfate (Gypsum) - CaSO4 . 2H2O
Calcium carbonate - CaCO3
2080 mg
53 mg
Barium Sulfate - BaSO4
2.3 mg
-
-
-
Table 3
Primary Variables in the Formation of Mineral Scales
Scale, or Deposit
Chemical Formula
Calcium Carbonate
CaCO3
Calcium Sulfate
CaSO4 . 2H20 (dehydrate)
CaSO4 (anhydrite)
Barium Sulfate
Strontium Sulfate
BaSO4
SrSO4
Iron Compounds:
Iron Carbonate
Iron Sulfide
Iron (II) Hydroxide
Iron (III) Hydroxide
Iron Oxide
FeCO3
FeS
Fe(OH)2
Fe(OH)3
Fe2O3
Primary Variables
-Partial pressure of CO2 (scale
increases as CO2 decreases)
-Less Soluble with Increasing
temperature
-Pressure Drops
-More soluble as TDS increases
-Less Soluble with Increasing
temperature
-Pressure drops
-More soluble as TDS increases
-More soluble as Temperatures
increase
-More soluble as TDS increases
-Corrosion by-products
-Bacterial activity
-Dissolved gases
-Depositions increase as pH
increases (especially true after an
acid job with spent acids)
Type
Detail
Probable
Origin
Table 4
Common Suspended Solids and their Probable Origins
Iron
Hydrocarbons
Carbonates
Sulfates
Compounds
-Oil Carry Over
-Iron sulfide
-Calcium
-Calcium
Insolubles/organics
-Formation fines
-Paraffin
Deposition
-Iron oxide
-Iron carbonate
carbonate
-Iron carbonate Magnesium
carbonate
sulfate
-Magnesium
sulfate
-Barium sulfate
-Sand
-Microbes Asphaltenes
-Separator
malfunction
-Truck treating
schedules
-Surface tank
damages
-Improper
equipment sizing
-Corrosion
-Incompatible
waters
-Oxygen
introduction
-Stimulation byproducts
-Scaling waters
-Corrosion
-Pressure drops
-Fluid property
changes
-Scaling water
-High
temperature
-Pump
outlets/shrouds
-Incompatible
waters
-Inadequate wellbore
cleanouts
-Pump intakes set
low
-Bacterial activity
-Natural, or induced,
formation of
asphaltenes
(CO2flood)
Figure 1 - Representative Water Analysis Indicating Scaling Tendencies
Figure 2 - Calcium Carbonate Scale
Figure 3 - Calcium Sulfate Scale
Figure 4 - Barium Sulfate Scale