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“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.
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
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”
Scale Scale in Heavy Sludge
Sludge Coated Scale in 15% HCL + Micel
Approximately 10 Seconds Later
QUALITATIVE ANALYSIS OF SOLIDS
Soluble in HCL
ComponentSoluble 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 Acid Color Smell Water Soluble
Hydrocarbons Yes No None No
CaCO3 No No Violent None None No
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
Scaling Tendency Calculations
Calcium Carbonate:
Multiply mg/liter of Bicarbonate by the mg/liter of CalciumIf the resulting product is:
Below 500,000 Tendency RemoteAbove 500,000 Tendency PossibleAbove 1,000,000 Tendency Probable
Calcium Sulfate (Gypsum):
Multiply mg/liter of Sulfate by the me/liter of Calcium
Below 5,000,000 Tendency Remote5,000,000 to 10,000,000 Tendency PossibleAbove 10,000,000 Tendency Probable
Cations Anions Other Properties
*Calcium (Ca) *Chloride (Cl) *pH *Magnesium (Mg) *Carbonate (CO3) *Temperature *Sodium (Na) *Bicarbonate (HCO3) *Specific Gravity *Iron (Fe) *Sulfate (SO4) *Dissolved Carbon Dioxide *Barium (Ba)
*Sulfide as H2S Strontium (Sr)
Resistivity Manganese (Mn)
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 2080 mg
Calcium carbonate -- CaCO3 53 mg
Barium Sulfate -- BaSO4 2.3 mg
Table 1 Primary Constituents of Oilfield Waters
Table 3 Primary Variables in the Formation of Mineral Scales
Scale, or Deposit Chemical Formula Primary Variables
Calcium Carbonate CaCO3
-Partial pressure of CO2 (scale increases as CO2 decreases)
-Less Soluble with Increasing temperature
-Pressure Drops -More soluble as TDS increases
Calcium Sulfate CaSO4 . 2H20 (dehydrate) CaSO4 (anhydrite)
-Less Soluble with Increasing temperature
-Pressure drops -More soluble as TDS increases
Barium Sulfate Strontium Sulfate
BaSO4
SrSO4
-More soluble as Temperatures increase
-More soluble as TDS increases
Iron Compounds: Iron Carbonate
Iron Sulfide Iron (II) Hydroxide Iron (III) Hydroxide
Iron Oxide
FeCO3
FeS Fe(OH)2
Fe(OH)3
Fe2O3
-Corrosion by-products -Bacterial activity -Dissolved gases
-Depositions increase as pH increases (especially true after an
acid job with spent acids)
Type Hydrocarbons Iron
Compounds Carbonates Sulfates Insolubles/organics
-Oil Carry Over -Iron sulfide -Calcium -Calcium -Formation fines
-Paraffin -Iron oxide carbonate sulfate -Sand Detail Deposition -Iron carbonate -Iron carbonate -
Magnesium carbonate
-Magnesium sulfate -Barium sulfate
-Microbes - Asphaltenes
-Separator -Corrosion -Scaling waters -Scaling water -Inadequate wellbore
malfunction -Incompatible -Corrosion -High cleanouts
-Truck treating waters -Pressure drops temperature -Pump intakes set
schedules -Oxygen -Fluid property -Pump low Probable -Surface tank introduction changes outlets/shrouds -Bacterial activity Origin damages
-Improper equipment sizing
-Stimulation by- products
-Incompatible waters
-Natural, or induced, formation of asphaltenes (CO2flood)
Figure 1 - Representative Water Analysis Indicating Scaling Tendencies
Table 4 Common Suspended Solids and their Probable Origins