Simplified models used to predict scale tendencies

2) Convert to an Equilibrium equation (shorthand form shown) 1) Write the reaction 3) Rearrange to test for solids 4) Simplify 1.0 by definition

Step 1) Na,ppm (mol/kg)Cl, ppm (mol/kg)Ksp C / P, atm)  Na  Cl 86,000 (3.74)145,000 (4.09)38.2 / 1)0.84 2) Solve) measured calculated (symmetry)

Na,ppm (mol/kg)Cl, ppm (mol/kg)Ksp C / P, atm)  Na  Cl 87,250 (4.30)134,549 (4.30) ,250 (6.41)225,414 (6.36) ,800 (0.47)19,850 (0.56)41.3 (symmetry) S (w/o  ) S (with  )

CationAnion Ionppm (mol/kg)  ionppm (mol/kg)  Na + 14,500 (0.63)0.665Cl - 22,510 (0.69)0.681 K+K+ 1,234 (0.032)0.612SO ,142 (0.022)0.087 Ca +2 1,420 (0.035)0.253HCO (4.0e -3 )0.555 Ba +2 2 (1.4e -5 )0.207CO 3 -2 (1.18e -5 )0.093 Assemble necessary variables (from software or literature) Plug in data and calculate Answer: ST=0.385 Step 1) Step 4) …

Assemble necessary variables (from software or literature) Plug in data and calculate Answer: ST=3.99 Step 1) Step 4) … CationAnion ppm (mol/kg)Act coef (25C, 1atm)ppm (mol/kg)Act coef (25C, 1atm) Na + 14,500 (0.63)0.665Cl - 22,510 (0.69)0.681 K+K+ 1,234 (0.032)0.612SO ,142 (0.022)0.087 Ca +2 1,420 (0.035)0.253HCO (4.0e -3 )0.555 Ba +2 2 (1.4e -5 )0.207CO 3 -2 (1.18e -5 )0.093

TPCaSO4BaSO4CaCO3NaCaBaClSO4CO3 CBarKsp  E E E E E E E E E E E E E E E TP, barCaSO4BaSO4CaCO3 CBarSSS

Hand calculations that include methods to predict K and 

K  CaCO3BaSO4CaSO4SrSO4 Langlier Saturation Index  Ryznar Stability index  Pukoris Scale Index  Stiff Davis  Oddo-Tomson 

* Well known, pre-computer method * Developed in 1936 to predict corrosion and CaCO 3 scale in municipal water * Results (next slide) * LSI>0 Scale forms * LSI<0 Scale dissolves * Limits * 6.5 – 9.0 pH * 4000 mg/l TDS * 95 o C pH meas = measured pH pH s = pH at saturated CaCO 3 pK 2 =HCO 3  CO 3 equilibrium const. pK sp = Calcite solubility product const. Ca +2 = Calcium conc. (mol/l) Alk = Alkalinity conc. (mol/l as HCO 3 - )

Start with this Rearrange Convert p to -log Multiply by -1 Take the exponent Insert CO 3 equation End with Solubility

LSIScaling and Corrosion TendencyTreatment +4Severe Scale FormingRequired +3Significant Scale FormingRecommended +2.0Moderate Scale-formingSuggested +0.5Slightly scaling and noncorrosive 0Balanced but corrosion possibleNo Treatment -0.5Slightly corrosive and non-scaling Mild CorrosionSuggested -2.0Moderate CorrosionRecommended -3Severe CorrosionRequired

Measured data Results Ionmg/lmol/l Na + 23, Ca Cl - 35, Alk C100C pK pK sp pH C100C LSI OLI (ScaleTend) Constants and pH

calculations/langlier.html These website contain an LSI calculator

Where: and: Ryznar (1942) refined LSI to distinguish waters with same LSI but different Ca/Alk ratios A ionic strength term is added to include salinity effects Primarily for corrosion, semi- quantitative for scale RSI is inverted: low values indicate scaling. High values indicate corrosion pK 2 and pK s are same as Langelier constants I=ionic strength M=concentration (mol/kg) z=ion charge and:

RSIScaling and Corrosion Tendency Heavy Scale Light Scale Little Scale or Corrosion Some Corrosion Heavy Corrosion 9.0+Corrosion Intolerable

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Where: & Based on water buffering capacity and the maximum quantity of precipitate that can form and:

Stiff and Davis (1952) refined LSI to correct for high salinity of oilfield waters All the corrections are contained within a Temperature / Salinity plot

Molar Ionic Strength K 0oC0oC 30 o C 50 o C 60 o C 70 o C 80 o C 90 o C 100 o C

Measured data Results Ionmg/lmol/lmz 2 Na + 23, Ca Cl - 35, Alk C100C pK pK sp pH C100C Stiff Davis OLI(ScaleTend)6.835 Constants and pH

 Initially developed in 1980’s  Available for CaCO 3, CaSO 4, BaSO 4, and SrSO 4  Includes gas phase for calcite scale  based on English units; F and psia Equations for Calculating CaCO 3 scaling with a gas phase present. Oddo-Tomson was a significant advancement over previous indices. Operators and service companies created spreadsheets with this model. Many are still in use.

J.E. Oddo, J.P. Smith, and M.B. Tomson Analysis of and solutions to the CaCO3 and CaSO4 problems in West Indonesia. SPE T in Fahrenheit, P in psia, I in mg/l TDS/58400

J.E. Oddo, J.P. Smith, and M.B. Tomson Analysis of and solutions to the CaCO3 and CaSO4 problems in West Indonesia. SPE T in Fahrenheit, P in psia, I in mg/l TDS/58400

J.E. Oddo, J.P. Smith, and M.B. Tomson Analysis of and solutions to the CaCO3 and CaSO4 problems in West Indonesia. SPE T in Fahrenheit, P in psia, I in mg/l TDS/58400

 Based on mild steel corrosion in Great Lakes water  Ratio of epm SO4 2- and Cl- to epm HCO 3 - +CO 3 -2  X< SO4 2- and Cl- will not interfere with film formation  0.8<X<1.2 - SO4 2- and Cl- may interfere with film formation. Higher corrosion rates anticipated  X>1.2 - high local corrosion rates expected epm=equivalents per million

 Multi Scale (Expro Petrotech)  ScaleSoftPitz (Rice University)  Downhole SAT (French Creek Software)  ScaleChem (OLI Systems)

 Equilibrium and steady- state  Equation of state *  Chemical Speciation *  Activity coefficient *  Phase equilibrium  Mass balance  Charge balance  Energy balance  Process simulation  non-Equilibrium and transient  Mass transfer  Phase velocity  Nucleation kinetics  Precipitation kinetics  Scale inhibition  Scale adhesion * - minimum required for accurate oilfield scale prediction

 Many scale prediction tools available  All based on fundamental equilibrium equations  Each have adjustments for temperature and salinity  All have some value  Low cost  Easy to use  Can calculate complex systems  Can model mixing and inhibition