Simplified models used to predict scale tendencies Ca

* Simplified models used to predict scale tendencies

Ca. CO 3 semi-empirical Langlier Saturation Index Theoretical Ba. SO 4, Ca. SO 4, Sr. SO 4 Halides Unique scales K* Mass Balance Ryznar Stability index Pukoris Scale Index Stiff Davis Oddo-Tomson Computer Software *T- * and P-dependent K value

Cation Ion Anion ppm (mol/kg) ion ppm (mol/kg) Na+ 14, 500 (0. 63) 0. 665 Cl- 22, 510 (0. 69) 0. 681 K+ 1, 234 (0. 032) 0. 612 SO 4 -2 2, 142 (0. 022) 0. 087 Ca+2 1, 420 (0. 035) 0. 253 HCO 3 - 158 (4. 0 e-3) 0. 555 Ba+2 2 (1. 4 e-5) 0. 207 CO 3 -2 (1. 18 e-5) 0. 093 Step 1) … Step 4) Assemble necessary variables (from software or literature) Plug in data and calculate Answer: * ST=0. 385

* Well known, pre-computer method * Developed in 1936 to predict p. Hmeas = measured p. Hs = p. H at saturated Ca. CO 3 corrosion and Ca. CO 3 scale in municipal water * Results (next slide) * LSI>0 Scale forms * LSI<0 Scale dissolves p. K 2 =HCO 3 equilibrium const. p. Ksp = Calcite solubility product const. Ca+2 = Calcium conc. (mol/l) Alk = Alkalinity conc. (mol/l as HCO 3 -) * Limits * 6. 5 – 9. 0 p. H * 4000 mg/l TDS * 95 o C *

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

LSI Scaling and Corrosion Tendency Treatment +4 Severe Scale Forming Required +3 Significant Scale Forming Recommended +2. 0 Moderate Scale-forming Suggested +0. 5 Slightly scaling and noncorrosive 0 Balanced but corrosion possible -0. 5 Slightly corrosive and non-scaling -1 Mild Corrosion Suggested -2. 0 Moderate Corrosion Recommended -3 Severe Corrosion Required * No Treatment

Measured data Constants and p. H Ion mg/l mol/l Na+ 23, 000 1. 0 Ca+2 400 Cl- 25 C 100 C p. K 2 10. 33 10. 09 0. 01 p. Ksp 8. 62 9. 28 35, 500 1. 0 p. H 7. 54 7. 47 Alk 610 0. 01 Results 25 C 100 C 6. 8 35 LSI OLI(Scale. Tend) * Calculate LSI for the following water

http: //www. lenntech. com/calculators/langelier/index/langelier. htm http: //www. csgnetwork. com/langeliersicalc. html http: //www. cleanwaterstore. com/technical/water-treatmentcalculations/langlier. html * These website contain an LSI calculator

� 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- Where: and: quantitative for scale � RSI is inverted: low values indicate scaling. High values indicate corrosion and: p. K 2 and p. Ks are same as Langelier constants I=ionic strength M=concentration (mol/kg) z=ion charge *

RSI Scaling and Corrosion Tendency 4. 0 -5. 0 Heavy Scale 5. 0 -6. 0 Light Scale 6. 0 -7. 0 Little Scale or Corrosion 7. 0 -7. 5 Some Corrosion 7. 5 -9. 0 Heavy Corrosion 9. 0+ Corrosion Intolerable *

http: //www. lenntech. com/calculators/ryznar/index/ryzn ar. htm *

Based on water buffering capacity and the maximum quantity of precipitate that can form Where: & and: * http: //corrosion-doctors. org/Corrosion-by-Water/Scaling-indices. htm

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

4 3. 5 0 o C 3 30 o. C K 2. 5 60 o. C 50 o. C 70 o. C 2 80 o. C 90 o. C 1. 5 100 o. C 1 0. 5 0 0 0. 4 * 0. 8 1. 2 1. 6 2 2. 4 2. 8 Molar Ionic Strength 3. 2 3. 6 4

Measured data Ion mg/l mol/l mz 2 Na+ 23, 000 1. 0 1 Ca+2 400 0. 01 0. 04 Cl- 35, 500 1. 0 1 Alk 610 0. 01 Constants and p. H 25 C 100 C p. K 2 10. 33 10. 09 p. Ksp 8. 62 9. 28 p. H 7. 54 7. 47 Results 25 C 100 C 6. 8 35 Stiff Davis OLI(Scale. Tend) *

§Initially developed in 1980’s §Available for Ca. CO 3, Ca. SO 4, Ba. SO 4, and Sr. SO 4 §Includes gas phase for calcite scale §based on English units; F and psia Oddo-Tomson was a significant advancement over previous indices. Operators and service companies created spreadsheets with this model. Many are still in use. * Equations for Calculating Ca. CO 3 scaling with a gas phase

* T in Fahrenheit, P in psia, I in mg/l TDS/58400 J. E. Oddo, J. P. Smith, and M. B. Tomson. 1991. Analysis of and solutions to the Ca. CO 3 and Ca. SO 4 problems in West Indonesia. SPE 22782.

* T in Fahrenheit, P in psia, I in mg/l TDS/58400 J. E. Oddo, J. P. Smith, and M. B. Tomson. 1991. Analysis of and solutions to the Ca. CO 3 and Ca. SO 4 problems in West Indonesia. SPE 22782.

* T in Fahrenheit, P in psia, I in mg/l TDS/58400 J. E. Oddo, J. P. Smith, and M. B. Tomson. 1991. Analysis of and solutions to the Ca. CO 3 and Ca. SO 4 problems in West Indonesia. SPE 22782.

Ø Multi Scale (Expro Petrotech) Ø Scale. Soft. Pitz (Rice University) Ø Downhole SAT (French Creek Software) Ø Scale. Chem (OLI Systems) *

Ø Equilibrium and steadystate Ø non-Equilibrium and transient q Equation of state * q Mass transfer q Chemical Speciation* q Phase velocity q Activity coefficient * q Nucleation kinetics q Phase equilibrium q Precipitation kinetics q Mass balance q Scale inhibition q Charge balance q Scale adhesion q Energy balance q Process simulation * Contain some-many of the following equations/models * - minimum required for accurate oilfield scale prediction

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