1. A combination of acids, bases, and pH

2. In Produced water, Alkalinity is usually bicarbonate The amount of acid or base that a water can tolerate before the pH changes significantly. Buffer capacity, Carbonate buffering, Carbonate alkalinity, Acid absorption capacity a.k.a.

4. BufferTitration productBufferTitration producct

5. * It keeps the water pH stable – pH will not change significantly if acids are added (like stimulation fluids) * It protects against corrosion because it reduces the amount of free acid (H + )

6. The acid volume needed to consume the buffer Any chemical that will absorb H + ions in the pH region where the buffer is active A buffer against Acid HCO 3 - + H + = H 2 CO 3 pH inflection point Acid consumed at inflection point pH buffer zone (~5.3-7.3pH)

7. Available Buffer Consumed Buffer Excess Acid HCO 3 - + H + = H 2 CO 3

8. Available Buffer Consumed Buffer Excess Acid HCO 3 - + H + = H 2 CO 3

10. BufferBuffering reactionBuffering Region (pH) 1 pH Titration End Point Conc. range in reservoir water CarbonateCO 3 -2 + H + = HCO 3 - 11.3 to 9.38.3<1 BicarbonateHCO 3 - + H + = H 2 CO 3 7.3 to 5.34.3 2 100-1000 FormateHCOO - +H + =HCOOH4.8 to 2.81.81-100 AcetateCH 3 COO - + H + = CH 3 COOH 5.7 to 3.72.720-2000 PropanateC 2 H 5 COO - + H + = C 2 H 5 COOH 5.9 to 3.92.91-100 BorateH 4 BO 4 - + H + = H 3 BO 3 + H 2 O 8.8 to 6.86.510-300 AmmoniaNH 3 + H + = NH 4 + 10.2 to 8.27.2<10 SulfideS -2 + H + = HS - 12 to 109<1 BisulfideHS - + H + = H 2 S8 to 651-100 1 – This represents about 90% of the buffering region 2 – Recognize this endpoint? This is the Total alkalinity endpoint we use in produced water analyses

11. * Acid consumes the buffer * Converting to an equilibrium equation * The Equilibrium constant determines the pH range where the buffer is active

12. HCO 3 -1 buffers between pH=7.3 and pH=5.3 (at zero salinity) HCO 3 - + H + = H 2 CO 3 pH region where the HCO 3 -1 buffer is active

13. CH 3 COO - + H + = CH 3 COOH pH region where the Acetate -1 buffer is active Acetate -1 buffers between pH=5.7 and pH=3.7 (at zero salinity)

14. HS - + H + = H 2 S pH region where the HS -1 buffer is active HS -1 buffers between pH=8 and pH=6 (at zero salinity)

15. B(OH) 4 - + H + = H 3 BO 3 +H 2 O pH region where the B(OH) 4 -1 buffer is active B(OH) 4 -1 buffers between pH=10 and pH=8 (at zero salinity)

16. A seawater analysis CationsMg/lAnionsMg/lNeutralsMg/l Na+110561Cl-118980SiO25 K+1380SO4-22648 Ca+2400Br-165 Mg+21272B(OH)4-133.6 Sr+213F-11.4 Ba+20.05I-10.05 Li+10.1HCO3-1144

17. Inflection point Titration endpoint 0.1L sample titrated using 0.1M HCl Two inflection points

18. First inflection PointSecond Inflection Point Acid volume (L)pH Acid volume (L)pH 0.000447.38 0.002455.12 0.000537.09 0.002545.00 0.000616.87 0.002634.85 0.000706.71 0.002714.66 0.000796.580.002804.42 0.000886.470.002894.18 0.000966.370.002983.98

19. * Calculate Total Alkalinity from total volume acid used * Calculate Alkalinity using acid volume from first titration point * Calculate Alkalinity using acid volume from first to second titration point * Compare all three results to seawater concentrations

20. Additional experimental data Acid Concentration0.1 moles/l Sample volume0.1 L

21. Total Alkalinity (total acid used) Alkalinity to first inflection (0.00053 L acid used) Alkalinity to second inflection (0.00289 L-0.00053 L = 0.00236 L acid used)

22. CationsMg/lAnionsMg/l Na+110561Cl-118980 K+1380SO4-22648 Ca+2400Br-165 Mg+21272B(OH)4-133.6 Sr+213F-11.4 Ba+20.05I-10.05 Li+10.1HCO3-1144 If the total alkalinity value us used to calculate the bicarbonate concentration, then other alkalinity contributing species should be removed. Otherwise the value will be incorrect. The bicarbonate alkalinity is 144 mg/l and not 176.3 mg/l.

24. 1 L sample titrated using 0.1M HCl

25. 6.8 ml The infection point is between 3.9 and 4.8 pH

27. Cations(mg/L) Anions(mg/L) Na+11800 Cl-13090 K+160 SO4-255 Ca+2220 HCO3-1400 Mg+230 Sr+260 Ba+210 Fe+22

29. 1 L sample titrated using 0.1M HCl

30. 13.5 ml The infection point is between 3.6 and 4.2 pH

32. Cation Conc (mg/l) Anions Conc (mg/l) Neutrals Conc (mg/l) Na+111600 Cl-119000 CO2110 K+1140 HCO3-1800 H2S70 Mg+2650 SO4-2200 Ca+2100 Sr+2125 Ba+21 Fe+210

34. 1 L sample titrated using 0.1M HCl

35. 13.5 ml The infection point is between 3.5 and 3.9 pH

37. Cation Conc (mg/l) Anions Conc (mg/l) Na+182000 Cl-1145000 K+1900 HCO3-1120 Mg+22050 SO4-290 Ca+28000 Br-1400 Sr+2250 Ba+2230

38. The Alkalinity titration represents the total alkalinity, not only bicarbonate

39. 0.001 mol (78 ppm) borate -1 0.003 mol (183ppm) bicarbonate -1 0.002 mol (120ppm) acetate -1 0.0002 mol (9 ppm) formate -1 Borate buffer Bicarbonate buffer Acetate buffer Formate buffer Notice how the buffers overlap Traditional Alkalinity Endpoint

40. Let’s test this out in ScaleChem Do Chapter 9 – Alkalinity And we’ll discuss the results

41. The amount of base added to consume the acid buffer Any chemical that will absorb a OH - ion in the pH region where the buffer is active A Buffer against base H 2 CO 3 +OH - =HCO 3 - + H 2 O