The Role of Modeling in Addressing Scaling with Triazine Based H 2 S Scavengers

Transcription

1 The Role of Modeling in Addressing Scaling with Triazine Based H 2 S Scavengers OLI Simulation Conference 2014 Tuesday 21 st October Dr Neil Goodwin Technical Manager New Developments and Engineering Services 1

2 How can we model and test scale inhibitor performance in this system? 2

3 Calcium carbonate Ca(HCO 3 ) 2 (aq) CaCO 3 (s) + CO 2 + H 2 O Common scale, made worse by: Drops in pressure Increases in temperature Increases in ph Scale prediction is useful tool 3

4 Hydrogen Sulphide Toxic gas Acidic Corrosive Typically limited to < 10 ppm in export lines Removed by Scrubbing or scavenging Typically using high ph chemicals Risk of CaCO 3 scale 4

5 Hydrogen Sulphide Toxic gas Acidic Corrosive Typically limited to < 10 ppm in export lines Removed by scrubbing or scavenging Typically using high ph chemicals Risk of CaCO 3 scale 5

6 Hydrogen Sulphide Toxic gas Acidic Corrosive Typically limited to < 10 ppm in export lines Removed by scrubbing or scavenging Typically using high ph chemicals Risk of CaCO 3 scale 6

7 Hexahydrotriazines OH N N HO N N OH 1,3,5-Tris (2-hydroxyethyl)hexahydrotriazine 1,3,5-trimethylhexahydrotriazine. N N Act by increasing ph to deprotonate H 2 S (aq) to form S 2- ions Trapped in solutions Reaction with Sulphide to form thioether Less toxic and less corrosive. 7

8 Scavenging reaction OH OH HO N N (A) N HO N HO H 2 S N H 2 S N S OH + H 2 N OH (B) + H 2 N S S (C) OH S S S H 2 S + H 2 N (D) OH 8

9 Question? Can we model the effect of triazine and all the scavenging reactions on the calcium carbonate scale risk? 9

10 Question? Can we model the effect of triazine and all the scavenging reactions on the calcium carbonate scale risk? What else can we do? 10

11 Topsides injection Scavenger is required to sweeten gas before export. ~5,000 mg/l Ca in formation water 14,000 or 6,000 mg/l of triazine with respect to water Spent scavenger is mixed with other aqueous streams 11

12 Scavenger injection Dry Gas Scavenger Condenser Separator Wet Gas + Produced water Separator Produced water Condensed water + Condensate 12

13 Topsides injection Scavenger is required to sweeten gas before export. ~5,000 mg/l Ca in formation water 14,000 or 6,000 mg/l of triazine with respect to water Direct injection into 3 phase flow resulted in increased ph (to ) and scale Partial neutralization of Triazine had little effect on produced water ph (and still scales) 13

14 What is controlling the ph? 14

15 What is controlling the ph? How can we mimic in the laboratory: Brine prediction modelling Produced water, condensed water, export water 4 different Triazines commercial and generic Fresh and part reacted with H 2 S Effect of temperature Effect of gas saturation N 2, CO 2, (1 and 3 Bara) 15

16 What is controlling the ph? Large excess of carbon dioxide Reduces the ph by coming into the aqueous phase and forming carbonic acid. Reduces the scaling tendency Increases bicarbonate level Increases the scaling tendency CO 2(g) + H 2 O CO 2(aq) + H 2 O H 2 CO 3 HCO H + H + + R 2 NH R 2 NH 2 + Increases ph and removes H + Reduced ph 16

17 Sparged ph at 60 C Effect of Carbon dioxide Effect of Generic Triazine Scavenger concentration and saturation gas ,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000 Concentration (ppm) Nitrogen sparged at 60 C Carbon dioxide sparged at 60 C 17

18 Effect of Carbon dioxide Scavenger dose 0mg/l 1,000mg/l 10,000mg/l LOW CO 2 ph ~6 ~9.5 ~9.5 Alkalinity Increases with scavenger dose [HCO 3- ] Limited by available carbon dioxide 18

19 Effect of Carbon dioxide Scavenger dose 0mg/l 1,000mg/l 10,000mg/l LOW CO 2 HIGH CO 2 ph ~6 ~9.5 ~9.5 Alkalinity Increases with scavenger dose [HCO 3- ] Limited by available carbon dioxide ph ~4 ~6.5 ~6.5 Alkalinity Increases with scavenger dose [HCO 3- ] Increases with scavenger dose 19

20 Downhole Low Ca low CO 2 1. Downhole injection into an oil well. 2. Low water cut 3. ~2,300 mg/l Ca 4. ~1,100 mg/l HCO 3 5. Up to 1,000 ppm of H 2 S scavenger with respect to water 6. Downhole ph and CaCO 3 scale risk will increase 20

21 How can we model and test scale inhibitor performance in this system? 21

22 Dynamic Scale Formation Tests Scale Dynamic Performance Test Rig Cation Pump DPT DPT Ca + Anion Pump Scaling Coil Inline Filter Bicarbonate

23 Downhole Low Ca low CO 2 Laboratory tests Optimised for mild scale within test rig 3 hours blank time Note ph is higher than expected in field. Triazine Concentration (ppm) Blank Time (mins) Ambient ph

24 Differential Pressure (psi) Downhole Low Ca low CO 2 Effect of adding Triazine to formation water (~2,300mg/l Ca, 1,120mg/l HCO 3 ) at 62 C 8 Blank - 0ppm triazine Blank - 250ppm triazine Blank - 500ppm triazine Blank 750ppm triazine Blank ppm triazine Time (minutes) 24

25 Downhole Low Ca low CO 2 Laboratory tests Optimised for mild scale within test rig 3 hours blank time Note ph is higher than expected in field. Triazine Concentration (ppm) Blank Time (mins) Ambient ph Scale inhibitor* MED (ppm) N/A *Generic vinyl sulphonated co-polymer Minimum Effective Dose 25

26 Modelling the System Our methodology: Saturate stock of scavenger in lab. Fresh or partially reacted Record titration curve with HCl and NaOH. Use ammonia as an equivalent base in the model and recreate the titration curve in the model Select appropriate ammonia concentration by visual curve fitting. Use the ammonia concentration in scale prediction 26

27 ph Titration Curve fitting Acid (mmole / l) Experimental data, 6,000mg/l Generic triazine Saturated with carbon dioxide Modelled data, 640mg/l ammonia saturated with 70% CO2 Modelled data, 600mg/l ammonia saturated with 70% CO2 Modelled data, 590mg/l ammonia saturated with 70% CO2 at 60 C Modelled data, 590mg/l ammonia saturated with 70% CO2 at 40 C 27

28 Effect of Carbon Dioxide - Low Calcium System Scavenger dose ppmv Modeled ph Nitrogen sparged (CO 2 starved system) Laboratory ph Scale time (mins) Modeled ph CO 2 sparged Laboratory ph Scale time (mins) > > > > >180 28

29 Comparison: Lab vs Model High Carbon dioxide system, FW containing ~5,000mg/l Ca, 60 C [Scavenger] (ppm) Laboratory ph 21 C 1bar 21 C 1bar Modeled ph Scaling of CaCO 3 In situ ph (dynamic 60 C) Scaling tendency Time to Scale dynamic 60 C (mins) > 180 6,

30 Question? Can we model the effect of triazine and all the scavenging reactions on the calcium carbonate scale risk? What else can we do? Use Ammonia as an Analogue Base 30

31 Laboratory conditions to mimic CO 2 Low CO 2 Nitrogen sparging of brines with scavenger present CO 2 sparging of brines, and injection of Nitrogen sparged scavenger stock Sparging with mixed Gas: 90 : 10 N 2 : CO 2 75 : 25 N 2 : CO 2 50 : 50 N 2 : CO 2 High CO 2 Sparging with CO 2 at ambient pressure Sparging with CO 2 at elevated pressures 31

32 Selecting lab conditions Best available field data Model conditions of interest High scale tendency / mass Lab tests covering a range of test conditions. Different gas mixtures / pressures Model lab tests to determine the in situ ph and bicarbonate levels Select the best lab conditions to mimic field conditions. 32

33 Conclusions Addition of triazine based scavengers has a significant impact on calcium carbonate scale formation. If alkaline scavenger is added, the available carbon dioxide determines the severity of the change. Laboratory conditions can mimic these. Modelling is required to get the best fit between lab and field conditions. 33

34 References SPE Williams, H., Dyer, S., Bezerra, M.C.M., et al The Effect of Sulphide Scavengers on Scaling Tendency and Scale Inhibitor Performance. Paper presented at the SPE International Conference on Oilfield Scale, Aberdeen, Scotland, UK, May doi: / ms. SPE Goodwin, N., Walsh, J.M., Wright, R., et al Modeling the Effect of Triazine Based Sulphide Scavengers on the in situ ph and Scaling Tendency. Paper presented at the SPE International Symposium on Oilfield Chemistry, The Woodlands, Houston, Texas, USA, 8-10 April. doi: / ms. Graham, G, Goodwin N., Davidson, J., et al Laboratory Simulation of Scaling in the Presence of Hydrogen Sulphide Scavengers, Paper presented at the 2010 Royal Society of Chemistry. Chemistry in the Oil Industry XII, Manchester, Uk, 7-9 November

35 Questions? 35

36 Alternative lab techniques Bicarbonate addition and ph buffering for dynamic tests Need to model increased bicarbonate formation due to addition of alkaline scavenger in the presence of carbon dioxide. Need to maintain bicarbonate concentration during test. Static tests Similar need to use sparge gas 36

37 Additional slide 1 Dry Gas Scavenger Condenser Separator Wet Gas + Produced water Separator Produced water Condensed water + Condensate 37

38 Additional slide 2 Sweet Gas Rich amine H 2 S / CO 2 Scrubber Regenerator Separator Lean Amine Sour Wet Gas Produced water & Condensate (HOCH 2 CH 2 ) 2 NH Diethanolamine 38