Selected environmentally benign iron compounds (synthetic magnetite and ferrous gluconate) have been evaluated as corrosion inhibitors for oil-well steel (N-80) in 50 mg/l sulphide concentration at various pH ranging from 5.5 to 11.5 and at High Temperature, High Pressure (HTHP) conditions by the weight loss method. The test temperatures were 150 °F, 275 °F and 350 °F respectively for pressures of 3 000 psi, 5 000 psi and 6 000 psi. The ferrous complex was found to be a better corrosion inhibitor compared to the synthetic magnetite. It exhibited up to 99.2% inhibition efficiency (IE) when the dose of the scavenger was doubled (i.e. when the sulphide to scavenger ratio was 1:2) irrespective of other factors such as pH, temperature and pressure. Whereas, the synthetic magnetite’s optimum inhibition efficiency (IE) was observed to be up to 75.1% only when the ratio of the sulphide to scavenger was 1:4 at the lowest pH of the experiment (pH 5.5) which is not desirable for a drilling mud. As the pH increases, the inhibition efficiency of the magnetite decreases and found to be lowest at the alkaline pH of 11.5.

1. STUDY OF CORROSION CONTROL EFFECT OF H2S
SCAVENGERS IN DRILLING FLUIDS
BY
Mutiu K. Amosa
Guest Consultant
Yusran Technical Services Limited
Port-Harcourt, Nigeria

2. Introduction
 Drilling Fluids
 Drill Stem
 Scavengers
 Health, Safety and Environmental (HSE)
Considerations

3. Introduction (Contd.)
Corrosion
• The destruction of a metal by chemical or
electrochemical reaction.
• Most drilling muds are corrosive. OBM's are the least
corrosive.
• The elevated temperatures and pressures encountered
downhole promote corrosion.
• Electrolytes and inorganic materials are more
corrosive than organic material.

4. Introduction (Contd.)
Figure 1: The circulating system for a mud Figure 2: Cuttings transport in
the annulus

5. Theory
 Sour gas has been reported in old fields where the
presence of hydrogen sulphide had not been
previously reported (Carter et al, 1979).
 The most HSE compliant scavenger in drilling fluids so far
is magnetite. This scavenger has a limitation of low
reaction rates in high pH but faster rates in low pH muds
(Garrett et al, 1979, KMC Oiltools, 2006).
 Whereas muds’ pH are not usually allowed to go below 9.5.
It is usually between 10 and 11.5 (M-I, LLC, 2001).
 Although commercial Zinc-containing compounds (ZCCs)
are very effective but pose rheological and environmental
problems (Ray et al, 1979).
 Zinc metal has been classified as a toxic substance,
concentrations as low as 0.15 ppm contamination could be
potentially hazardous, hence, rendering the ZCCs as
environmentally non-viable (Martin, 2005).

6. Theory (Contd.)
 Efficiencies of some organic compounds like Acrolein,
Formaldehyde, and chelates like EDTA, NTA etc as
sulphide scavengers have been reported. Their
reactions with H2S are too complex to be predicted,
and besides, there are outstanding questions
concerning HSE, especially the health aspects of
reactants and reaction products of the organic
compounds/chelates. Formaldehyde has been clearly
confirmed to be carcinogen (Nasr-El-Din et al, 2002).
 These organic compounds and chelates usually
renders themselves easily for sweetening purposes
rather than application in muds (Sitz et al, 2003).

7. Theory (contd.)
Description of An Ideal Scavenger
An Ideal Scavenger has to meet the following
requirements (Garrett et al, 1979):
 Complete, fast, and irreversible reaction with H2S
under all mud conditions;
 Should be able to undergo a quantitative reaction with
sulphide;
 pH stability of up to and beyond 11.5;
 Non-corrosive to metals;
 Easy and safe to handle and non-polluting to the
environment;
 Non detrimental to mud’s rheology;
 Must have a good environmental acceptability before
and after reaction with sulphide.

8. Theory (Contd.)
Controlling Corrosion
• The fluid should be non corrosive to the:
– Drill string
– Casing
– Surface equipment
• Corrosion can lead to:
– Wash outs
– Twist offs
– Pump failure
– Surface Leaks
Corrosion leads to loss of
&

9. Theory (Contd.)
 Complexes of iron in the Fe2+ oxidation state are usually
less sensitive to pH values (Shriver et al, 1999).
 Fe2+, ferrous ion is a necessary trace element used by all
known living organisms. It is also used in fertilizing aquatic
plants (Anonymous, 2007).
 Gluconic acid is generally recognised as safe (GRAS). Also,
sodium, calcium and iron salts of gluconic acid have been
confirmed mild, non-volatile, non-corrosive and non-toxic.
They are stable up to alkaline pH values and are also stable
at high temperatures. A metal gluconate is comparatively
better than EDTA, NTA and other chelators
(Ramachandran et al, 2006).
 Most metal gluconates are confirmed HSE compliant
materials especially the iron, sodium, zinc and calcium
salts of gluconic acids which are used for medicinal
purposes in both humans and animals (Ramachandran et
al, 2006).
 The inhibitive effect of calcium gluconate on carbon steel
in neutral aqueous media has been put to test due to its
non-toxic and eco-friendly nature and found satisfactory
(Shibli and Kumary, 2004).

10. Theory (Contd.)
H2S Stability and pH
H2S  H+ + HS-  2H+ + S2-
……………….……..………………..….(1)
Effects of H2S on Oil-well steel
H2S + Fe2+ → FeS + 2H+..............................................................(2)
 At the anode: Fe → Fe2+ + 2e- …………………………………….…….. (3)
 At the cathode: 2H+ + 2e- → H2 …………………………..………………..(4)
Probable reactions of the scavengers with sulphides:
Synthetic Magnetite (Fe3O4)
Fe3O4 + 6S2- → 3FeS2 + 4O2- …………………………………………………..(5)
Ferrous Gluconate
Fe (C6H12O7)2 + S2- → FeS + 2 [C6H12O7]- …(6)
Ferrous gluconate + Sulphide →Ferrous sulphide + gluconate

11. Experimental
Materials and Instruments
 Commercially available ferrous gluconate and magnetite
were used as scavengers. The water based mud used is
saturated brine mud. Analar grade reagents of Potassium
hydroxide, HCl, sodium sulphide pellets were used.
 Instruments such as pH meter (model OMEGA PHH-3X),
Precision Weighing Balance (model GD-503), Corrosion
Autoclave (model CORTEST 12.45) were used. Oil-well steel
coupon (N80 Steel) specimens of specification 50 x 12 x 2
mm were used for the corrosion tests using the weight loss
method. (Chemical Composition of the N80 steel (%): Fe –
97.237, C – 0.44, Mn – 1.74, P – 0.019, S – 0.014, Si –
0.24, Cr – 0.12, Ni – 0.02, Mo – 0.20)

12. Experimental (Contd.)
Procedure for Corrosion Inhibition Tests
Figure 3: Procedure for corrosion inhibition tests
Pre-Weighed
Polished
Coupons
Sulphide
Contaminated
Mud (with different
pH values)
Time-frame for
Corrosion processes
At different conditions
Coupon Removal
& Analysis
Washing &
Re-Weighing Drying

13. Results and Discussion
Control
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Control at pH 5.5 Magnetite at pH 5.5 Control at pH 7.5 Magnetite at pH 7.5
Control at pH 9.5 Magnetite at pH 9.5 Control at pH 11.5 Magnetite at pH 11.5
60 80 100 120 140 160 180
Corrosion Rate, mm/y
Temperature, deg. C
Figure 4: Dependency of corrosion rate on temperature and pH in 50 mg/l sulphide and 50 mg/l of
magnetite.

14. Control (Contd.)
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Control at pH 5.5 Fe Gluconate at pH 5.5 Control at pH 7.5 Fe Gluconate at pH 7.5
Control at pH 9.5 Fe Gluconate at pH 9.5 Control at pH 11.5 Fe Gluconate at pH 11.5
60 80 100 120 140 160 180
Corrosion Rate, mm/y
Temperature, deg. C
Figure 5: Dependency of corrosion rate on temperature and pH in 50 mg/l sulphide and 50 mg/l of ferrous gluconate.

15. Corrosion Inhibition
100
90
80
70
60
50
40
30
20
10
20 40 60 80 100 120 140 160 180 200 220
Corrosion Inhibition Efficiency, %
Scavenger Concentration, mg/l
pH 11.5
pH 9.5
pH 7.5
pH 5.5
pH 5.5
pH 7.5
pH 9.5
pH 11.5
Ferrous Gluconate
Magnetite
Figure 4: Corrosion inhibition efficiency of the two scavengers in 50 mg/l sulphide at various scavenger concentrations, 1500F and 3000 psi.

16. Corrosion Inhibition (Contd.)
100
90
80
70
60
50
40
30
20
20 40 60 80 100 120 140 160 180 200 220
Corrosion Inhibition Efficiency, %
Scavenger Concentration, mg/l
pH 11.5
pH 9.5
pH 7.5
pH 5.5
pH 5.5
pH 7.5
pH 9.5
pH 11.5
Ferrous Gluconate
Magnetite
Figure 5: Corrosion inhibition efficiency of the two scavengers in 50 mg/l sulphide at various scavenger concentrations, 2750F and 5000 psi.

17. Corrosion Inhibition (Contd.)
100
90
80
70
60
50
40
30
20
20 40 60 80 100 120 140 160 180 200 220
Corrosion Inhibition Efficiency, %
Scavenger Concentration, mg/l
pH 11.5
pH 9.5
pH 7.5
pH 5.5
pH 5.5
pH 7.5
pH 9.5
pH 11.5
Ferrous Gluconate
Magnetite
Figure 6: Corrosion inhibition efficiency of the two scavengers in 50 mg/l sulphide at various scavenger concentrations, 3500F and 6000 psi.

18. Corrosion micrographs of the coupons
Plate 1: Pitting at 3500F, pH 5.5, Plate 2: Pitting at 3500F, pH 5.5,
50 mg/l sulphide. (CONTROL) 200 mg/l sulphide (CONTROL)
focus: x100 focus: x100

19. Plate 3: Reduced pitting at 3500F, pH 5.5, Plate 4: Clean coupon surface at 3500F,
200 mg/l sulphide, 800 mg/l pH 5.5, 200 mg/l sulphide, 400 mg/l
magnetite. focus: x100 ferrous gluconate. focus: x100

20. Conclusions
 The investigated corrosion rate of N-80 steel in the H2S
contaminated mud is very rapid; it can reach 2.5 mm/y (100
mpy).
 The corrosion rate is dependent on the hydrogen sulphide
concentration, pH of the medium and the temperature
condition of the environment.
 Ferrous gluconate can reduce the corrosion of drill string
and mud lines. Its corrosion inhibition efficiency reached
almost 100% when the dose was doubled, thus preventing
drill strings from pitting corrosion, hydrogen
embrittlement and sulphide stress cracking. Magnetite had
its highest inhibition efficiency (about 70 %) at the lowest
pH when the magnetite to sulphide ratio 4:1.
 Ferrous gluconate has the advantages of being more readily
available and cheaper than synthetic magnetite.

21. Recommendations
 This information needs to be translated into the
realistic rig-site corrosion inhibition tests.
 More research should be conducted on the existing
organic products to identify their true corrosion
inhibition capabilities under realistic wellbore drilling
conditions.
 Optimization of the corrosion inhibition processes of
the ferrous gluconate should be looked into.

22. THANK YOU