STUDY OF CORROSION CONTROL EFFECT OF H2S SCAVENGERS IN DRILLING FLUIDS

BY

Mutiu K. Amosa

Guest ConsultantYusran Technical Services Limited

Port-Harcourt, Nigeria

Introduction Drilling Fluids Drill Stem Scavengers Health, Safety and Environmental (HSE)

Considerations

Introduction (Contd.)Corrosion• The destruction of a metal by chemical or

electrochemical reaction.

• Most drilling muds are corrosive. OBM's are the leastcorrosive.

• The elevated temperatures and pressures encountereddownhole promote corrosion.

• Electrolytes and inorganic materials are morecorrosive than organic material.

Introduction (Contd.)

Figure 1: The circulating system for a mud Figure 2: Cuttings transport in the annulus

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).

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).

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 H2Sunder all mud conditions;

Should be able to undergo a quantitative reaction withsulphide;

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.

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

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).

Theory (Contd.)H2S Stability and pH

H2S H+ + HS- 2H+ + S2-……………….……..………………..….(1)

Effects of H2S on Oil-well steelH2S + 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

ExperimentalMaterials 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)

Experimental (Contd.)Procedure for Corrosion Inhibition Tests

Figure 3: Procedure for corrosion inhibition tests

Pre-WeighedPolishedCoupons

SulphideContaminated

Mud (with different pH values)

Time-frame forCorrosion processesAt different conditions

Coupon Removal& Analysis

Washing & DryingRe-Weighing

Results and DiscussionControl

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

60 80 100 120 140 160 180

Cor

rosi

on R

ate,

mm

/y

Temperature, deg. C

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

Figure 4: Dependency of corrosion rate on temperature and pH in 50 mg/l sulphide and 50 mg/l of magnetite.

Control (Contd.)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

60 80 100 120 140 160 180

Cor

rosi

on R

ate,

mm

/y

Temperature, deg. C

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

Figure 5: Dependency of corrosion rate on temperature and pH in 50 mg/l sulphide and 50 mg/l of ferrous gluconate.

Corrosion Inhibition

10

20

30

40

50

60

70

80

90

100

20 40 60 80 100 120 140 160 180 200 220

Cor

rosi

on In

hibi

tion

Effic

ienc

y, %

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.

Corrosion Inhibition (Contd.)

20

30

40

50

60

70

80

90

100

20 40 60 80 100 120 140 160 180 200 220

Cor

rosi

on In

hibi

tion

Effic

ienc

y, %

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.

Corrosion Inhibition (Contd.)

20

30

40

50

60

70

80

90

100

20 40 60 80 100 120 140 160 180 200 220

Cor

rosi

on In

hibi

tion

Effic

ienc

y, %

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.

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

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/lmagnetite. focus: x100 ferrous gluconate. focus: x100

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.

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.

THANK YOU