Two environmentally safe iron compounds (synthetic magnetite and ferrous gluconate) have been evaluated as sulphide scavengers at temperature conditions of 25 °C, 35 °C, 45 °C and 55 °C at pH of 12 in a sulphide-contaminated drilling mud. The ferrous complex was found to be a better scavenger than synthetic magnetite. It exhibited 100 % scavenging efficiency within the first 40 minutes of agitation. The same concentration of the reagents, which is 700 mg/l scavenger vs. 700 mg/l sulphide, was employed (i.e. sulphide concentration to scavenger concentration ratio was 1:1). Whereas, the synthetic magnetite’s scavenging efficiency was only about 30% even after 2 hours of agitation. Addition of the ferrous complex to the drilling mud was not found to be detrimental to the rheological properties of the mud. Its inclusion brought about the stabilization of mud’s rheological properties.

1. TOWARDS ENVIRONMENTAL – FRIENDLY ADDITIVES FOR
SULPHIDE SCAVENGING IN OIL AND GAS DRILLING
OPERATIONS
Mutiu K. Amosa
Guest Consultant (R&D/HSE)
Yusran Technical Services Limited
Port-Harcourt, Nigeria

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

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

4. 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).

5.  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).

6. 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.

7.  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).

8. 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

9. 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), Agitator (model
INBS 3000), Chemetrics Vacuettes Kit (Vacuettes K-
9510B), Filter Paper (Whatman No. 50), Fann
Viscometer (Fann 35A Model) and the API Filter Press
were used during the experiments.

10. Experimental (Contd.)
Procedure (Chen et al (1985), Ray et al (1979))
Na2S
Scavenger
Brine at pH 12 Agitation
Settling & Filtration
Residue
Filtrate
Figure 3: Procedure for the desulphurization tests
Sulphide
Content
Determination

11. Results and Discussion
Desulphurization Tests
750
700
650
600
550
500
450
400
350
300
250
200
150
100
50
0
Gluconate at 25 deg. C Gluconate at 35 deg. C Gluconate at 45 deg. C Gluconate at 55 deg. C
Magnetite at 25 deg. C Magnetite at 35 deg. C Magnetite at 45 deg. C Magnetite at 55 deg. C
0 20 40 60 80 100 120 140
Sulphide Content in Mud,mg/l
Time Taken, mins
MAGNETITE
FERROUS GLUCONATE
25 deg. C
35 deg. C
45 deg. C
55 deg. C

12. Results and Discussion
Rheology Tests

13. Conclusions
 Ferrous gluconate can be used in a wide range of mud pH
values. It provides efficient desulphurization as the mud
was tested “zero-sulphide” content after the
desulphurization tests. This indicates that it can protect
operating personnel from the toxic H2S. Magnetite was
only able to remove about 30 % of the sulphide from the
mud.
 Ferrous gluconate generally has no adverse effect on mud
properties, even at larger concentrations. It can even serve
as a rheology modifier in muds. Magnetite in larger
concentrations can cause unwanted weight additions to the
mud.
 Ferrous gluconate has the advantages of being more readily
available and cheaper than synthetic magnetite.

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

15. ACKNOWLEDGEMENT
 MEMBERS OF STAFF AND MANAGEMENT OF
THE MI-SWARCO DRILLING FLUID
ENGINEERING COMPANY, PORT-HARCOURT,
RIVERS STATE.
 ENGRS. MUAZU MAGAJI AND BASHEER BELLO
OF SHELL PETROLEUM DEVELOPMENT
COMPANY - SPDC (EAST), PORT-HARCOURT,
RIVERS STATE.
 CHEMICAL ENGINEERING DEPARTMENT,
AHMADU BELLO UNIVERSITY, ZARIA.

16. THANK YOU