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

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

Considerations

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).

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.

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).

H2S Stability and pHH2S 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), Agitator (model INBS 3000), Chemetrics Vacuettes Kit (Vacuettes K-9510B), Filter Paper (Whatman No. 50), FannViscometer (Fann 35A Model) and the API Filter Press were used during the experiments.

Experimental (Contd.)Procedure (Chen et al (1985), Ray et al (1979))

Figure 3: Procedure for the desulphurization tests

S2Na

Brine at pH 12 Agitation

Settling & Filtration

Scavenger

Residue

FiltrateSulphideContent

Determination

Results and DiscussionDesulphurization Tests

050

100150200250300350400450500550600650700750

0 20 40 60 80 100 120 140

Sulp

hide

Con

tent

in M

ud,m

g/l

Time Taken, mins

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

MAGNETITE

FERROUS GLUCONATE

25 deg. C

35 deg. C

45 deg. C

55 deg. C

Results and DiscussionRheology Tests

Conclusions Ferrous gluconate can be used in a wide range of mud pH

values. It provides efficient desulphurization as the mudwas tested “zero-sulphide” content after thedesulphurization tests. This indicates that it can protectoperating personnel from the toxic H2S. Magnetite wasonly able to remove about 30 % of the sulphide from themud.

Ferrous gluconate generally has no adverse effect on mudproperties, even at larger concentrations. It can even serveas a rheology modifier in muds. Magnetite in largerconcentrations can cause unwanted weight additions to themud.

Ferrous gluconate has the advantages of being more readilyavailable and cheaper than synthetic magnetite.

Recommendations

This information needs to be translated into realisticrig-site hydrogen sulphide scavenging tests.

More research should be conducted on the existingorganic products to identify their true scavengingcapabilities under realistic wellbore drillingconditions.

Optimization of the scavenging processes of theferrous gluconate should be looked into.

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.

THANK YOU