H2S Standards

Minimum standard and general practices for carrying out operations in presence of H2S.

1. In general for oil and gas tubular selection, a sour environment is defined if H2S partial pressure > 0.05 psi and is H2S found on a:

a. Low pressure gas well (P < 5 bar)

b. Low pressure oil wells (< 15 bar) and low GOR (< 80 m3/m3) oil wells.

2. It is the responsibility of the subsurface department to identify and define the possibility to encounter sour gas during the drilling of a well.

3. It is the responsibility of the Well Operations Manager to ensure that any Well Operation Programme takes into account the presence of H2S when prognosed by the subsurface departments and that measures will be taken to mitigate risks to a level as low as practicably reasonable

4. If during well operations activities, the level of H2S exceeds the prognosed level, then the activity shall cease and the well shall be made safe.

5. Operations shall not resume until the risks have been fully reassessed and deemed to be ALARP.

6. When using water based mud in an H2S environment the following precautionary measures shall be taken:

a. Maintain alkalinity of mud above 11

b. Measure the global sulphides in mud and filtrate

c. Elimination of the soluble sulphides by using scavengers.

7. Such measures do not prevent the requirement to provide H2S resistant equipment.

8. In known H2S environments the use of oil based mud should be avoided.

9. Drill pipe shall not be used for testing or stimulation operations in a potential H2S environment.

10. All specific H2S procedures should be detailed in a dedicated contingency plan. This plan must be jointly approved by Well Operations and HSE Department Managers, Drilling Contractors, etc..

RISK LEVELS

The risk of encountering Hydrogen Sulphide (H2S) will be assessed and classified as:

CATEGORY 0: NEGLIGIBLE RISK:

• Applies only to appraisal or development wells in known areas with no record of H2S.

CATEGORY 1: LOW RISK:

• Any exploration wells with a bottom hole pressure greater than 5000psi.
• Any appraisal or development well which may intersect water that might contain H2S or any well where there is sulphides that could generate H2S by contact with acid.

CATEGORY 2: HIGH RISK:

• Any exploration wells in an area known to have H2S
• Appraisal or development wells that penetrate known H2S bearing formations
• Well intervention work on wells with known H2S potential
• Testing of formations where H2S is suspected

The risk category shall be included in the Well Operations Plan.

DESIGN CRITERIA

CATEGORY 0 – NEGLIGIBLE RISK

1. No special criteria apply to selection of casing, tubing, wellhead or tree.

2. No specific measures or procedures are required.

3. No specific BOP or rig requirements.

CATEGORY 1 – LOW RISK

1. The first mechanical surround must be rated for the envisaged sour service.

2. Sufficient chemicals will be on site to allow pH and corrosion control if needed. Sulphide detection equipment will be on hand.

3. Well control equipment shall be H2S trimmed.

4. A fixed H2S gas detection alarm system set at 5ppm shall be in place with detectors on the mud logging unit degasser, flow line, mud return pit and the rig floor. Two portable detectors shall be available.

5. Canister masks equipped with H2S filtration cartridge shall be available in the dog house, mud room, derrick and mud logging cabin (in sufficient number for working shift personnel).

6. 4 self contained breathing apparatus of medium capacity shall be available in offices or living quarters (with refilling pump).

7. An efficient intercom system should exist between all strategic points of the rig. Onshore a phone link should be operational between camp and rig.

8. Procedures in case of H2S alarm must be clearly defined and attached to the operation programme.

9. Rig evacuation drills are without change.

10. Briefing on perfect maintenance and use of canister masks and portable breathing systems.

CATEGORY 2 – HIGH RISK

1. The first mechanical barriers must be H2S resistant.

2. Specific mud treatment procedures will be in place and used.

3. A contingency plan shall be included in the Well Programmes detailing actions to be taken.

4. Well control equipment shall be H2S trimmed, flare booms or horizontal vent lines shall be installed, blowers to be installed in areas where H2S might accumulate and accommodation ventilation shall be equipped to prevent the ingress of H2S i.e. Detector and automatic shut down of the vent system.

5. H2S sensors must be installed as a minimum on the trough close to the bell nipple, shale shakers, return mud pit, cellar deck, rig floor and any critical place where H2S can accumulate (will depend on rig configuration and ventilation).

6. An independent detection system will be installed offshore at air intakes of living quarters (may be considered onshore at the rig camp).

7. Specialised Service company shall be contracted to ensure proper training of the personnel and to supply the required protection equipment

8. Dedicated portable H2S detectors given to tool pusher, mud engineer, mud logger, company man, barge master, supply captains.

9. Two portable tube gas detector pumps with H2S and SO2 detector tubes shall be maintained on the rig together with lead acetate paper.

10. Sufficient escape masks shall be available to equip all personnel onboard plus 50% excess. Spare filter cartridges shall be available at the safe areas.

11. A sufficient number of portable escape S.C.B.A. (self contained breathing apparatus) shall be available in dog house, mud room, derrick and mud logging cabin (in sufficient number for working shift personnel).

12. Isolating S.C.B.A. of medium capacity shall be available next to the offices or living quarters offshore or at the rig camp on a land rig.

13. A Cascade breathing air system for long term work in H2S environment. Each work place should be equipped (including cranes). Two air compressors should be located at the main safe area.

14. In addition to an efficient rig intercom system (and camp/rig phone link onshore), a public address system connected on backup power supply shall be installed.

15. Chalkboards or notepads shall be available to assist communication.

16. Appropriate caution signs and flags shall be placed onshore at all possible location entrances.
Additional wind direction devices should be visible from any work or breathing areas.

H2S CONTINGENCY PLAN

1. A contingency plan shall be drawn up, supported by operational procedures, to cover occurrence of H2S during well operations.

2. The plan will classify Rig areas as below:

• "H2S 0” area: areas where H2S should not be encountered (due to wind, relative height or pressurisation); should of course be periodically checked.
• “H2S 1” area: areas normally safe but should be systematically checked; escape masks have to be worn by personnel and ready to be used.
• “H2S 2” area: areas with potential danger once a trace of H2S has been detected Access should be permitted only by rig tool pusher or OIM, all personnel entering these zones must be equipped with S.C.B.A. and accompanied.

3. The Plan will describe alert conditions as below:

• ALERT LEVEL 0 - Normal Condition with no H2S detected in the air but Sulphides may have been detected in mud.
• ALERT LEVEL 1 - H2S has been detected in the atmosphere at any concentration < 10 ppm.
• ALERT LEVEL 2 - H2S concentration in the atmosphere is exceeding 10 ppm somewhere on the rig.

4. The plan will describe:

a. H2S specific procedures for normal conditions (alert level 0): tripping, coring, kick control, WL operations, well testing

b. The emergency procedures - alert level 1

c. The emergency procedures - alert level 2

d. The environmental limitations versus operations

e. Organisation, training of personnel and periodicity of safety drills

f. Stand-by vessels communication and safety procedures

g. Neighbouring rigs or platforms communication and safety procedures

h. The platform evacuation procedures

i. The local population communication and evacuation plan (onshore)

j. The verification of suitability for H2S equipment (directly used and back-up) including the working limitations such equipment shall have in an H2S environment (rating, working cycle’s number).

k. The procedures to work safely within the specified limits.

An H2S specialist instructor shall be in charge of the training of all personnel (technical and supporting), and the arrangement and maintenance of all detection and protection equipment.

EFFECTS OF H2S

This table represents the physiological consequences of an exposure to different H2S concentrations. These concentrations are given in ppm (0.0001% in volume).

REFERENCES

NACE MR 01 75 Rev 94

API RP 49 – 2nd edition

API RP 13 B1 and B2

API SPEC 6A – 16th edition

API SPEC 16A – 1st edition