During drilling and workover operations the consequences of leaks or kicks with sour gas or crude may be very serious. Personnel can be incapacitated by relatively low concentrations of H2S in a very short time and equipment can suffer catastrophic failure due to H2S embrittlement.

The H2S risk depends primarily on the H2S content of the formation fluids, the formation pressure, and the production flow rate. This information is used to assess the level of danger that H2S presents and the requirement for special equipment, layout, and emergency procedures for the drilling or workover operation.

1 Planning of operations

• A study should be made of the geological and geographical information of the area, including history of adjacent wells, in order to predict the expected area where H2S may be encountered. Information should be obtained and taken into consideration about the area and known field conditions, including temperatures, pressures, proposed well depth, and H2S concentrations.
• A mud programme should be drawn up which will cater for the pressures expected to be encountered, but should also include the use of an H2S scavenger to reduce the reaction of H2S on the drillstring and related equipment and to control the processing of H2S at surface. Normal practice is to maintain a higher than normal pH (i.e. 10.5 - 11) and to treat the mud with a suitable scavenger as soon as dissolved sulphides are analysed. H2S contamination of water based muds can deteriorate the mud properties at a fast rate. It is advisable to keep the mud moving with immediate treatment to maintain the desired properties.
• Maintaining a high pH or using a scavenger is not suitable to safeguard drilling equipment against H2S, since in a kick situation the wellbore may become partially/fully evacuated of drilling fluid, thus reducing or eliminating the ability to contact drillstring and wellhead and BOP components with scavenger. H2S resistant materials should be considered for this well control condition. BOPs must be made to NACE specifications.
• Prior to reaching the H2S-bearing formations the emergency equipment (blowout preventer, degasser, etc.) and response procedures should be tested in an exercise simulating a kick.
• Wind direction should be taken into account for the layout of equipment such as shale shakers, choke manifold, mud tanks and particularly vents such as flare lines, degasser vents, mud-gas separator vents, and diverter lines. Wind socks on the site or platform should enable identification of upwind assembly points. Offshore, each assembly point should allow easy evacuation from the installation.

2 Drilling equipment selection

Equipment should be selected after consideration of metallurgical properties which will reduce the chances of failure from H2S embrittlement.

The following recommendations are to be followed for H2S designated wells:

a. BOP stack

a.1. Metallic materials for sour service

All pressure containing components of the BOP stack with the potential to be exposed to H2S should be constructed of material which meets the standard of the NACE MR-01-75 and API RP 53. These components, as a minimum, include annular preventer, rams, drilling spools, the hydraulic operated choke line valve and gaskets.

a.2. Non-metallic materials for sour service

Non-metallic materials for sour service should conform to API RP 53, Section 9.A.8. Fluoropolymers, such as Teflon or Ryton and fluoroelastomers, such as viton or Kalrez are acceptable materials.

a.3. Welding for sour service

Where welding is required for component fabrication, the weldment and the heat affected zone of the welded components should possess essentially the same chemical and physical properties as the parent metals of the subcomponents. These include hardness properties and impact properties where appropriate. The weldment is also required to be free of linear defects such as cracks, undercutting, and lack of fusion.

a.4. Sour service identification

Components should be marked in a manner that shows their suitability, under NACE MR-01-75, for sour service.

Identification stamping procedures as detailed in NACE MR-01-75, Section 5.4 should be followed.

a.5. Transportation, rigging up, and maintenance

During transportation, rigging up, and maintenance of BOP stacks, operating practices should be used which avoid cold work and hence, hardening of equipment components. Material control for replacement parts for the BOP stack should have specifications and quality control equivalent to the original equipment.

b. Flange, bonnet cover, bolting and nut material

Each of these intended for H2S use should meet requirements prescribed in API Spec 6A section 1.4 (14th edition).

c. Choke manifold

Piping, flanges, valves, fittings, and discharge lines (flare lines) used in the composition of the choke manifold assembly should contain metals and seals in accordance with API RP 53.

d. Degassers/mud-gas separator

The degasser should be capable of effectively removing entrained gases from contaminated drilling fluid circulated back to the surface. The vent outlet on the degasser should be extended so that the extracted gas can be routed to a remote area for flaring or connected into the choke flare line. A mud-gas separator used to extract gas containing H2S from drilling fluids should be tied into a vent line for burning, so as not to release the gas into the atmosphere close to the rig.

e. Flare lines

Flare lines should be installed from the degasser, choke manifold, and mud-gas separator according to API RP 49. All flare lines should be equipped with the means for constant or automatic ignition.

f. Drillpipe

Because of the direct contact of drillpipe with H2S in the wellbore where various temperature and pressure conditions exist, the lower grades of pipe should be used so as to minimise hydrogen embrittlement or sulphide stress corrosion cracking (SSCC). Means of control to minimise hydrogen embrittlement and SSCC of drillpipe can also be found in API RP 49. Consideration may be given to the use of a drillstring equipped with special tool joint material.

g. Monitoring equipment

Each drilling rig operating in an area known or suspected to produce H2S gas should have adequate H2S monitoring and/or detection equipment. Should the need for specific continuous monitoring and/or detection instruments be deemed necessary, it is recommended that this equipment should be installed 350 metres and/or 1 week prior to drilling into the H2S zone. H2S concentrations should be continuously monitored at strategic sampling positions, e.g. shale shaker, mud ditch, mud tank area, etc., and results transmitted both to the driller's console and to the toolpusher's office. Audible and visible alarms should indicate both locally and remotely when H2S concentration reaches 10 ppm. Sulphide tests should be carried out as part of the mud testing programme in areas where hydrogen sulphide gas (H2S) might be encountered.

h. Mud logging unit

The mud logging unit and equipment should be located away from the shaker tank and a minimum of 50 metres away from the well head.

i. Venting system

Weatherised rigs equipped with partitions permanent in nature, should be provided with a ventilation system sufficient for the removal of accumulated H2S.

3 Training

When drilling in an area where hydrogen sulphide gas (H2S) might be encountered, training of personnel must be carried out in the characteristics and hazards of H2S, the action to be taken in the event of alarm, the use of safety equipment and escape procedures whatever the likelihood of encountering H2S. Emergency procedures must be practised regularly using realistic drills.

4 H2S contingency planning

A contingency plan should be drawn up when H2S is anticipated while drilling. The contingency plan should be developed prior to the commencement of drilling operations and should include the following:

• Information on the physical effects or exposure to hydrogen sulphide (H2S) and sulphur dioxide (SO2).
• Safety and training procedures to be followed and safety equipment that will be used.
• Procedures for operations when the following conditions exist:
  • pre-alarm condition;
  • moderate danger to life;
  • extreme danger to life.
• Responsibilities and duties of personnel for each operating condition.
• Briefing areas or locations for assembly of personnel during extreme danger condition should be designated. At least two briefing areas shall be established on each drilling facility. Of these two areas, the one upwind at any given time is the safe briefing area.
• Evacuation plan.
• Authorities to be notified in case of emergency, by whom and at what stage.
• A list of emergency medical facilities, including locations and/or addresses and telephone numbers.
• In a pre-spud meeting, the Company drilling supervisor should review the drilling programme with the drilling contractor and service contractors, outlining each party's responsibility in drilling a well where H2S can be encountered.
• All personnel should be fully trained and the H2S-related equipment should be in place when drilling at 350 metres above and/or one week prior to encountering a hydrogen sulphide zone.
• Available literature should be carefully studied before drawing up H2S procedures. Recommended references are: API RP49 'Safe Drilling of Wells Containing Hydrogen Sulphide'.