During drilling and workover operations, the consequences of leaks with sour gas or crude may be devastating. Drilling H2S-bearing formations poses one of the most challenging and dangerous problems to humans and equipment. Personnel can be injured or even killed by relatively low concentrations of H2S in a very short period. Equipment can experience terrible failure due to H2S gas-induced material failure. This risk depends primarily on the H2S content, formation fluids, formation pressure, and the production flow rate. This information is used to assess the level of risk from the presence of H2S.
In addition, if this risk is known or anticipated, there are particular requirements to follow the International Association of Drilling Contractors (IADC) rules and regulations. All information will ultimately lead to the requirement for special equipment, layout, and emergency procedures for drilling and/or workover operations.
How to Tackle H2S Drilling Wells
The presence of H2S can be anticipated from previous field or region data. For a wildcat, all precautionary measures should be taken, following IADC rules, as if H2S will be encountered. The following steps and plans should be followed when H2S gas is discovered.
H2S Well Planning In Drilling
A study should be done on the geological and geographical information of the area. This study should include the history of adjacent wells to predict the expected area where H2S may be encountered. Information about the area and known field conditions, including temperatures, pressures, proposed well depth, and H2S concentrations, should be obtained and considered.
A drilling fluid program should be drawn up, providing different pressures expected to be encountered. However, an H2S scavenger should also be included to reduce the reaction of H2S on the drill string and related equipment to control the processing of H2S at the surface. Standard practice is maintaining a higher-than-normal pH (i.e., 10.5–11) and treating the mud with a suitable scavenger as soon as dissolved sulfides are analyzed.
The contamination of water-based mud due to H2S can deteriorate the mud properties at a fast rate. It is advisable to keep the mud moving with an 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 well kick situation, the wellbore may become partially/entirely devoid of drilling fluid, thus reducing or eliminating the ability to contact the drill string and wellhead and BOP stack components with the scavenger. H2S-resistant materials should be considered for this well-controlled condition. The BOPs must be made to NACE specifications that conform to the presence of H2S.
Before reaching the H2S-bearing formations, the emergency equipment (blowout preventer, degasser, etc.) and response procedures should be tested in an exercise that simulates a kick.
Wind direction should be considered for equipment layouts such as shale shakers, choke manifolds, mud tanks, and particularly vents such as flare lines, degasser vents, mud-gas separator vents, and diverter lines. Windsocks on the site or platform should enable the identification of upwind assembly points. Each assembly point should allow easy evacuation from the installation for offshore operations.
Drilling Equipment Selection In H2S Wells
Equipment should be selected after consideration of metallurgical properties, thus reducing the chances of string failure from H2S-induced corrosion. The following recommendations are to be followed for H2S-designated wells:
- During transportation, rigging up, and maintenance of BOP stacks, operating practices should be used to avoid cold temperatures that might induce the hardening of equipment components. Material control for replacement parts for the BOP stack should have specifications and quality control equivalent to the original equipment.
- All pressure-containing components of the BOP stack with the potential to be exposed to H2S should be manufactured with the material that meets the standard of the NACE MR-01-75 and API RP 53. These components include annular preventers, rams, drilling spools, the hydraulic operated choke line valve, gaskets, etc.
- As detailed in NACE MR-01-75, Section 5.4, identification stamping procedures should be followed.
- 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.
- Where welding is required for component fabrication, the welding 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 welding must also be free of linear defects such as cracks, undercutting, and lack of fusion.
- Metallic materials for sour-gas service should be employed.
- Sour-gas service identification should be performed.
- Components should be marked to show their suitability, under NACE MR-01-75, for sour service.
- Welding should conform to sour-gas service.
- Transportation, rigging up, and maintenance should conform to sour-gas requirements.
Flange, Bonnet Cover, Bolting, And Nut Material
Each of these intended for H2S use should meet the requirements prescribed in API Specification 6A section 1.4 (14th edition).
Choke Manifold While Drilling H2S Wells
Piping, flanges, valves, fittings, and discharge lines (flare lines) used in the composition of the choke manifold assembly should contain metals and seals following API RP 53.
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 to the choke flare line. A mud-gas separator is used to extract gas containing H2S from drilling fluids. This separator should be tied into a vent line for burning so that it cannot release the gas into the atmosphere close to the rig side area.
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.
Drill Pipe For H2S Wells
Because of the direct contact of the drill pipe with H2S in the wellbore where various temperature and pressure conditions exist, the pipe’s lower grades should be used to minimize hydrogen embrittlement or sulfide stress corrosion cracking (SSCC). Means of control to reduce hydrogen embrittlement and SSCC of drill pipe can also be found in API RP 49. Consideration may be given to using a drill string equipped with unique drill pipe tool joint material.
Each drilling rig type operating in an area known or suspected to produce H2S gas should have adequate H2S monitoring and detection equipment. It is recommended that this equipment should be installed 350 meters and one week before 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 should be transmitted to the driller‘s console and the toolpusher‘s office. Audible and visible alarms should indicate both locally and remotely when the H2S concentration reaches ten ppm. Sulfide tests should be carried out as part of the mud testing program in areas where hydrogen sulfide gas (H2S) might be encountered.
Mud Logging Unit
The mud logging unit and equipment should be located away from the shaker tank, and a minimum of 50 meters of distance should be kept from the wellhead.
Weatherized rigs equipped with permanent partitions should have a ventilation system to remove accumulated H2S.
When drilling where H2S gas might be encountered, rig personnel training must be carried out on the subject matter. Action should 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 practiced regularly, using realistic emergency drills.
H2S Contingency Planning
A contingency plan should be drawn up when H2S is anticipated while drilling. The contingency plan should be developed before the commencement of drilling operations and should include the following:
- Responsibilities and duties of personnel for each operating condition.
- Information on the physical effects or exposure to H2S and sulfur dioxide (SO2).
- Safety and training procedures should be followed, and safety equipment will be used.
- Procedures for operations when the following conditions exist:
- Pre-alarm condition
- Moderate danger to life
- Extreme danger to life
- All personnel should be fully trained, and the H2S-related equipment should be placed when drilling 350 meters above and one week before encountering a hydrogen sulfide zone.
- The evacuation plan should be in place and well-rehearsed.
- A plan must be in place for who would notify the authority and at what stage of the incident.
- In a pre-spud meeting, the company drilling supervisor should review the drilling program with the drilling contractor and service contractors, outlining each party’s responsibility in drilling a well where H2S may be encountered.
- A list of emergency medical facilities, including locations, addresses, and telephone numbers, must be in place.
- Available literature should be carefully studied before drawing up H2S procedures. Recommended references are API RP49 “Safe Drilling of Wells Containing Hydrogen Sulphide.”
- Briefing areas or locations for the assembly of personnel during hazardous conditions should be designated. At least two briefing areas should be established in each drilling facility. Of these two areas, the one upwind at any given time is the safe briefing area.
Drilling Engineering Problem & Solutions – A Field Guide Engineers & Students – M.E. Hossain – M.R. Islam