Shallow gas is defined as the gas encountered at such a depth that shut- in of the well cannot be carried out due to the potential for break down of the conductor shoe.This document provides the minimum requirements for operations (onshore and offshore) in areas with a high risk of shallow gas. A detailed procedure shall be prepared for each specific well specifying the procedures to be taken to mitigate the risk of shallow gas flows.
PLANNING
- A shallow seismic survey shall be carried out at any new location to determine the potential presence of shallow gas in the following circumstances:
- a. For exploration in new areas
- b. For a new location in a documented high risk or suspected high-risk area.
- Such a survey shall be interpreted by a competent person/agency and the results shall be provided in the well prognosis.
- For operations conducted in known and documented areas with sufficient and reliable offset data indicating the absence of shallow gas there is no requirement to carry out shallow gas seismic.
- If shallow gas is known or suspected at a location then every effort shall be made to move the location to an area where the shallow gas risk is as low as possible.
- Any well to be drilled in an unknown location where there is a possibility of encountering shallow gas shall be designed and planned with specific procedures to mitigate risks of a shallow gas kick.
- A pilot hole drilled in small diameter (12 ¼” maximum) is highly recommended.
- A risk analysis shall be applied to all operations and authorisation from Head Office and from Drilling Contractor management shall be obtained prior to commencing any operations.
- Any well encountering shallow gas, or indications that shallow gas is present, shall be terminated immediately and made safe unless the requirement of the above paragraph have been met
RIG SELECTION
- Bottom supported rigs are not recommended for use in shallow gas prone areas. Where water depth permits, a semi-submersible or drill ship rig should be used.
- The use of a diverter is compulsory on bottom-supported rigs and land rigs on:
- Any well where there is a risk of shallow gas.
- On the first four wells on a cluster or platform.
- On any combined installation where SIMOPs provisions apply.
- On floating rig operations riserless drilling is mandatory up to the 20” installation.
- For water depth less than 150 meters, a gas plume simulation shall be conducted and the use of a riser and drilling with mud shall be assessed, if prevailing current and wind conditions does not guarantee the safety of the personnel and the unit.
- On anchored floating rigs an emergency release system is compulsory and should be capable of moving off location in a blackout condition.
- It is prohibited to drill with only seawater in shallow gas prone areas whether or not a riser is used.
- When drilling in a high-risk area the following criteria shall apply to the drilling unit:
- A rig shall only be used if it is properly equipped; the crews’ adequately trained and adequate procedures are in place.
- Toolpushers and Drillers shall have experience of drilling in shallow gas areas.
- Electrical equipment shall be checked to ensure it is explosion proof and meets the applicable isolation standard.
- Gas detection and kick/loss detection equipment must be in good working order.
- The diverter equipment shall be thoroughly tested and checked.
- No hot work will be allowed on the rig at any time and for any reason.
- Well operations shall not commence before all deficiencies have been rectified.
DRILLING PRACTICES
- For offshore drilling a standby vessel shall be on location throughout the shallow gas phase of drilling and this vessel shall be equipped with adequate fire fighting and rescue equipment.
- Rig manning shall be kept at a strict minimum when spudding in a suspected shallow gas area.
- The use of a sacrificial pilot hole prior to spud is not mandatory (unless stated otherwise by local regulation) but shall be assessed on a case by case basis. The decision to drill a pilot hole is made by the Affiliate Well Operations Manager.
- The use of a specialized vessel (geotechnical coring vessel) shall be chosen for drilling such a hole.
- Pilot hole is mandatory in the following circumstances:
- Prior to installing a new platform or wellhead cluster.
- At a high-risk location with a rig that could become unstable if a shallow gas blowout occurs.
- The use of a non-sacrificial pilot hole (opening up later) shall be determined on a case by case basis, especially in case of a shallow deviated well.
- During the drilling of the top hole section, no crew change shall be permitted.
- A specific shallow gas plan shall be in place for each well that is to be drilled in a high-risk environment. Such a plan shall include:
- a. Organisation and responsibilities
- b. Emergency procedures.
- c. Minimum crew.
- d. Any restrictions on night operations.
- e. Testing of safety equipment.
- f. Drills and training.
- g. Suspension of hot work permits (including those on support barges close to the rig).
- h. Preparations including provision of pre-made volumes of kill mud, mud pump alignment.
- i. BHA design
- j. Specific drilling procedures
- k. Specific tripping procedures.
- l. Specific cementing procedures
- On platform or well clusters in a high-risk area batch drilling and casing of shallow gas zones shall be carried out.
- When drilling shallow gas zones or possible shallow gas zones it is mandatory to pump out of the hole to prevent swabbing.
- A float sub shall always be used in the BHA.
WELL CONTROL
- In the event of a shallow flow the dynamic kill procedure shall be immediately used as outlined below:
- Increase pump rate to a maximum.
- Close diverter (overboard line is simultaneously open).
- Switch to pumping pre-prepared water based mud (10ppg minimum). Switch to seawater when running out of mud.
- Initiate ESD procedure on the combined installation (if applicable).
- Take actions as soon as possible to reduce to minimum personnel onboard/onsite.
- If the flow cannot be stopped, or the diverter starts to fail or bubbling occurs from the seabed, evacuate the rig.
- Under no circumstances will shallow gas be diverted through the BOP and mud-gas separation equipment.
- For deepwater floating rigs the procedure is the same except that:
- The riser should be prepared immediately for move off.
- Do not open the subsea dump valve unless riser collapse is an issue.
- Do not bring gas to surface.
- On riserless rigs control the well for as long as is safely possible then move off location.