Shallow gas is defined as any hydrocarbon-bearing zone which may be encountered at a depth close to the surface or mudline. Generally it is not possible to close-in and contain a gas influx from a shallow zone because weak formation integrity may lead to breakdown and broaching to surface / mudline.

This situation is particularly hazardous when drilling from a fixed installation or jack-up.

Shallow gas-bearing zones are usually normally pressured. However, the effective increase in pore pressure due to gas gradient can lead to underbalance when a shallow gas zone is first penetrated.

Probability and Risk

Identification and avoidance of shallow gas shall be a principal objective in well planning and site survey procedures. All Drilling Programmes shall contain a clear statement on the probability and risk of encountering shallow gas This shall be based on seismic survey and interpretation together with offset geological and drilling data.

Onshore Operations

Consideration should be given for carrying out shallow seismic surveys in areas of shallow gas risk. In the absence of such survey, assessment should be based on the exploration seismic data, historical well data and the geological probability of a shallow gas trap.

If shallow gas is a likelihood at the proposed drilling location, a shallow gas plan specific to Company and the Drilling Contractor must be prepared prior to spudding the well. Special consideration should be given to:

• crew positions
• training
• evacuation plan
• emergency power shut down.

Offshore Operations

The presence of shallow gas can be extremely hazardous. especially if no specific plan of action is prepared prior to spudding of the well. The Driller will be instructed in writing on what action to take if a well kicks while drilling surface hole.

The problem of drilling shallow hole is that normal indications of a kick are not reliable; for example:

• penetration rates vary tremendously
• mud volume is continuously being added to the active system.

The most reliable indicator is the differential flow sensor. Due to the difficulties of early detection and the depth of shallow gas reservoirs, reaction time is minimal. Extreme caution and alertness are required.

Diverter Procedures While Drilling Top Hole

At first sign of flow:

1. DO NOT stop pumping

2. OPEN diverter line to divert/close diverter (both functions should be interlocked;

3. INCREASE pump strokes to maximum (DO NOT exceed maximum pump speed recommended by the manufacturer or maximum pressure allowed by relief valve)

4. SWITCH suction on mud pumps to heavy mud in the reserve pit. ZERO stroke counter.

5. RAISE alarm and ANNOUNCE emergency using the PA system and/or INFORM the rig superintendent. POST personnel to look for gas (Jack-up)

6. If the well appears to have stopped flowing after the heavy mud has been displaced STOP pumps and OBSERVE well

7. If the well appears to continue to flow after the heavy mud has been pumped CARRY ON pumping from the active system and PREPARE water in a pit for pumping and/or CONSIDER preparing pit with heavier mud. When all mud has been consumed, SWITCH pumps to water. DO NOT stop pumping for as long as the well continues to flow.

Shallow Gas General Drilling Guidelines

The following guidelines shall be adhered to while drilling :

• Consideration shall be given to drilling a pilot hole with the 8 ½” or smaller bit size when drilling explorations wells. The BHA design shall include a float valve and considerations given to deviation and subsequent hole opening. The major advantages of a small pilot hole are:
• The ROP shall be controlled to avoid overloading the annulus with cuttings and inducing losses.
• All losses shall be cured prior to drilling ahead. Drilling blind or with losses requires the approval from Head of Operations.
• Pump pressure shall be closely monitored and all connections (on jack-up) shall be flow checked.
• Pipe shall be pumped out of hole at a moderate rate to prevent swabbing.