The gas readings are obtained by the mud loggers from gas analysis equipment and are classified according to the operation that releases the gas into the borehole (background, drilled, connection, trip, etc.)

1. Definitions

Drilled gas:

Gas that is produced from the volume of cutting drilled. It is usual to record peak drilled gas level on the mud log.

Back ground gas:

The gas that enters the bore hole during drilling or circulating (correspond to dynamic well bore conditions). It is the average gas level excluding peaks.

Connection Gas:

The gas that enters the borehole when circulation is stopped (static conditions) during the time taken for a typical connection.

Flow check gas:

The gas that enters the borehole when circulation is stopped (static conditions) during the time taken for a typical flow check (i.e. 15 min,  1/2 hr…etc)

Trip gas:

Gas that enters the bore hole when the drill string is tripped out of the hole, or partially trip out of the hole (i.e. short trips)

Re circulated gas:

Gas which is not removed by surface equipment and re circulated into the bore hole. May be detected when re circulated back to surface.

Swab gas:

Gas that is introduced into the Well bore when the hydrostatic bottom hole pressure is temporary less than the gas bearing formation pore pressure at the point of interest. The temporary reduction in pressure is caused by the swabbing effect of the drilling string moving upward.

Kick gas:

Gas that enters into the bore hole when the reservoir pore pressure exceeds the mud hydrostatic pressure, either while circulating or under static conditions. Reservoir pressure may include virgin reservoir pressure or pressure surcharged by mud losses.

2.Acceptable gas level

The meaning acceptable gas level can be defined as:

A level (%) of gas in the drilling mud, as measured by gas analysis equipment, that does not require any modification on the on going operation, or adjustment to the mud weight.

In determining that the level is acceptable the following criteria will be assessed

• Gas cutting: The mud should not be heavily gas cut because the process of passing through the surface processing equipment shakers …etc should allow any entrained gas to be released from the mud, and the mud should return to its planned weight.
• Free combustible gas: the amount of free combustible gas in any area where the mud passes through should stay below the lower alarm setting for the combustible gas detectors.
• Get wetness ratio: ratio of heavier gas elements to lighter gas elements to indicate proximity of transition zone.