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1. n. [Drilling]

An average or baseline measure of gas entrained in circulating mud. This baseline trend pertains to gas that is liberated downhole while drilling through a uniform lithologic interval at a constant rate of penetration. The gas is typically obtained from a suction line above the gas trap located immediately upstream of the shale shaker screens, where the gas evolves out of the mud. Oil-based mud systems tend to produce higher background gas values than do water-based muds.

Deviations from the background gas trend likely indicate changes in porosity or permeability, or changes in drilling conditions; any of which merits further investigation. A drift or gradual shift of the background gas trend toward higher values may indicate a slow gas influx into the mud column, which can eventually lead to a kick or blowout. When annotated on mud logs, background gas is usually abbreviated as BGG.

Alternate Form: background gas