1. n. [Geology, Shale Gas]

The gaseous phase present in a reservoir or other contained area. Gas may be found either dissolved in reservoir fluids or as free gas that tends to form a gas cap beneath the top seal on the reservoir trap. Both free gas and dissolved gas play important roles in the reservoir-drive mechanism.

See: gas cap,  solution gas

2. n. [Well Testing]

Gas that exists in the reservoir in the gaseous phase rather than in solution. As soon as formation pressure drops below the bubblepoint, gas is evolved. This is referred to as free gas while it is in the reservoir.

See: formation pressure