1. n. [Well Testing]

The portion of the volume of a reservoir drained by a well. In a reservoir drained by multiple wells, the volume ultimately drained by any given well is proportional to that well's production rate: V_i = V_t x q_i/q_t, where V_i is the drainage volume of Well i, V_t is the entire drainage volume of the reservoir, q_i is the production rate from Well i, and q_t is the total production rate from the reservoir.

See related terms: drainage area,  well production rate