Total open pore volume around the well where reservoir flow is initiated by production or injection from a given well or group of wells.

There are three major types of the drainage volumes:

Dynamic drainage volume of a well which is formed dynamically depending on the nearby well production and injection which can be formed in case a given well is working alone on the whole field (no interference with other wells) which is formed dynamically by production and injection of all wells

For homogeneous reservoir it can be estimated as:

 V_e = A_e \, h_e \, \phi_e

where