The radius of a around a producing or injecting well where reservoir fluid is forced moving by the well operation.
In other words, the area, where reservoir fluid is moving along the streamlines which either stem from injecting well or end up at producing well.
See also
Physics / Fluid Dynamics / Percolation
Petroleum Industry / Upstream / Subsurface E&P Disciplines / Field Study & Modelling / Drainage (fluid flow)