The transient volume of investigation extends the concepts of radius and depth of investigation (for radial and linear flow, respectively). However, it is far more general, with much broader applicability. Unlike the specific cases of radius and depth of investigation, the volume of investigation is independent of flow regime, geometry, and model. Thus, the transient volume of investigation is applicable even when no recognizable flow regime is present.
The transient volume of investigation is based on fundamental properties common to all constant-terminal-rate solutions to the diffusivity equation. It is defined as the minimum reservoir volume necessary to provide sufficient pressure support to produce the observed rate of change of sand-face pressure as a function of time.
The volume of investigation is a paradigm-changing concept. It is the only meaningful reservoir characteristic that is both easily estimated and model independent. It provides a rigorous lower bound on fluid in place investigated as a function of time during a test, independent of flow regime, geometry, model, and even permeability heterogeneity or anisotropy.