Wellbore sandface pressure after a given well is shut-in for t_e hours
(1) | p_e = p_{wf}(t_e) \bigg|_{q = 0} |
This definition is based on the practical observation of wellbore pressure in shut-in wells for well intervention purposes.
It is the simplest and the most popular definition of formation pressure and is widely used in all upstream industry applications.
The definition of shut-in time t_e is specific to each field or sometimes field area depending on the rock properties and the past well intervention experience.
Some conventions are to pick t_e at the end of the radial flow, as the most common number from the past pressure tests, which makes this definition close (but still not equal) to the (Drain-boundary formation pressure @ model:1)