...
The choice of reference pressure is made by engineer depending on common sense.
The popular choice usual practise is to:
select
as the bottom=hole pressure at initial time moment. LaTeX Math Inline body --uriencoded--p_%7Bref%7D = p(t=0)
select
as the bottom=hole pressure at initial time moment. LaTeX Math Inline body --uriencoded--p_%7Bref%7D = p_e(t)
...