Well flow regime with constant rate and constant delta pressure between wellbore and formation does not change in time:
(1) | q_t(t) = \rm const |
(2) | \Delta p(t) = | p_e(t) - p_{wf}(t) | = \Delta p = \rm const |
The exact solution of diffusion equation for PSS:
(3) | p_{wf}(t) = p_e(t) + J^{-1} q_t |
where
| constant productivity index | ||
| varying formation pressure at the external reservoir boundary |
and is shown as unit slope on PTA diagnostic log-log plot
During the PSS regime the formation pressure linearly declines