Well flow regime with constant rate and constant delta pressure between wellbore and formation does not change in time:
| q(t) = q = \rm const |
| \Delta p(t) = | p_e(t) - p_{wf}(t) | = \Delta p = \rm const |
The exact solution of diffusion equation for PSS:
| p_{wf}(t) = p_i + \frac{q_t}{ V_{\phi} \, c_t} \ t |
and is shown as unit slope on PTA diagnostic log-log plot
During the PSS regime the formation pressure linearly declines