Normalised dimensionless difference between the actual drawdown wellbore pressure p_{wf}(t) and a model of a full-entry vertical well with homogeneous reservoir and non-damaged near-reservoir zone p^*_{wf}(t):
| (1) | S_G = \frac{2 \pi \sigma}{q_t} \left( p_{wf}(t) - p*_{wf}(t) \right) |