Pressure at the top of the well, i.e., at its wellhead. It is measured by pressure gauges of the wellhead fittings. There are static and dynamic wellhead pressures.
Static wellhead pressure is gauged in the abandoned well and depends on reservoir pressure, well depth and density of the filling medium. Numerically it equals the difference between reservoir pressure and hydrostatic pressure of the liquid column from wellhead to reservoir.
Dynamic wellhead pressure is measured in the operating well and depends on the same parameters as the static pressure + on the well rate or on injection agent flow rate, as well as on the pressure in the pipeline near the well and on the pressure differential in shut-off elements of the wellhead fittings.