Well control operations are performed at reduced pump speeds or slow circulation rate in order to:
- Allow weighting up and degassing of the drilling fluid
- Reduce loads on surface equipment
- Increase reaction time for correct choke adjustments.
Operation of the pump at a pressure too close to the setting of its relief valve is dangerous. If the relief valve opens, drilling fluid will flow back through the drill pipe and standpipe manifold.
Circulation pressures at the reduced pump speeds must be known in advance of a well-killing operation. Before starting to drill with a new bit and at the beginning of each shift, the driller should therefore observe pump pressures at selected slow circulation rates and record the results in the daily drilling report and kick control worksheet (check also: Kick warning signs).
Circulation pressures may differ considerably when using a different equipment hook-up (e.g. circulating head/chicksans vs. kelly or top drive TDS top drive in drilling rig). SCR should be taken with the equipment hook-up which is planned, or most likely to be used during the well killing operation.
Slow circulation rates and pressures can be plotted as a straight line on a graph using log-log paper. Appropriate mud pump pressures for any pump rate can then be estimated using this graph, provided the drilling fluid properties and drill string components configuration do not change.
When using different drilling fluid properties, the slow circulation rates should be repeated. If necessary, provided the rheology has not changed too greatly, pressure losses ( check also: PRESSURE LOSS CALCULATIONS IN DRILLING) may be approximated using the original figures as follows:
P2 = P1 x ρ2 / ρ1
P1 = circulating pressure with original drilling fluid
P2 = circulating pressure with new drilling fluid
ρ1 = original drilling fluid gradient
ρ2 = new drilling fluid gradient
Ref: Shell Well Control Manual