Drilling S Well Services Trai rung
If this start up procedure is used on a floating rig with significant choke line friction losses, bottom hole pressure would increase by a pressure equal to the friction losses. This overpressure could cause damage to the formation. There are two methods that can be implemented to compensate for this over pressure (choke line friction losses).
As the pump is being brought up to kill speed, the casing pressure is being reduced by an amount equal to the choke line friction losses. With the pump at kill speed, drill pipe pressure will be at the required initial circulating pressure. This method can be difficult to implement.
Two The kill line can be used as a pressure monitor, assuming the fluid density in the kill line is the same as that in the well bore. As the pump in being brought up to kill speed, the kill line pressure at the surface is held constant. There will be no pressure losses in this line, as no mud is being circulated through it. See Figure 10. With the pump at kill speed drill pipe pressure will be at the required initial circulating pressure, casing pressure will have reduced by an amount equal to the friction losses.
If choke line friction losses are greater than shut in casing pressure, then the well will be subjected to an over pressure. The amount of over pressure will be the pressure difference between the CLFL and SICP. With the pump at kill speed the choke device will be fully open. In order to avoid or to minimise over pressures, consideration should be given to slower pump rates, if the pump speed is reduced, friction losses will reduce. In some cases consideration maybe given to circulating through both choke and kill lines. Using both lines increases the flow area thereby reducing the friction losses. Using this would normally require the addition of a dedicated pressure system on the stack back to surface as the kill line could not be used as a pressure monitor.
Choke line pressure losses will change as the kick is brought to surface. When the influx enters the choke line, friction losses will reduce, if the influx is gas the reduction will be significant, and will have to be compensated for, by choke manipulation. When the mud displacing the influx enters into the choke line, the friction loss returns. When kill mud enters the choke line friction loss will further increase. These changes will require choke adjustments and should be planned for.
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