Since most blow-outs actually occur during trips, extra care must be taken during tripping. Before tripping out of the hole the following precautions are recommended:
(i) Circulate bottoms up to ensure that no influx has entered the wellbore
(ii) Make a flowcheck
(iii) Displace a heavy slug of mud down the drillstring. This is to prevent the string being pulled wet (i.e. mud still in the pipe when the connections are broken). The loss of this mud complicates the calculation of drillstring displacement.
It is important to check that an influx is not taking place and that the well is dead before pulling out of the hole since the well control operations become more complicated if a kick occurs during a trip. When the bit is off bottom it is not possible to circulate mud all the way to the bottom of the well. If this happens the pipe must be run back to bottom with the BOP’s closed. This procedure is known as stripping-in and will be discussed later.
As the pipe is tripped out of the hole the volume of mud added to the well, from the trip tank, should be monitored closely. To check for swabbing it is recommended that the drillbit is only pulled back to the previous casing shoe and then run back to bottom before pulling out of hole completely. This is known as a short trip. Early detection of swabbing or incomplete filling of the hole is very important.