The following precautions should be observed to ensure that primary control is maintained, whilst tripping, circulating bottoms up after a trip, drilling and circulating on bottom.
Records:
Accurate records of all operations shall be maintained, particularly of any unusual occurrences. These shall describe all flowline shows. The duration of the occurrence, mud return gradients, chloride content, etc. shall be also noted.
Hole:
The hole shall be kept full at all times.
Volumes:
When tripping the drillstring or running the casing, the trip tank must be used for accurate measurement of mud volume changes, to insure that the mud mud volume is equal to the appropriate pipe displacement. Pulling mud-filled pipe should be avoided if possible. A trip sheet should be kept of trip tank volumes against pipe displacements of each stand, to identify any discrepancies (e.g. gains or losses) as soon as possible.
Flow check:
A flow check shall be made before pulling out, in the last casing shoe, and before starting to pull the drill collars. For exploration wells it is recommended to make a short check trip prior to pulling out completely to verify that the overbalance is sufficient to prevent swabbing the well.
Running speeds:
Pipe pulling or running speeds should be controlled to limit pressure surges which otherwise could cause a kick, losses, or formation damage. Pressure surges will depend on the hole dimensions, string configuration, mud characteristics and pipe velocity. For normal hole conditions and pulling speeds, surge pressures are of a similar order of magnitude to annular friction losses whilst circulating.
Swabbing:
In order to avoid swabbing effects, the drillstring shall be circulated whilst pulling through critical open hole sections. In circumstances where it is suspected that swabbing may have occurred, the pipe shall be run back to bottom and bottoms circulated up and the mud conditioned if necessary. If the well is flowing after swabbing, it should be closed in and be killed at the prevailing bit depth. If the distance is too far off bottom, then the pipe should be stripped in. Do not attempt to run back to bottom with an open flowing well.
Time out of hole:
The time out of the hole should be kept to the minimum, consistent with operational requirements and with regard to the hole stability. The hole should be kept full and monitored continuously.
Repairs and servicing:
If rig equipment has to be repaired, the pipe should be run or pulled to the shoe of the last casing string before repairs begin. In well servicing operations, the pipe should be run to sufficient depth to ensure that the well can be controlled.
Circulation at intermediate stages:
Except for specific reasons (e.g. when retrieving long and tight formation cores), circulation should preferably not be carried out at intermediate stages when tripping drillpipe or running casing into the hole. A flow check is preferred.
Testing of casing float equipment:
Immediately after making up the shoe track at the drillfloor the casing float equipment should be function tested. This should be done prior to running additional casing.
The recommended procedure to function test the shoe track is as follows:
- fill the shoe track with mud;
- pull back the shoe track high enough to drain the shoe track;
- lower the shoe track into the hole again to check if the float equipment is properly closed.
Mud logging:
If a mud logging unit is available, it shall be ensured that all sensors and instruments are fully operational and are fit for the location installed. It shall also be ensured that the mud logging engineer is in communication with the driller and has clear instructions concerning conditions and parameters which are to be notified directly.
Mud tank transfer:
Mud shall not be transferred from the reserve tanks to the active mud system or vice versa, unless the driller and mud logger (if present) has been informed with details of volumes and time intervals.
Swabbed-in gas influx:
The effects of a swabbed-in gas influx being circulated up in an open annulus should be understood. The first indication of expanding gas or a kicking well is an increase in mud returns at the flowline.
Pit level alarms:
It shall be ensured that the high and low level alarms on the pit level recorder (and flowline sensor) are operative and set as closely together as practicable to ensure that a maximum influx volume of 1 m3 (6 bbl) is detected prior to closing in the well.
Mud volume changes:
The driller and mud logger shall be advised of any proposed change in the volume of active mud. He should be informed as soon as such a change in volume has been completed.
Drilling break, flow check:
A drilling break is a distinct change in penetration rate which not always appear as a sharp change, but may be a gradual increase over a fairly large interval. A decrease in penetration rate can also be seen as a drilling break, depending on the type of formation which is being drilled (e.g. drilling from salt into carbonate).
When a drilling break is encountered, a flow check shall be made to confirm that formation fluids are not entering the well bore. The duration of the flow check depends on several factors and should be defined locally.
Following a negative flow check (no flow observed), normal drilling may be resumed.
If the flow check is not conclusive, the hole should be circulated bottoms-up before deciding to drill ahead.