Pilot mills are used for milling the following equipment/ material: adapters, liners, casing, swaged casing, stuck drillpipe, wash pipe.
The procedure for milling with a pilot mill is as follows:
1. Rotate the string above the fish to establish the free rotating torque.
2. While rotating approximately 30 rpm, slowly enter the fish. Torque shall increase when the mill blades start milling on the fish.
3. Add approximately 2 000 lbs weight and stop rotating suddenly. A gradual slowdown or spin indicates that the mill has entered the fish and is properly aligned. If there is a stinger below the mill, stopping rotation suddenly may cause backing off below the mill.
Note: Determine the optimum weight and rpm as the job progresses.
4. After establishing the penetration rate, the string shall not be picked up again. Continue until the mill is worn out.
General Guidelines
The following general guidelines should be used when milling with a pilot mill:
In selecting a pilot mill, the blade OD should be about 6mm larger than the OD of the tooljoint or coupling of the fish to be milled. The pilot OD should be the same as the drift diameter of the fish.
The best speed and weight to run a pilot mill must be determined for each job. Conditions may change from one pilot milling job to the next in the same well so that different speeds and weights may be required at different times. In the absence of experience, an initial speed of between 100 and 150 rpm and 2 000 - 7 000 lbs of weight should be used for best results.
When milling liner or casing that has been gun perforated, damaged with a spear, or collapsed, use about 70 rpm with the order of 2 000 lbs or less.
If when milling swaged casing a sudden drop off in milling rate is experienced, the trouble may be a loose ring of steel formed at a joint or weld that is turning with the pilot mill. The pilot mill should be spudded gently to break up the ring and help position it for milling.
If cutting stops altogether (after trying the method above) when milling wash pipe, casing, or liner and there is no appreciable increase in torque, there is a good chance the fish is turning. If this is the case, pull the mill and make a run with a spear.
Considerations When Milling Liner, Hangers and Adapters
On most liner milling jobs, a pilot mill is used to mill the liner hanger, or adapter, and then the liner. In some cases the liner hanger, or adapter, is milled with a junk mill, and then the liner is milled with a pilot mill. This latter method is preferred if there is hard cement behind the liner or if the hanger has numerous bow-springs, slips, etc. A pilot mill with blades that will cut just over the pipe couplings should be selected and a minimum of cement will be encountered.
Pilot Mill Ideal for Wash Pipe
The pilot mill is the most efficient tool for milling stuck wash pipe. If drillpipe or drill collars are inside the wash pipe, however, they shall first be milled with a junk mill or smaller pilot mill.
Drillpipe and Drill Collars
Drillpipe and drill collars are sometimes milled with pilot mills if the ID is open. If the drillpipe is cemented inside casing, particularly in deviated holes, the pipe probably is lying to one side with its centre eccentric to the casing. This often makes the job extremely difficult for a pilot mill. Under these conditions, a full gauge junk mill is recommended. A pilot mill will do a reasonable job on drill collars provided the cuttings can be removed as the milling progresses. If cuttings tend to fall into the ID and plug it, then a junk mill must be used.
Casing
If casing above has been cut and pulled, then the lower remaining casing can be milled with a pilot mill in the same manner that washpipe is milled.