To effectively remove the cuttings while milling stuck pipe, it shall be ensured that:

- the yield point of the mud is increased as high as possible prior to commencing milling, taking advice from the fluids engineer.

- the flow is turbulent to prevent cuttings from blocking the annulus, (a minimum annular velocity of 115 ft/min is recommended at BOP’s riser)

Rotation shall start a minimum 1 ft above the fish. WOB and rpm shall be  adjusted to find the best milling rate while noting the rotating torque.

Confirm casing has been cut prior to commencing milling.

Milling shall commence with a low rpm ie., 70 rpm (increasing to a maximum of 140 rpm), dependent on the torque exhibited by the fish and guidelines from Milling Contractor.

A constant milling weight shall be maintained. The tool shall not be allowed to drill off.

In order to provide a high circulation rate all subs and auxiliary tools shall be full bore where possible. The tools shall be picked up, circulated and rotated at regular intervals.

BOP cavities shall be flushed upon completion of milling.

To avoid plugging, do not circulate through choke whilst milling.

Consider running (special) jars in compression for milling.

A minimum of two ditch magnets shall be used at the shale shakers or flowline. These magnets shall be cleaned regularly and the weight of steel recorded.

Trough/flowline screens and flowline design shall be considered for an extensive casing milling job.

When milling inside casing smooth tool joints shall be used and not newly hard faced ones. For deviated wells only, lubricants shall be added to the mud to reduce casing wear.

When milling washpipe, liner or casing, cuttings can accumulate inside the fish. When cuttings removal is not optimum inside the fish, the well shall be periodically cleaned out to bottom with a junk mill or the circulation reversed. Consider using the wellhead wing valve to flush out cuttings at high rate every two hours for 15 minutes.

The wear pattern on the mill will tell a lot about the performance of the mill in the hole. If the blades have a hook wear pattern, milling is progressing satisfactorily, If the blades are tapered it usually means too much weight was used which flared the top of the fish.

To minimise potential casing wear, optimise the BHA to keep the drill pipe in tension.

Reading Cuttings

Ideal cuttings are about 1/32in thick and up to 3in long.

Hair-like cuttings with low penetration rates mean the weight should be increased.

Fish-like cuttings when pilot milling or section milling (common when milling
H-40 pipe) indicate that the penetration rate can be improved by decreasing the weight and increasing the rotary table speed.

It is normal to expect cuttings finer than other grades of pipe when milling P-110 pipe.

Recording

While milling, a record shall be kept of the:

• progress made
• torque
• pump pressure
• weight on mill
• rpm
• circulation rate
• metal recovery from magnets and shakers.

Stabilisers

Under gauge stabilisers shall be run to centralise the mill, but the number of stabilisers shall be kept to a minimum to prevent excessive torque and bird nesting of cuttings. Normally, a stabiliser is run one or two collars above the mill.

Where stabilisers are placed inside casing during milling, consideration shall be given to the use of soft blade.

String Casing Protectors

Further casing protection can be achieved if required by the use of non-rotating drillpipe protectors, particularly across build up/drop off intervals. Follow general guidelines from the supplier.

General Operating Recommendations

The following table gives the recommended rpm’s and weight’s for the different types of milling tools used by Company.

Note:    Best ROP’s are made when a constant weight is applied.

Expected Milling Rates

The following table gives the expected milling rates for each milling tool milling different materials:

Factors Affecting Milling Rates

The factors affecting milling rates are:

• type and stability of fish
• weight on mill
• speed of mill
• hardness of cement when milling casing
• well cemented or uncemented casing.

Rubber

Rubber in association with a fish can cause trouble during milling operations. If rubber is reducing milling rates, the pump pressure should be reduced or the pump stopped for short intervals. The mill should be spudded frequently to help the mill get some bites on the rubber.

Rough Operation

Any mill works best when it runs smoothly. To do this, the following procedures should be used:

1. To stop bouncing or rough running, LOWER the table speed to about 50 rpm and REDUCE weight to about 3 000 lbs.

2. RUN for an hours or two, gradually increasing weight.

IF ..the mill runs smoothly and penetration rate is good THEN..CONTINUE at this speed.

IF ..the penetration rate is low THEN ..INCREASE speed.

IF ..the mill starts to run rough again THEN..BACK OFF on the speed until the mill runs smoothly again and HOLD that speed.

3. When milling casing, a rough running or bouncing mill - particularly a pilot mill or a section mill - often indicates that the casing is not well cemented and is wobbling. Reducing table speed should help to overcome this condition.

Centralisers and Scratchers

Centralisers and scratchers behind casing sometimes cause trouble in milling, although less difficulty is generally encountered if the casing is well centred. For best results, a mill with blades that extend completely over the centralisers (+/- 1.1/2in + casing collar OD) and scratchers shall be used.