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Taper Mills

Taper mills can be used for reaming out collapsed or stuck pipe such as casing and liners. They can also be used to clean up permanent whipstock windows, to mill through jagged or split guide shoes and to enlarge restrictions through retainers and adapters.

Procedures

The following procedure shall be used when milling with taper mills:

1. ROTATE while entering the fish.

Notes:

i. The weight on the mill shall be kept low (approximately 2.0 tonnes).

ii.         Do not torque up exceeding 6 000 ft.lbs and 75 rpm.

2. After the restriction has been enlarged, the rpm can be increased to 80 - 100 rpm while reciprocating the mill through the interval several times.

Optimising Taper Mills

  • Taper milling table speeds are governed by torque encountered, but should seldom exceed 75 rpm
  • Never start a taper mill on the fish. The fish should be entered with a rotary speed of 75 rpm or less.
  • Use less weight when running a taper mill than a junk mill or pilot mill. After entering the fish, the weight should be increased slowly to 1 000 - 2 000 lbs. Torquing up should be monitored.
  • When milling with a taper mill, the diameter of the taper mill shall be equal to the desired enlargement required.

Whipstock Windows

A taper mil can be used to clean a whipstock window using the following guidelines:

1. USE a taper mill of the same diameter as the largest mill used to mill the window or slightly larger than the bit to be used.

2. RUN the taper mill in the hole to within 5 ft of the top of the window.

3. START the rotary table and ROTATE at approximately 40 rpm down the face of the whipstock.

4. KEEP the weight under 1 000 lbs as excessive weight may cause the taper mill to slip out of the window prematurely.

5. ROTATE slowly with light weight down the full length of the face of the whipstock DO NOT attempt to make hole with this tool.

6. To clean up all rough edges REPEAT the above procedure several times until the mill runs smoothly for the full length of the whipstock.

Reaming Out Collapsed Casing

The following guidelines should be used when reaming out collapsed casing:

  • Determine the approximate diameter of an impression block or bit that will pass through the collapsed interval. Do not use a taper mill if collapse has passed centre.
  • Use a taper mill about 6mm larger than the minimum ID of the collapsed section and mill out the collapsed interval by stages. In other words, if the collapse is great, use several different sizes of mills to bring the ID of the pipe to full gauge. This will minimise any tendency to sidetrack.
  • A string taper mill can be used if there is any danger that sidetracking may occur. The length and diameter of the string taper mill will be governed by the casing conditions. If used, the action is pure reaming.
  • Begin milling at a table speed of about 50 rpm.
  • The milling weight is governed by the torque encountered. In most cases, milling weights of around 1 000 - 3 000 lbs are used.
  • Where the pipe is collapsed greatly, the lower portion of the collapsed interval may act as a whipstock. The taper mill, in this case, may cut through the upper portion of the collapsed interval and be deflected into the formation by the lower section of the damaged casing. In some cases of extreme collapsed pipe, it is better to run a stabilised, rigid hookup with a junk mill and use very light weight with a table speed of about 150 rpm to mill out the collapsed portion and enter the undamaged casing below. A string taper can be used, as previously mentioned if there is an opening large enough to get a tubing or “macaroni” lead through the opening.

Enlarging Restrictions Through Retainers and Adapters

The following guidelines should be used when enlarging restriction through retainers and adapters with taper mills:

  • Use a taper mill with a diameter equal to the desired enlargement (usually the drift ID of the casing).
  • Mill at about 70 rpm and with weight from 2 000 - 7 000 lbs. Both the speed and the weight should be governed by the torque. If the torque is high, the speed and weight should be reduced until the mill turns freely.
  • After milling through the restriction, increase speed to between 80 and 100 rpm and rotate up and down through the interval several times until it is smooth and free.