Fishing Technology (6)

Using Magnets for Fishing

Purpose

Fishing magnets are primarily used to pick up bit cones, but they can pick
up all types of small objects with magnetic properties.

Guidelines for using magnets

Magnets should be run on pipe instead of wire line so that you can circulate
and rotate. Fishing magnets run on wire line are usually made inside the
casing with clear fluid in the hole. There are adapters available to run
magnets on a wire line.
Note: Most, but not all tungsten carbide bit inserts are non-magnetic.

Checking magnet capacity

Check to see if the magnet is charged properly by attaching a 20-lb.
sledgehammer to the face of it. If the magnet will support the weight of the
sledgehammer, it will likely hold any small pieces of junk in the well bore.
The lifting capacity of the magnet is a function of its size, area, and charge.

Verifying size

The OD of the magnet should be ¼" to ½" less than the hole size.

Figure 2-1:
Fishing Magnet with a Cripple Type Guide Attached

Using a Junk Mill

Description

Junk mills mill up and break up large junk into smaller pieces that can be
recovered in a boot basket. Junk mills provide the surest method for
eliminating junk in the hole.

Types of mills

The following are examples of types of mills that are used for eliminating
junk:
1- insert type mill
2- crushed tungsten carbide mill
3- skirted flat bottom or concave type mill
4- blade type mill
5- cone buster mill
6- cement mill.

 

Figure 2-2: Cone Buster and P Mill

Figure 2-3:  Packer or Cement Mill

Milling guidelines

Some important guidelines for milling with junk mills include:
1- Provide adequate circulation to flush the cuttings and cool the mill.
2- A boot basket must be run directly above the mill to catch the metal
cuttings.
3- Always mill by torque and not by weight. If the mill is producing torque
and making progress, then the weight applied to the mill is sufficient.
Most people run too much weight on mills. A standard rule of thumb is
not to exceed 1,000 pounds per OD inch of the mill.
Note: Mills with stabilizer pads should be used inside the casing.
IF you are milling pipe that causes the drill string to bounce and vibrate,
THEN consider using a Bowen Shock Sub to cut down on vibration.

Calculating milling rates

The milling rates for the carbide mills are specified by the manufacturer in
Surface Feet Per Minute (SFPM). The surface feet per minute must be
converted to revolutions per minute (RPM) because the operator does not
have a way to measure surface feet per minute.
The rpm may be derived from the SFPM by multiplying the mill diameter
by the milling RPM. This result is then multiplied by a constant, 0.262. For
example, a certain mill’s diameter is 4.5" and the milling rate in RPM is
120. You would calculate the SFPM in this way:
4.5 x 120 x 0.262 = 142 SFPM.
Table 2-2 lists some standard milling rates. Other rates may be computed
by converting the milling rates in revolutions per minute to surface feet per
minute.

Collecting the junk

Mills and boot baskets go hand-in-hand. Boot baskets are always run with
mills because they collect the pieces of the junk that the mill grinds up.