Composite Plug Milling: 4.5” Casing

In my previous milling article, here, I highlighted the two most popular choices for milling composite plugs in 5.5” Casing. The two choices are really centered around what type of roller cone bit, Mill Tooth or TCI, to choose. In my experience in 5.5” Casing these are really the only two options. In 4.5” casing the decision is a little more difficult. Roller Cone bits for 4.5” 13.5#/ft (114.3 mm 20 kg/m) have an OD of only 3.75” (95.25 mm). They perform well however due to their smaller size the bearings are smaller, the protection for the bearings is smaller and they generally do not last as long as their counterparts for 5.5” casing. Due to this fact many operators choose to run carbide mills or PDC bits.

Roller Cone Bits

There are two types of roller cone bits, Mill Tooth and TCI. The mill tooth bits have teeth forged directly into the cone structure that are then hard faced for strength. These bits mill through a gouging action, the teeth penetrate what they’re milling, grab, stretch, and then pull the material apart. The TCI bits have tungsten carbide inserts that act as teeth. These bits are characterized by a crushing action. The TCI are not as sharp as the mill teeth but are very hard and crush what they’re milling.

The typical failure mode for a roller cone bit is a bearing that is compromised by sand or debris. During milling this debris wears down the bearing until it eventually fails. To extend the life of a roller cone bit manufacturers work to ensure the gauge protection of the cone stays intact. Providing this protection ensures the gap between the cone arm and cone is kept from direct contact with the sand/debris in the well. If this protection fails or wears away, as the milling continues the sand will be forced between the arm and the cone causing the bearing seals to fail and ultimately the cone to fail. This is more likely to happen in 4.5” because everything on the bit is smaller and weaker.

In my experience the hybrid bits that include TCI teeth and mill teeth perform the best in the smaller size. Part of this may be that the TCI teeth on the outer row are less prone to wear and keep the gap between the cone and arm away from the sand on the bottom of the well. One customer communicated they had the best results with the Varel Slipstream roller cone bit (a hybrid roller cone bit), saying that it was consistently milling 55 plugs in one run without issues. It had also done 73 RzrFRAC plugs in one run as well.

Carbide Mills

Carbide mills are a steel body that has carbide welded, or brazed, onto its surface. This carbide provides a scraping action for milling away the target. There are several different options for designing the mill for your application including the type of carbide, concave/convex/flat face, mill taper, reverse/standard clutch, mill length, flow course size, etc. Each one of these options can be optimized based on the plug being milled, the type of milling (coil/stick pipe), the preference of the operator. 

A couple issues I’ve experienced in milling composite plugs with carbide mills is that no two mills are the same. The welding of the carbide is extremely important and the quality of this weld depends on the skill of the welder. Also, typically the outside diameter of the mill is not hardfaced and can wear down throughout the course of the milling operation. This can lead to the later plugs being milled with an undergauged mill, resulting in larger cuttings.

The picture to the right shows a mill used for milling RzrFRAC Composite plug. This mill used a reverse clutch with custom carbide and performed quite well. As you can see the gauge OD of the bit was dressed with carbide as well and was full OD after the mill up. This well had 57 RzrFRAC plugs that were milled out using this mill in an average of 5 minutes each.  

PDC

The last option is a PDC mill for milling in smaller casing sizes. PDC bits are characterized by a matrix body with diamond inserts. Their milling action is like a carbide scraping action, but the cutters are larger and they make larger cuts.

My experience with PDC bits has been mixed. If they don’t work it as well is because they tend to core through the plug leaving larger pieces up against the casing. In one case we worked with the operator who changed the aggressiveness of the cutters and reduced the coring/cutting sizes. 

We recently had success in 4.5” 13.5# casing using a 5 bladed PDC bit shown here along with the cuttings. The milling and bit selection was performed by Conquest Completion Services.

For your mill selection in smaller casing sizes lean on your milling provider to discuss what has worked for them. Typically, they have experience with your plug in that casing size and will know what combination of mill type, design, and milling parameters will bring you the most success.

In any casing size the RzrFRAC has been optimized to bring you value through the entire Plug & Perf operation and certainly will provide more value by reducing the amount of debris, milling speed, and efficiency.