Composite Plug Milling: Bit Selection for 5.5" Completions
The use of composite frac plugs in North American horizontal completions has grown exponentially over the past year. Not only are customers drilling more wells, they’re breaking each well into more zones. And, since the downturn, operators in the US have largely shifted their completion methodology to plug & perf due to a number of reasons. Increasing the stage count in wells has brought heightened focus on the milling bottom hole assembly (BHA) selection for removing the composite plugs from the well. One of the major choices when designing the BHA for a milling operation is the bit type. In my experience, the bit choice can greatly affect the speed and efficiency of the milling operation. With efficiency being the size of the cuttings experienced, (high efficiency = small cuttings). There are three bit types used to mill plugs in North America; roller cone, polycrystalline diamond cutters (PDC), and a junk mill. The most common choice is the roller cone bit. For most of the 5.5" completions in the US, a roller cone bit is used to mill up the plugs. Even when selecting a roller cone bit, there are a few variants that can have a significant impact on the bit performance.
Roller Cone Bits
Roller cone bits typically used for milling composite plugs are made up of three cones that are connected to the arms of the bit with a sealed bearing. The cones are typically made of a forged steel and contain teeth for performing the milling action. There are two types of teeth on roller cone bits, mill teeth and tungsten carbide inserts (TCI).
Mill Tooth Bit
The structure of the teeth on the cones are forged into the body of the cone on a mill tooth bit. The cone and teeth are then coated with hard facing for additional wear resistance. The integral mill teeth are sharper than that of the TCI. Their cutting action is characterized by gouging. This action of the tooth is to penetrate, stretch, and then pulls what it is milling loose. This is very effective for composite and cast-iron material, but isn’t as effective on very hard materials like large hardened buttons. Larger hardened buttons tend to wear down the outside teeth of a mill tooth bit, which could compromise the bearing and lead to a bit failure.
If there is no concern with the materials from the composite frac plug wearing down the mill teeth on the cones, the mill tooth bit is the most efficient type of roller cone bit to choose for milling a composite plug.
TCI Bits
TCI bits have cones that are forged and have rows of holes milled in them where the teeth will be. The tungsten carbide inserts are then press fit into the holes of the cone locking them in place. The inserts are typically more rounded (less sharp) than the mill teeth and the cutting action is characterized by crushing. Though these inserts are not as efficient as mill teeth, their extremely hard nature makes them more resistant to wear when encountering larger hardened inserts.
As more composite plugs with larger hardened inserts came on the market, the drill bit manufactures have innovated by developing bits that have TCI on the outer row of the cones and mill teeth on the inner rows. The goal of this design is to reduce the wear on the outside edges of the cone, while getting the efficiency of the mill teeth from the inside rows.
These hybrid bits are more effective than a pure TCI bit, but still not as efficient as the mill tooth bit. If you are running a composite plug with larger pieces of hardened material the TCI or Hybrid TCI is the right choice, this will insure that the bearings remain competent during the entire milling operation.
RzrFRAC enables the Most Efficient Bit Selection
At Rubicon we recommend choosing a mill tooth bit for milling the RzrFRAC Composite Frac Plug. The RzrFRAC has proven to mill up quickly and efficiently without causing any damage to the milling BHA including the bit. We recently had a customer mill up 76 RzrFRAC plugs and the bit came back looking “brand new”. There was very little scarring on the side of the bit and the bearings were still tight. The 100% composite construction and small size/amount of buttons enable the milling BHA to perform its job without much wear at all.
