How Coiled Tubing Milling Operations Are Performed?

Milling with coiled tubing is often used to remove various obstructions from the well.

Milling operations are similar to cleanouts but require a different type of bottom hole assembly (BHA) and slightly different procedures.

Types of Coiled Tubing Milling Operations

Bridge Plug Milling – after the well is fractured, bridge plugs are usually milled out to allow hydrocarbons to flow to the surface.

Frac Port Millout – in some cases frac ports are milled out to allow hydrocarbons to flow more easily to the surface. In other cases, frac ports are left in the well and milled out later as the well gets older and production decreases due to sand and debris buildups around the frac ports.

Cement Milling – is often performed to remove the cement from the wellbore. Sometimes cement gets inside the casing because of unsuccessful cement jobs.

Casing Deformation Milling – sometimes casing gets damaged during fracturing operations and its internal diameter needs to be increased to allow downhole tools to get past the deformation area.

Related: Top 5 Chemicals Used for Coiled Tubing Operations

Typical Milling Downhole Tool Components

• Coiled Tubing Connector – used to connect BHA to the end of the coiled tubing.
• Check Valve – used for preventing backflow of liquids up the coil in the case of pressure loss due to coil damage.
• Jars – are used when coiled tubing gets stuck at the bit.
• Hydraulic Disconnect – used in case of emergency when the coil tubing is stuck downhole at the bit.
• Vibrational Tool – used in extended-reach operations to decrease the friction coefficient between the coiled tubing and the casing and allow the coil to get deeper in the well.
• Mud Motor – used to provide rotational force to the milling bit by converting the power of fluid flowing through it.
• Milling Bit – used for milling various obstructions in the wellbore.

Related: 18 Most Common Coiled Tubing Applications

Example of Basic Coiled Tubing Milling Procedures

• Function test the BHA to ensure it works as expected.
• Start running in the hole while circulating fluid at minimum rates until coiled tubing enters the horizontal section of the well.
• In the horizontal section increase the pump rate and frequency of pull tests. Pull tests are performed to ensure that there are no debris buildups in the heel and coiled tubing can pull free if needed.
• If there is an overpull due to debris buildup, increase the pump rate to the max and wait for the debris to come to the surface before continuing with the milling operation.
• Continue running in the well to the first obstruction. Increase the pump rate and start milling on obstruction. To avoid creating large chunks of debris it is a good idea not to apply too much weight on a drilling bit.
• While milling it is also important to watch out for motor stalls. Motor stalls will create pressure spikes that can be visible at the surface. When stalls are detected, the coil is usually pulled a few meters above the obstruction and the pump rate is decreased to avoid damaging the BHA. After the pressure drops back to normal, the operation is continued.
• Similarly, the rest of the obstructions are milled out in the well.
• Some companies like to pump gel pills after milling through each obstruction to help the fluid lift debris in the vertical section of the well. However, circulating fluid at a high rate is usually a better way to ensure that the wellbore is free. The problem with too much gel is that it increases the viscosity of the fluid and in turn decreases its turbulence and makes it harder to remove the sand from the horizontal section of the well.
• Often wiper trips are performed after milling a certain number of obstructions to ensure that the wellbore is clean from any debris or sand.
• When milling obstructions in the well it is very important to pay attention to the returns at the surface. If a very small amount of debris is being produced, it might be worth performing extra wipers to ensure that there is no debris accumulation behind the BHA that can lock up the coiled tubing on the way out of the hole
• After all the obstructions are milled out and coiled tubing gets to the target depth, the coil is usually stopped and a couple of bottom ups are performed to ensure that the well is clean before the coil is pulled to the surface.
• Most stuck-in-hole situations happen when the coil is getting pulled to the surface at the end of the job.
• That’s why it is a good idea to go slow to ensure that all the debris and sand are brought to the surface instead of getting accumulated in the heel or other sections of the well.

Related: What Are Bridge Plugs In Oil And Gas?

Milling on Low-Pressure Wells

If the well doesn’t have enough pressure to hold the column of water (under pressurized formation) which is common in older fields, nitrogen can be used to lower the hydrostatic pressure in the well.

Usually, a mix of nitrogen and fluid is used for milling on these low-pressure wells.

Most downhole motors cannot properly operate on straight nitrogen and require fluid.

Chemicals Used on Coiled Tubing Milling Operations

Gel – is used to increase the viscosity of the fluid to improve its lifting capacity in the vertical section of the well.

Metal to metal friction reducer – pumped on the deeper wells where coiled tubing is struggling with getting to the target depth.

Hydraulic friction reducers – are used to decrease pumping pressures.

H2S inhibitors and scavengers – pumped on sour wells.

Biocide – pumped to kill the bacteria in the water and protect the coil.

Foamers and defoamers – pumped during nitrogen foam cleanouts.

Surfactants – sometimes pumped to improve fluid’s sand sweeping efficiency.

Read next: How Coiled Tubing Wellbore Cleanout Operations Are Performed?