Hello guys! Today we're going to take a look at the coiled tubing milling operations. We're going to
discuss common coiled tubing operations, we'll take a look at the procedures as well as
chemicals used on coiled tubing operations and a typical configuration for coiled tubing
milling bottom hole assembly. So milling operations with coiled tubing are often used to
remove various obstructions from the wellbore. Milling operations are kind of very similar to cleanouts
however different downhole tools are used and procedures are slightly different as well.
So first let's take a look at the types of coiled tubing milling operations.
First, we have bridge plug milling. After the well is fractured bridge plugs are usually milled out
to allow the hydrocarbons to flow to the surface. Frac ports mill out - in some cases frac
ports are milled out after the well is completed to allow hydrocarbons to flow easily to the
surface while in other cases frac ports are left in the well and milled out later as the production
on the well starts to decrease because of the sand or wax buildups around the froc ports.
Cement milling and it's often performed on the wells to remove cement from the wellbore.
Sometimes cement gets inside of the casing during the improper cement jobs and
milling is also performed to fix the casing deformation.
Sometimes casing gets damaged during fracturing operations and its internal diameter
needs to be increased to allow the downhole tools to get past the deformation area that's where
mill outs with coiled tubing are often performed. Now let's take a look at the typical downhole tool
components of milling bottom hole assembly. First, we have a coiled tubing connector and it's used
to connect the bottom hole assembly to the end of the coiled tubing. The check valve is used to prevent the
backflow of liquids up the coil in the case of pressure loss due to the coil damage.
Jars are used when coiled tubing gets stuck at the bit to get it free. The hydraulic disconnect is used
in the case of emergency when the coiled tubing gets stuck at the bit and there is nothing that can be
done to get coil free and disconnect basically as the name implies disconnects and leaves the bottom
hole assembly in a well that allows coiled tubing to get free so it can be pulled to the surface
Vibration tools are used on extended reach operations to decrease the friction coefficient
between the coiled tubing and the casing and allow the coil to get deeper and well.
Mud motor is used to provide rotational force to the milling bit while converting the power of the
fluid flowing through it and lastly, we have the actual milling bit and it's used for
milling various obstructions in the wellbore. Now let's take a look at the sample
coiled tubing milling procedures. The first step is to function test the bottom hole assembly
to make sure it operates as expected and then you start running in the hole while circulating
the fluid at the minimum rate until coiled tubing enters the horizontal section of the well.
In the horizontal section usually the fluid pump rate is increased and the frequency
of the pulltests is increased as well. Pull tests are performed to ensure that there is no debris buildup
in the heel and the coiled tubing is able to pull free if needed. If there is an overpull due to the debris buildup, usually the pump rate is increased to the max and coil is stopped
the fluid is circulated until all the debris comes to the surface and after the well is
free that's when the milling operation continues. Then you continue running in a hole in the well
with coiled tubing to the first obstruction and when you get to the area of the first obstruction the pump
rate is increased and coiled tubing starts milling on this obstruction. To avoid creating large chunks of
debris while milling on obstructions in the well it's usually a good idea not to apply too much
weight on a drilling bit. While milling it's also important to pay attention to the motor stalls.
Motor stalls will create a pressure spike that can be visible on the surface. When
stalls are detected coil is usually pulled a few meters above the obstruction and pump rate is decreased
to avoid damaging the bottom hole assembly. As the pressure drops back to normal operations continues.
In a similar manner, the rest of the obstructions in the well are milled out. Some companies like to pump
gel pills after milling through each obstruction to help fluid lift the debris in the
vertical section of the well however circulating fluid at a high rate is
usually, a better way to make sure that the wellbore is free of all the debris and sand. The problem with
pumping too much gel is that it increases the viscosity of the fluid and in turn
decreases its turbulence makes it harder to remove the sand and debris 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. While milling obstructions in the well it is
also important to pay attention to the returns at the surface and if a very small amount of debris
is being returned it's usually a good idea to perform a few wiper trips to make sure that
there is no debris accumulation anywhere in the well and at the same time it's also good
idea to perform pull tests more frequently. After all the obstructions are milled out and
coiled tubing gets to the target depth coil is usually stopped and a few bottoms up are performed
with the fluid to make sure that the well is clean before coiled tubing is pulled back to the surface
and usually problem with pulling coiled tubing to the surface is that most
stuck in hole situations actually happen at the end of the operation when the
coil is pulled to a surface. That's why it's a good idea to go slow
and watch out for any debris coming to surface and also watch out for any overpulls
to make sure the coil is not going to get stuck in the well. If the well doesn't have enough pressure
to hold the column of water for example in case of under pressurized formation which is common
on 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 and one thing to keep in mind is
that most downhole tools cannot operate properly on the straight nitrogen and require some fluid. Lastly
I wanted to discuss the chemicals used in coiled tubing milling operations. Gels are pumped to increase the
viscosity of the fluid and improve its lifting capacity in the vertical section of the well.
Metal to metal friction reducers are pumped on the deeper wells where coiled tubing is struggling
getting to the target depth and basically they work by decreasing the friction between the coiled tubing
and the casing. Hydraulic friction reducers are used to decrease the pumping pressure.
H2S inhibitors and scavenges are pumped on the wells with h2s. Biocides are used to kill the bacteria in the
water and to protect the coil from microbial corrosion. Foamers and defoamers might be used
if the foam is pumped during the milling operation and there are a few more chemicals specialized
chemicals that are used but the ones we discussed today are the most commonly used. So guys thank you
for watching this video, hopefully, you learned something new and I'll see you guys later. Bye!