A stuck string inside tubing is a problem with a narrow set of solutions. The fish cannot be pulled, cannot be washed over without clearance, and cannot be left because it blocks the well. A hydraulic inside thru-tubing cutter works from the inside: it passes through a small restriction, opens blades against the tubing wall, and cuts the fish so the upper part can be recovered. The tool is simple in principle and unforgiving in practice, because the cut has to be clean enough to fish. On the rig, the tool is chosen after the fish is measured, not before, because a cutter that will not pass the restriction wastes a trip.
When a Thru-Tubing Cutter Is the Right Tool
The cutter is used when the fish is tubing or a tubular component that can be severed from the inside, and when the string above it can be rotated. It runs on coiled tubing with a mud motor or on a threaded pipe string, which makes it usable on a workover rig without a coiled tubing unit. The choice between coil and pipe depends on what is on location, and the cutting procedure is the same once the tool is at depth. It is not a substitute for a fishing tool when the fish is a tool body that will not pass the blades, and it is not a substitute for a casing cutter when the cut is outside the tubing.
Blade Actuation at 150 to 250 Psi
The blades are opened by differential pressure across the tool, and the tool needs roughly 150 to 250 psi to extend them against the wall. Pump pressure drives an internal piston down, the piston pushes the cutter head out, and the blades, tipped with carbide inserts, contact the tubing. Flow is routed over the blades to clear cuttings, which is why the pump rate is held steady through the cut rather than pulsed. The crew holds the pump steady and watches the pressure, because a pressure drop mid-cut usually means a blade has broken rather than that the cut is finished.
Sizes and Tubing Ranges
The range covers 43 mm (1.69 inches) outside diameter for 2-3/8 to 2-7/8 inch tubing, up to 112 mm (4.41 inches) for 5-1/2 inch tubing. Connections include 1 inch AMMT, 1.5 inches AMMT, 2-3/8 inches TBG and 2-3/8 or 2-7/8 inches REG. Flow rate is 10 to 20 liters per second and rotation is 20 to 40 rpm, rising to 80 to 130 rpm on the largest body where the cut diameter is greater. The tool body has to pass the smallest restriction above the fish, and the blade reach has to cover the wall of the tubing being cut, so the two limits are checked against each other.
Cutting Parameters on the Rig
The cut is made with the pump pressure held at the value that keeps the blades extended, and the string rotated at a steady speed. Too little pressure and the blades skip; too much and the tool stalls or the blades chip. The crew watches the torque and the returns: a steady torque with fine cuttings means the cut is progressing, while a sudden drop in torque usually means the fish has parted and the string has to be picked up carefully. The returns are the other indicator: fine steel cuttings mean the blades are cutting, while rubber or scale means the tool is working on something else.
Confirm the tool outside diameter passes every restriction above the fish.
Hold pressure within the blade range and rotate at a constant speed.
Watch the returns for metal cuttings before assuming the cut is complete.
Thru-Tubing Cutter Recovery After the Cut
When the cut is finished, the pump is stopped, the differential pressure falls, and a reset spring pulls the blades back into the body. That retraction is what allows the tool to be pulled out of the hole without hanging up on the cut edge. If the blades do not retract, the tool is worked gently while circulating, because forcing it is how a successful cut becomes a second fish. Temperature rating is 120 degrees C (250 degrees F), which covers most workover conditions. If the blades hang on the cut edge, the crew works the string with circulation rather than pulling hard, because a jarring load on a retracted blade can bend the head.
A thru-tubing cutter is a controlled cut in a place where nothing else fits. Size it to the restriction, hold the pressure in range, and the fish comes out in one piece instead of becoming two problems.
For more information, please contact China Vigor at info@vigorpetroleum.com or call +0086 29 81161513.
