Tubing-End Locator

Key Takeaways

• The most common application of the tubing-end locator is as a reference depth-setting tool before running plug-setting or perforating assemblies on slickline or coiled tubing in wells where the measured depth from surface cannot be trusted due to accumulated depth uncertainty from cable stretch, sheave calibration errors, or wellbore trajectory in deviated wells: by first locating the tubing shoe (whose depth is known from the tubing tally record of pipe joints run into the hole) and confirming that the tool depth counter reading at the tubing shoe matches the tubing tally depth, the operator can calculate a depth correction (pick-up or lay-down adjustment) that brings the tool string depth counter into agreement with the tubing tally, and all subsequent depth references in the same run are then corrected by the same amount; this practice of using the tubing-end locator as a depth calibration reference is particularly important in high-angle or horizontal wells where the cable tension and tool weight vary significantly with inclination and the standard cable stretch correction formulas designed for vertical wells give poor results.
• Mechanical tubing-end locators (also called fishing bell guides or shoe finders in some service company terminologies) consist of a tool body slightly smaller in OD than the tubing drift diameter, with a set of spring-loaded fingers or a tapered nose that contacts the tubing end when the tool is lowered past the tubing shoe from inside the tubing: as the tool passes the tubing shoe from above to below, the spring fingers, which were held inward by the tubing bore, spring outward to their natural diameter (larger than the tubing ID) when the constraint of the tubing wall is removed, and the sudden change in the tool's effective outer envelope creates a detectable jerk or slack in the slickline that the surface operator feels and records as the tubing-shoe depth; more sophisticated mechanical designs use a weight bar assembly where the additional tool weight engaging below the tubing shoe creates a measurable decrease in surface line tension (pick-up weight) that is recorded by the surface weight indicator and interpreted as the tubing-shoe depth; these mechanical designs are inexpensive and reliable in clean tubing but can give erroneous signals in tubing with heavy paraffin deposits, scale, or mechanical deformation (flattened OD from squeeze or collapse) that may partially arrest the spring finger expansion before the tool is fully below the tubing shoe.
• Electromagnetic tubing-end locators use a casing collar locator (CCL) type sensor adapted for the specific task of detecting the tubing shoe, operating on the principle that the magnetic flux linkage through the sensor changes when the tool transitions from a region where both tubing and casing wall surround the tool (two concentric steel cylinders creating a high-flux-path environment) to a region where only casing surrounds the tool (one steel cylinder, lower flux): the transition at the tubing shoe creates a distinctive signal in the CCL output that appears as a negative deflection (loss of flux) as the tool descends below the tubing end into the casing-only environment; the electromagnetic signal is recorded on the surface memory gauge or the electric line surface acquisition system alongside the tool depth counter, creating a permanent depth record that is compared to the tubing tally; electromagnetic tubing-end locators have the advantage of operating without mechanical contact, making them suitable for use in wells with distorted, heavily scaled, or paraffin-plugged tubing ends where mechanical designs may give erratic results, but they require careful baseline calibration because the magnitude of the electromagnetic signal at the tubing shoe depends on the tubing-casing annular geometry (annulus size and tubing wall thickness), which varies between well configurations.
• Slickline operation with a tubing-end locator follows a standard field procedure: the tool string is assembled with the TEL at the bottom, a sinker bar (weight bar assembly) of appropriate weight above, and the slickline attached at the top; the tool is lowered through the wellhead lubricator into the tubing at a controlled speed; as the tool approaches the expected tubing shoe depth (from the tubing tally), the descent is slowed and the operator watches the surface weight indicator or line-out recorder for the characteristic signal of the tubing end; after detecting the tubing shoe signal, the tool is cycled up and down over the shoe depth two or three times to confirm the reading is reproducible and not a mechanical artifact (paraffin bump, scale ledge, or dog-leg in the tubing); the confirmed tubing shoe depth from the tool run is compared to the tubing tally depth and any discrepancy is noted and applied as a depth correction to all subsequent tool positions in the same well intervention; if the discrepancy exceeds an acceptable threshold (typically 1 to 2 meters for most well intervention standards), the cause is investigated before proceeding with the primary intervention operation.
• Production logging and completion operations that rely on tubing-end locator depth referencing include: setting bridge plugs or mechanical packers to isolate specific zones below the tubing (where the plug must be set in the casing at a precise depth relative to the perforations, requiring the tubing shoe depth as a reference); perforating through the tubing on electric line (where the gun position in the casing below the tubing must be confirmed relative to the perforating interval target depth); production log interpretation (where the flow profile measurement is made both inside the tubing and in the casing below the tubing, and the transition zone at the tubing shoe is a reference marker for correlating the two measurement environments in the same logging run); and tubing extension or milling operations (where the existing tubing string length must be confirmed before the new section is installed or the tubing end is milled to allow a production packer to be set below it); in all these applications, a depth error of 1 to 3 meters at the tubing shoe translates directly into equivalent errors in the target depth of the primary intervention, potentially resulting in plugs set across perforations, perforating intervals missing the target zone, or other costly depth errors.