Tubing End Locator (TEL)

A tubing end locator (TEL) is a wireline tool run inside a production tubing string to precisely locate the bottom end (tubing shoe) of the production tubing, providing a depth reference that is essential for planning through-tubing intervention operations where tools must be accurately positioned relative to the tubing end; the TEL typically uses a mechanical, acoustic, or electromagnetic sensing mechanism to detect the transition from the tubing interior to the open casing bore below the tubing shoe, signaling to the wireline engineer at surface exactly where the tubing ends; the depth of the tubing shoe is often not accurately known from the completion records alone because tubing stretch (caused by the weight of the tubing string and thermal expansion during production), settlement of the tubing hanger, and inaccuracies in the original tally of tubing joints all contribute to uncertainty in the actual tubing shoe depth relative to the depth recorded in the well file; for through-tubing operations where tools must pass through the tubing shoe and into the casing or perforated interval below, knowing the actual tubing shoe depth prevents the tool from inadvertently extending into the open casing where the lack of tubing restraint could allow the tool to swing laterally and become stuck, and allows the operator to confirm that any tool positioned "below tubing" is actually in the desired location in the open casing rather than still inside the tubing bore.

Key Takeaways

The depth uncertainty of the tubing shoe in a production well arises from multiple sources that individually contribute small errors but can accumulate to significant total error over a long tubing string: tubing joint tally errors (each joint is measured to the nearest tenth of a foot or centimeter, but inaccuracies in measurement and recording compound over hundreds of joints), tubing stretch under its own weight (a 10,000-foot string of 2.875-inch tubing weighing approximately 25 pounds per foot stretches several feet from the hanging weight), thermal expansion during production (the tubing heats up from cold completion conditions to production temperature, expanding several feet for a 200°F temperature change), and tubing hanger setting depth variations (the hanger may land at a slightly different depth than the nominal wellhead depth due to manufacturing tolerances); the cumulative effect of these factors can place the actual tubing shoe 5-20 feet away from the depth recorded in the completion tally, an amount that matters significantly for operations requiring precise below-tubing positioning of tools in the narrow transition zone between the tubing shoe and the first open perforation.
The collar locator function in a TEL provides the definitive depth tie to known reference points by detecting the casing collar locator (CCL) signal from the collars in the casing string as the TEL traverses upward through the well, allowing the wireline engineer to correlate the measured depth of the tubing shoe against the known depths of the casing collars from the original casing tally; casing collars are permanent, dimensionally stable features whose depths from the original running log are generally accurate (casing pipe is more dimensionally uniform than production tubing and the original depths were recorded under more controlled conditions during the initial well construction); by combining the TEL's detection of the tubing shoe with the CCL signal from adjacent casing collars, the wireline engineer calculates the tubing shoe depth relative to casing collar positions, providing a more accurate and reliable depth reference than the tubing tally alone; this CCL-anchored tubing shoe depth is then used as the reference for all subsequent through-tubing intervention operations in the well.
Through-tubing perforating operations are the most common context where accurate TEL data is critical: when a new producing interval is to be opened below the existing tubing, the perforating guns must be positioned at the exact depth of the target formation, and the TEL survey provides the depth correction needed to place the guns accurately after accounting for the actual tubing shoe depth; without a TEL survey, the gun depth would be set based on the nominal tubing shoe depth plus the calculated distance to the target formation, which could misplace the guns by enough to miss the target interval entirely or to perforate the wrong zone; the cost of a misplaced perforating job (including the wireline service, the rig time to detect the error, and the additional runs needed to correct it) far exceeds the cost of the TEL survey that prevents it; for wells being reperforated or produced from multiple intervals in a phased completion sequence, the TEL survey is a standard first step in the intervention planning process.
Mechanical TEL designs use a spring-loaded finger or drag mechanism that contacts the tubing inner wall and seats against the tubing coupling (which has a slightly larger ID than the tubing body), providing tactile detection of tubing joints as the tool moves upward through the string and a distinct mechanical indication when the tool passes through the tubing shoe into the larger-ID casing bore below; the distinction between the tubing ID and the casing ID (which is always larger) creates an unmistakable transition that the TEL's sensing mechanism records as the tubing shoe location; electronic TEL designs use electromagnetic sensors or Hall effect detectors that respond to the change in metal distribution (from tubing-enclosed to open casing) at the tubing shoe, providing a signal that is recorded on the wireline log without requiring the tactile mechanism; both approaches are effective for locating the tubing shoe, with the electronic approach providing a more quantitative depth marker that is easier to interpret from the wireline log compared to the mechanical approach that requires interpretation of the force signature on the tension log.
The TEL survey is often combined with a downhole camera run, a collar count log, or a casing integrity log in a single wireline trip to maximize the information obtained from a single trip in hole: a downhole camera can visually confirm the tubing shoe position by showing the transition from the tubing interior (smaller ID, may have scale or corrosion visible) to the open casing below (larger ID, different surface condition); a collar count log using the CCL provides the depth tie to the casing tally; and a casing integrity log (multi-finger caliper or electromagnetic inspection tool) assesses the condition of the casing below the tubing shoe in the zone that has been exposed to produced fluids since completion; combining these measurements into a single well entry minimizes intervention cost and rig time while providing the complete baseline dataset needed for subsequent well intervention planning.

Fast Facts

The tubing end locator survey became a standard practice in through-tubing intervention programs during the expansion of coiled tubing and through-tubing wireline operations in mature oil and gas fields in the 1980s and 1990s, when operators recognized that the depth discrepancies between nominal and actual tubing shoe depths were causing repeated positioning errors in perforating and plug-setting operations. Prior to the widespread use of TEL surveys, field engineers often relied on a rule-of-thumb depth correction for tubing stretch (typically 1-2 feet per 1,000 feet of tubing depth) that was too imprecise for critical positioning operations in long or thermally cycled tubing strings. The standardization of TEL surveys as a prerequisite for through-tubing perforating represented a recognition that the cost of depth uncertainty was systematically higher than the cost of accurate downhole depth verification.

What Is a Tubing End Locator?

A tubing end locator does exactly what its name says: it finds the bottom of the tubing. That sounds simple, but in a producing well where the tubing may have stretched, settled, or expanded from completion conditions, the bottom of the tubing can be several feet from where the completion record says it is. For a through-tubing perforating job where guns must be placed precisely at the target formation depth, those several feet of uncertainty can mean the difference between a successful perforation in the intended reservoir and an unsuccessful job that misses the target or opens the wrong zone. The TEL survey, typically a quick wireline run that takes hours rather than days, provides the actual tubing shoe depth that serves as the reference for all subsequent through-tubing positioning. It is one of the simplest and most cost-effective quality control steps in through-tubing well intervention.

A tubing end locator is also called a tubing shoe finder or a bottom-of-tubing (BOT) tool. Related terms include tubing shoe (the bottommost joint of the production tubing string, whose depth the TEL is designed to locate accurately for through-tubing intervention planning), collar locator (the electromagnetic logging sensor that detects casing or tubing collars, used in conjunction with the TEL to provide accurate depth correlation against the casing tally), through-tubing (describing intervention operations performed with tools run through the production tubing bore without removing the tubing string, for which accurate tubing shoe depth is a prerequisite), wireline (the conveyance method for TEL surveys, using a single-strand or multi-conductor cable to lower the tool to depth and record its measurements continuously as it is pulled back to surface), and depth correlation (the process of tying wireline log measurements to known depth reference points such as casing collars, the critical step that converts raw cable depth measurements to accurate subsurface depths).

Why Knowing Exactly Where the Tubing Ends Changes How You Plan Everything That Follows

Through-tubing well intervention is precise work. Guns must fire at the right depth. Plugs must set at the right depth. Chemical treatments must be spotted at the right depth. All of that precision starts with knowing where the tubing shoe actually is — not where the completion record says it was when the tubing was run several years ago, but where it is today with the stretch and thermal history of production. The TEL survey provides that anchor point, the measured datum from which all through-tubing depths are calculated. It costs a fraction of what a misplaced perforating gun or a wrongly set plug will cost to diagnose and correct. The engineers who run TEL surveys as a matter of routine policy on every well before a significant through-tubing intervention are the ones whose operations go right the first time.