Junk Sub

Key Takeaways

• The reverse-circulation junk basket design catches debris by reversing the direction of fluid flow within the tool: in conventional forward circulation, mud pumped down the drill string exits through the bit nozzles or mill ports and returns uphole in the annulus; in the junk basket, a portion of the return flow is diverted inward through a ported collar into the bore of the basket body, where centrifugal effects and reduced fluid velocity allow heavy metal chips to settle out of the fluid stream and accumulate in the basket, while the cleaned fluid continues uphole through the drill string bore; the basket is retrieved to surface at the end of the milling run and the collected debris is inspected to determine how much material was milled (by weight and composition), whether the milled object has been completely destroyed (no large pieces remaining), and whether any unexpected materials were encountered (non-metallic debris indicating cement, rubber, or formation rock milled unintentionally); the reverse-circulation design can capture non-magnetic materials (aluminum, bronze, lead, rubber) that magnetic junk subs cannot attract, making it more versatile for mixed-debris milling operations.
• Permanent magnet junk subs use high-strength rare earth magnets (neodymium-iron-boron or samarium-cobalt) mounted in the tool body to attract and hold ferromagnetic debris from the circulating mud stream as the mud passes through or around the magnetic section: the magnet assembly is sized to create a strong enough magnetic field in the annular flow path to capture steel chips and swarf traveling uphole in the mud, with the captured debris accumulating on the magnet surface in a growing mat that must not become so large that it restricts the mud flow path or creates a bridge that cannot be broken loose; permanent magnet junk subs are highly effective for capturing fine steel swarf (small chips and filings generated by milling), scale particles containing iron compounds, and any other ferromagnetic debris that might damage pump valves or choke components if it reaches the surface; the magnets are demagnetized and the captured debris removed by spinning the tool on the rig floor after retrieval, with the debris weighed and inspected; high-temperature applications (above 150-175 degrees C) may demagnetize neodymium magnets, requiring samarium-cobalt or electromagnetic designs.
• Mill selection for junk removal operations pairs with the junk sub design to ensure that all milled material is recovered: flat-bottom mills (used for milling the top of a fish with a flat or irregular profile) generate large chips that benefit from a reverse-circulation junk basket with a large debris capacity; taper mills and pilot mills generate finer swarf that is better captured by a magnet junk sub; section mills (used to mill a window through casing for a sidetrack) generate long ribbon-like steel strips (spiraling sections of milled casing) that can bridge in the junk basket if the basket capacity is insufficient, requiring careful monitoring of mill torque and weight on bit (WOB) to detect basket bridging before it stops the milling operation; the combination of a positive-displacement motor with a section mill and an appropriately sized junk basket above the motor is the standard BHA for casing window milling, with the junk basket sized to hold at least one joint's worth of milled casing ribbons between trips out of the hole to empty the basket.
• Junk identification from basket contents provides important operational information about the milling progress and wellbore condition: the composition of the recovered debris (steel alloy type from spectroscopic analysis, rubber compounds from packer element fragments, lead from lead impression blocks, brass from valve components) reveals what was milled and in what order; the size and shape of the debris indicate whether the milling is aggressive (large chips suggest fast penetration rate with efficient cutting) or has stalled (very fine powder suggests the mill is spinning on already-milled material without forward progress); the presence of formation cuttings in the junk basket indicates that the mill has cut through the fish and into the formation below, which is a criterion for stopping the milling operation before the bit damages the wellbore; unexpected materials in the junk basket (such as aluminum or nonferrous metals not expected in the fish being milled) alert the operator to the presence of additional unknown objects in the wellbore that were not identified before the fishing operation began.
• Wellbore cleanliness after milling is confirmed by running a dedicated junk sub or magnet sweep on a clean-out run before resuming normal drilling or completing the well: even with a junk basket in the milling BHA, some fine metallic debris may escape the basket and settle in the rat hole (the sump below the fish) or in irregularities in the borehole wall; a clean-out run with a junk sub dressed with fresh magnets or with a freshly serviced reverse-circulation basket, combined with a slow-rotary clean-out pass at reduced WOB through the milled interval, recovers residual debris before the subsequent bit run encounters it at the bottom of the hole; in critical well operations (well abandonment with regulatory requirements for wellbore cleanliness, re-completion requiring precise perforation depth correlation, or secondary recovery operations where metallic debris could plug injection perforations), multiple magnet sweeps are run until the basket comes back clean over two consecutive runs, confirming that all recoverable metallic junk has been removed.