Mandrel

Key Takeaways

• The side-pocket mandrel (SPM) is one of the most mechanically elegant applications of the mandrel concept in petroleum completions: it consists of a specially machined tubing joint with an offset pocket machined into one side of the body (the "side pocket") that is dimensioned to accept a standard-profile valve or equalizing plug retrieved and installed by wireline kickover tools; when the wireline kickover tool (which has a pivoting arm that deflects into the side pocket rather than passing through the main bore) is run to the SPM depth and actuated, the kickover arm swings a retrieving or installing tool into the pocket, where it latches, retrieves, or installs the valve without interrupting the integrity of the main tubing bore; the side-pocket mandrel allows gas lift valves (which inject gas from the casing annulus into the tubing bore through an orifice in the valve body) or chemical injection valves (which inject scale inhibitor, corrosion inhibitor, or other chemicals into the flow stream) to be installed or replaced by wireline without pulling the tubing string, saving the cost of a workover rig and maintaining production during the valve change operation; SPM spacing along the tubing string is designed to optimize the gas lift valve positions for the expected liquid gradient profile of the well, and multiple SPMs are typically installed at different depths to allow adjustment of the gas lift injection point by plugging off the upper valves and operating from a lower valve as reservoir pressure declines.
• Packer mandrels bear the mechanical loads imposed on the packer during setting, during production, and during well control events: in compression-set packers (set by applying weight from above), the mandrel shortens under compressive load to actuate the slip-and-cone system that grips the casing and the element system that seals against the casing ID; in tension-set packers (set by applying tension from above), the mandrel elongates slightly to pull the cone upward and expand the elements; in hydraulic-set packers (set by applying fluid pressure to a piston in the mandrel bore), the mandrel provides the structural reference against which the hydraulic piston actuates; during production, the packer mandrel carries the weight of the tubing string hung below the packer, the tension or compression imposed by thermal expansion or contraction of the tubing string, the differential pressure load between the annulus and the tubing bore, and the dynamic loads from fluid slug flow and production choke fluctuations; packer mandrel ratings specify the maximum tension, compression, and differential pressure loads the mandrel can withstand without permanent deformation or failure, and the tubing string design must confirm that these limits are not exceeded for the worst-case combination of production, injection, and well control conditions over the life of the well.
• The landing nipple and lock mandrel system uses the mandrel concept to provide selective depth control for wireline-retrievable downhole tools: a landing nipple is a tubing joint with a precision-machined internal profile (the "no-go" and "lock groove") that accepts a lock mandrel carrying a plug, safety valve, or test tool; the lock mandrel's running keys engage the lock groove when the mandrel is positioned at the correct depth and rotated, holding the carried tool in place against the differential pressure and flow forces that would otherwise displace it; multiple landing nipple profiles machined to different sizes are spaced along the tubing string to allow different tools to be selectively landed at different depths without accidentally landing a large-profile lock mandrel in a small-profile nipple (the "no-go" shoulder in each nipple prevents a mandrel sized for a larger nipple from passing through a smaller nipple); the "selective" system (where all nipples have the same profile but tools are selective by the number of key rotations required for locking) and the "non-selective" or "standard" system (where each nipple has a unique profile matched to a specific lock mandrel) represent two approaches to ensuring that each wireline-run tool lands only at its intended nipple depth.
• Expansion mandrels and drift mandrels are used to verify or restore the internal diameter (ID) clearance of casing and tubing strings: a drift mandrel is a precisely dimensioned cylindrical bar run through the casing or tubing to verify that the full ID is clear and unobstructed, confirming that downhole tools of a specified maximum outer diameter can pass freely through the string; API casing drift specifications define the minimum drift diameter for each casing size and weight (slightly smaller than the casing ID to account for manufacturing tolerances and minor deformation from thread makeup), and drifting the casing before perforating, logging, or running completion tools verifies that the wellbore is open to the required clearance; an expansion mandrel (also called a casing roller or casing swage when in contact with deformed casing) is run with applied force to plastically deform inward-protruding obstructions in the casing, restoring the drift diameter by pushing dents, collapsed sections, or scale buildup outward to restore clearance; expansion mandrels are used in casing remediation programs before tubular cleaning, and to restore production tubing ID after collapse from reservoir pressure depletion in weak formations (chalk, diatomite, unconsolidated sand) that compact around the producing tubing.
• Perforating gun mandrels serve as the central structural body for shaped charge assemblies: the gun mandrel (also called the gun body or carrier) is a thick-walled steel tube with charge holes drilled at the specified phasing (angular spacing between successive charges in the spiral pattern) and density (charges per foot); shaped charges are mounted in the charge holes in a charge carrier or charge holder assembly that positions the charge tip at the correct standoff from the casing (typically 0 to 5 mm for through-casing guns that fire through the casing wall and cement) and orients the charge axis for maximum penetration perpendicular to the casing axis; the gun mandrel must withstand the detonation pressure of all charges simultaneously (peak pressures of 50,000 to 200,000 psi inside the mandrel during detonation) without rupturing or fragmenting in a way that could jam the completion string or obstruct the wellbore; expendable guns (where the mandrel is destroyed or severely deformed by the detonation) are an alternative to retrievable guns (where the gun mandrel and carrier are recovered to surface after firing) for through-tubing perforating applications where the debris from an expendable gun can be flushed from the wellbore.