A tool string that runs vertically has no reason to bend, so it is built as straight and stiff as its sensors allow. The moment a well turns horizontal, that stiffness becomes the problem: a long rigid assembly cannot follow the trajectory past the kickoff, and the remaining options are to add centralizers, shorten the string, or accept a real risk of jamming. A logging flexible sub addresses the problem at its source by putting a controlled bend into the string itself rather than working around it at surface.
Why a Rigid String Struggles Past the Kickoff
In a vertical hole the string sees almost no bending load, and rigidity helps: it keeps sensors on axis and keeps depth correlation clean. In a deviated or horizontal section the geometry reverses. Every casing joint, reducer and extender is a step change in direction, and the string has to negotiate all of them while being pushed from surface through friction it cannot see. The limiting dimension is the maximum straight length between articulations. The longer that length, the larger the side load needed to force the string through a bend, and the higher the chance that it hangs up at a joint instead of sliding past it.
Logging Flexible Sub Design and Bend Envelope
The Vigor sub uses a multi-joint design produced by monolithic hollow-core machining, which gives a wide bend angle without turning the assembly into a chain of pinned links that would need maintenance between runs. Published bend angle is 0 to 15 degrees. Four body sizes are offered - VFS 38, VFS 43, VFS 73 and VFS 85.8 - at 38, 43, 73 and 85.8 mm outside diameter, equal to roughly 1.50, 1.69, 2.875 and 3.375 inches. Connection length is deliberately short, which matters because the sub has to add flexibility without adding materially to the string length that has to be handled and racked at surface.
Effect on String Rigidity and Centralization
Three effects follow from inserting the sub into the string, and all three reduce operating risk:
Maximum straight length is reduced, so the string passes casing joints and kickoff points rather than hanging up on them
Linear rigidity drops, so the string follows the wellbore trajectory instead of forcing the wellbore to accommodate the string
The centralizing requirement falls, because a string that can bend needs less help staying off the low side of the hole
The practical result is a higher probability of completing the log once the string has entered the horizontal section, which is exactly where runs are lost and where the cost of a failed trip is highest.
Working Pressure, Tensile Load and Temperature Range
Ratings are graded by body size and by option. Maximum working pressure is quoted at 103 MPa, about 15,000 psi, with a 138 MPa option equivalent to 20,000 psi. Maximum tensile load is approximately 55 kN on the smaller bodies and about 180 kN on the larger ones. Maximum temperature is 175 degrees Celsius, or 347 degrees Fahrenheit, across the range, and connections are customisable so the sub can be matched to the string rather than forcing an adapter. An H2S-resistant build is available as an option, which is the specification that decides whether the sub can be used at all in sour wells.
Logging Flexible Sub Selection by String Size
Selection is driven by the weakest rating in the string, not by the largest tool in it. A flexible sub rated below the pressure or tensile limit of the tools around it becomes the limiting component of the whole assembly, and one rated far above them adds length, weight and cost for no operational benefit. The usual approach is to match the sub body size to the tool string outside diameter, then confirm that the bend angle and the tensile rating cover the worst-case trajectory planned for the well. Where the trajectory includes a short-radius kickoff, the bend angle is the specification that decides whether the run is possible at all.
Flexibility is a string property, not a tool property. A logging flexible sub with a 0 to 15 degrees bend angle, ratings to 20,000 psi and 175 degrees Celsius and a short connection length lets a conventional string work in a trajectory it was never built for.
For more information, please contact China Vigor at info@vigorpetroleum.com or call +0086 29 81161513.
