When a wireline string stops moving, the operator has one question to answer before anything else: is the cable stuck, or is the tool stuck? The two conditions demand opposite responses, and the wrong choice either parts the cable at an uncontrolled point or leaves a recoverable tool string in the hole. A tension sub placed at the top of the string supplies the downhole load reading that turns that question from an inference into a measurement.
Where the Tension Sub Sits in the String
The sub is normally made up between the lower end of the cable head and the top of the telemetry section. Any axial tensile or compressive force acting on the string passes through the body and is transmitted to the load cell, which generates an electrical signal that the telemetry section sends to surface. Because it sits above the rest of the string, it sees the total load going into the tools rather than the load at one point inside them, and that total is exactly the number needed when deciding whether to keep pulling or to start a release sequence. It can be combined with a wide range of logging and intervention tools without changing the string architecture.
Separating a Stuck Cable From a Stuck Tool
Field procedure starts by holding normal logging tension and recording four things: current depth, cable type and size, surface tension just before the string stopped, and the cable head weakpoint rating. The cable is marked at the rotary table and fitted with a clamp so that if it parts, the surface end is retained. An incremental pull is then applied and the movement of the surface mark is measured, which gives elastic stretch and therefore the length of free cable:
Free cable length equals logging depth: the tool is stuck, not the cable
Free cable length is shorter than logging depth: the cable is stuck at a higher point
With the tool stuck, further pull either frees it, breaks the weakpoint so the cable is recovered, or parts the cable at its maximum tension point
Downhole load sharpens every step, because it removes the friction and buoyancy terms that make surface tension an unreliable proxy in a deviated well.
Tension Sub Load Range and Overload Margin
The Vigor unit measures both tension and compression to 25,000 lbf in each direction, with an overload rating of 150 percent, so a transient spike does not destroy the sensing element before the operator has seen it. Calibration is published as a sensitivity of 2.5027 mV/V at plus 20,000 lbs in tension and minus 2.4973 mV/V at minus 20,000 lbs in compression, against a maximum load cell excitation of 15 VDC. That symmetry is what lets the same readout be trusted in compression during set-down and perforating work as in tension during recovery. Body diameter is 3.375 inches with a makeup length of 42.4 inches, roughly 1.08 meters, short enough to add to a string without forcing a re-spacing of the tools below it.
Pressure and Temperature Operating Limits
The sub is rated to 20,000 psi, about 138 MPa, and operates from minus 20 degrees Celsius to 175 degrees Celsius, the latter being roughly 347 degrees Fahrenheit. That range covers the majority of open-hole and cased-hole logging programmes, including HPHT work where a load reading is most valuable. Temperature matters to the load cell as much as to the electronics, because excitation and sensitivity both drift with it, so a sub specified with margin will hold calibration through a long run where one specified at the limit will not.
Turning Load Data Into a Decision
Real-time monitoring is the point of the exercise. Surface tension alone cannot separate cable friction from tool drag, and in deviated or horizontal wells the gap between the two grows with every dog-leg. A downhole load channel lets the crew see stick-and-pull events as they begin, slack off before a string corkscrews, and hold overpull inside the weakpoint discipline the cable head was built around. Industry practice keeps weakpoints at no more than 50 percent of cable breaking strength and treats anything above 75 percent of the weakpoint rating as requiring explicit approval, which is a rule that can only be followed if the load being pulled is actually known.
Stuck-string decisions are made in minutes, with incomplete information. A tension sub reporting true downhole load in both directions, rated to 150 percent overload and 20,000 psi, gives the operator the one measurement that changes the response.
For more information, please contact China Vigor at info@vigorpetroleum.com or call +0086 29 81161513.
