Free Point

Key Takeaways

• The free-point indicator tool (FPI) operates on the principle that a free pipe under applied surface tension and torque stretches and twists measurably, while a stuck pipe below the sticking point does not stretch or twist regardless of the surface load applied: the FPI is run inside the drill string on wireline to a depth suspected to be near the free point, two slips or magnetic anchors are set against the pipe wall at a known distance apart (typically 30 feet), and a surface load (tension of 10,000-100,000 pounds, depending on the pipe size) is applied and released while the tool measures the relative displacement between the two anchors; if the pipe section between the anchors is free, the applied tension stretches the pipe by a calculated amount (E = FL/AE, where F is the applied force, L is the reference length, A is the cross-sectional area, and E is Young's modulus), and the tool records this stretch as movement between the anchors; if the pipe section is stuck, no relative movement is detected between the anchors regardless of the surface load; the free-point depth is determined by running the tool at successive depths and finding the transition from detectable stretch (free pipe above) to zero stretch (stuck pipe below); the torque test works analogously, applying and releasing surface torque and measuring the relative angular twist between the anchors to confirm the free-point location; the combination of tension and torque tests provides a more reliable free-point determination than either test alone, because differential sticking can sometimes allow small tensile movement but no rotational movement (or vice versa), and the location of zero movement in both modes most reliably identifies the true sticking point.
• Free-point accuracy determines whether a backoff operation succeeds or fails: if the FPI indicates a free point at 8,000 feet but the actual sticking point is at 7,500 feet, a backoff attempted at 7,900 feet (just above the indicated free point) will try to back off in a section of pipe that is actually still stuck, preventing rotation at the backoff point and resulting in a failed shot; if the FPI indicates a free point at 8,000 feet and the backoff is attempted at 8,100 feet (just below the indicated free point), the backoff may succeed in unscrewing the tool joint but may leave a section of free pipe below the backoff point that now must be fished as a separate assembly; the standard practice is to back off 1-2 joints above the measured free-point depth (to provide a safety margin against measurement uncertainty and to ensure that the backoff location is definitely in free pipe), accepting that slightly more pipe is lost above the backoff than strictly necessary; the FPI measurement uncertainty (typically ±1-2 joints in favorable conditions, ±3-5 joints in adverse conditions such as highly deviated wells, crooked holes, or pipes with worn or unusual joint configurations) must be accounted for in the backoff depth selection to minimize the risk of a failed shot in stuck pipe while also minimizing the length of free pipe abandoned above the backoff.
• Differential sticking — the most common cause of stuck pipe in overbalanced wells where the drill string is pressed against a permeable formation by the hydrostatic overbalance pressure — creates a characteristic free-point location at the depth of the permeable zone against which the drill string is differentially stuck, with the transition from stuck to free occurring over a relatively short depth interval (the length of the permeable zone) rather than progressively: differential sticking occurs when the drill pipe rests against the filter cake on the borehole wall of a permeable formation while stationary (during a connection or a log-while-drilling measurement pause), and the differential pressure between the wellbore (hydrostatic head of the mud column) and the formation (lower pore pressure) presses the pipe against the filter cake with a force proportional to the contact area and the differential pressure; once stuck, the pipe cannot be pulled free because the contact force is renewed by the differential pressure whenever the pipe tries to move; the FPI in this case finds a well-defined free point at the top of the permeable zone (the shallowest point where the differential sticking force acts), and a backoff at this depth leaves the bottom hole assembly (BHA) and the portion of the drill string stuck against the permeable zone to be fished using an overshot or a spear run on the subsequent fishing trip.
• String shot backoff using the free-point determination employs a string shot — a length of primacord or equivalent high-velocity detonating cord attached to a wireline and detonated inside the drill pipe at the joint just above the free point — to apply a brief, high-magnitude impact torque to the pipe threads at the selected joint while surface equipment applies left-hand (counterclockwise) torque to the surface portion of the string, causing the joint to spin off (back off) and separate the free portion of the string from the stuck portion: the string shot detonation creates a pressure pulse that momentarily increases the pipe diameter at the detonation point, briefly reducing the make-up friction in the thread connection and allowing the applied surface torque to spin the joint open; the primacord charge weight is selected based on the pipe size and the connection type (API or premium) to apply enough energy to loosen the threads without shattering the pipe or damaging adjacent joints; the success of the backoff depends on having sufficient left-hand torque stored in the free portion of the string above the backoff point (accumulated by winding up the string in the left-hand direction from the surface before the shot is fired) to spin the joint open when the connection friction is momentarily reduced by the pressure pulse; the stored torque is calculated from the free-pipe twist equation (torque = (GJ/L) x theta, where G is the shear modulus, J is the polar moment of inertia, L is the free pipe length, and theta is the surface turn measured in radians) and must be sufficient to overcome the minimum break-out torque of the connection.
• Free-point calculations can be made analytically from the theoretical stretch and twist of free pipe under applied loads, providing a preliminary estimate before the FPI tool run and a cross-check on the tool measurement: the theoretical stretch of a free pipe section of length L under applied tension T is delta = TL/(AE), and the theoretical twist is theta = TL/(GJ) under applied torque T (different T in this context); if the measured stretch at depth D corresponds to the predicted stretch for a free pipe length from surface to D, the pipe is free from surface to D; if the measured stretch at depth D corresponds to a shorter free pipe length than D, the pipe is partially stuck between the free-pipe equivalent length and D; this analytical cross-check is particularly useful in situations where the FPI tool measurement quality is uncertain (due to poor coupling, irregular pipe surfaces, or electronic noise in the tool) and an independent estimate of the free-point depth is needed to plan the backoff safely; differences between the analytical estimate and the tool-measured free point of more than 5-10 joint lengths should prompt a re-run of the FPI tool with improved coupling conditions or an alternate tool type (electromagnetic FPI rather than mechanical stretch FPI) to resolve the discrepancy before the backoff shot is fired.