Holdup Depth

Key Takeaways

• Drift runs are the primary method for establishing holdup depth and characterizing the restriction: a drift mandrel is a cylindrical bar machined to a precise outside diameter (typically 1/8 inch smaller than the nominal drift diameter specified in API 5CT for the casing weight and grade, representing the minimum acceptable inside diameter for that casing string) that is run into the wellbore on wireline or tubing to determine the maximum tool size that can pass through the wellbore; if the drift passes freely to total depth, the wellbore is cleared for tools up to the drift diameter; if the drift hangs up (the weight indicator on the surface shows a sharp increase as the drift contacts a restriction and forward motion stops), the measured depth at the point of hang-up is the holdup depth for that drift size; running a series of progressively smaller drifts (if the first drift hangs up) establishes the maximum clearance diameter at the restriction point, which is the critical parameter for selecting tools that can pass through the restriction without hanging up; wireline drift runs are preferred over drillpipe drift runs for precision because wireline depth measurement is more accurate and the drift can be re-set and run multiple times to confirm the holdup depth with less rig time cost than drillpipe trips.
• Common causes of holdup depths shallower than the planned tool depth include casing damage or deformation (buckled or collapsed casing caused by formation movement, thermal expansion, compaction, or wellbore pressures exceeding casing yield strength), cement invasion into the casing interior through cracks or splits (uncommon but possible when cement is over-pressured during primary cementing), scale or paraffin buildup on the casing interior (calcium carbonate, barium sulfate, iron sulfide, or paraffin wax deposits that accumulate over time in producing wells and progressively reduce the effective inside diameter), dropped or lodged equipment (a failed wellbore component such as a packer element, perforating gun section, or wireline tool that separates from the work string and comes to rest in the wellbore, creating a fish that blocks passage below its depth), and casing splits or perforations caused by corrosion or erosion that allow formation material to intrude into the wellbore (sand, shale fragments, or scale that enters through a damaged casing section and accumulates as a bridge or plug at the damage site); each cause requires a different remediation approach, and the holdup depth alone does not identify the cause, requiring additional diagnostic runs (caliper logs, video surveys, or electromagnetic casing inspection) to characterize the nature of the restriction before selecting the appropriate remediation method.
• The practical consequence of a holdup depth significantly above the target zone is that tools designed for the planned operation (perforating guns, packers, plugs, pumps) cannot reach the interval, requiring the operator to either remediate the wellbore to restore clearance or redesign the operation using smaller-diameter tools that will pass through the restriction at the cost of reduced performance (smaller perforating guns create smaller perforations with less penetration depth, smaller pumps deliver less lift capacity, and smaller packers may have reduced pressure ratings or reduced reliability in sealing the annulus); the economic impact of an unexpected holdup depth in a well completion or workover is significant because the rig or workover unit is already on location and mobilized, forcing either an unplanned milling or fishing job (adding days to the operation at $50,000 to $200,000 per day of additional rig time) or acceptance of a compromised completion that may not achieve the planned production rate; in wells with high expected recoveries (long horizontals in prolific shale formations or high-productivity offshore wells), the revenue impact of a compromised completion because the planned perforating guns could not be run to depth can far exceed the cost of the remediation.
• Holdup depth in horizontal and highly deviated wells requires additional interpretation beyond the simple contact depth recorded for vertical wells: in a vertical well, gravity pulls the drift against any restriction it contacts and forward motion stops abruptly, making the holdup depth unambiguous; in a horizontal well, the drift or tool lies on the low side of the casing and may slide past a restriction on one side of the casing while hanging up on an obstruction on the other side, or may stop at a deviation dogleg where the curvature exceeds the tool's flexibility limits rather than at a physical restriction of the casing inside diameter; the distinction between a holdup caused by a casing restriction (requiring milling or fishing) and a holdup caused by a dogleg severity limitation (requiring a more flexible tool assembly or a different deployment method) is critical for selecting the correct remediation approach; torque-and-drag modeling of the planned tool string through the wellbore trajectory is used to predict whether a holdup in a deviated well is caused by mechanical drag at the deviation (which can sometimes be overcome with additional weight or applied torque) or by a physical obstruction (which requires wellbore intervention regardless of applied force).
• Prevention of unexpected holdup depths is better than remediation: best practices for maintaining wellbore clearance include running drift runs at the completion of primary cementing before the wellbore is left for extended periods (confirming casing integrity before any obstruction has time to develop), running periodic production well inspections (caliper or imaging logs on wells with known scale or paraffin deposition tendencies) to detect developing restrictions before they reach the severity of a holdup, maintaining chemical injection programs for scale and paraffin inhibition in susceptible wells, and documenting all holdup depths (even those that do not immediately prevent planned operations) in the well history for future reference by completion and workover engineers planning subsequent operations; a holdup at a depth that is above the zone planned for one workover may be exactly the depth of the zone planned for the next workover, making the documentation of current holdup depths essential for future operation planning even when the restriction does not affect current operations.