Best Practices and Guidelines for Milling a ‘Cast Iron Bridge Plug’

Bridge plugs are used for zonal isolation or to plug and abandon wells. If reentry is required, the plug must be either retrieved or milled. Cast iron bridge plugs are used for high-pressure, high-temperature, or corrosive well conditions. Most bridge plugs have bidirectional slips to hold them firmly in place. Procedural steps for drilling or milling a bridge plug must be meticulously designed based on well configuration and conditions. This careful planning will make you feel prepared and in control, even in the face of several risks and challenges.

  1. Milling and drilling

  • Review the plug specifications, material composition, and manufacturer recommendations for milling the bridge plug. Depending on the manufacturer's recommendations, select a tri-cone bit or a flat-bottom mill of the appropriate size and grade.

  • Depending on the well configuration and type of bridge plug, washing over to mill the top set of slips and then grabbing the mandrel with a grapple can be reviewed.

  • There is a high probability that the plug can break and fall in the well once slips are milled. If a liner is below, the partial plug could fall on top of it.

  • Depending on the well configuration, review strategies for milling the plug at the top of the liner and circulating out the debris. The plug may move down when weight is applied, so it may be necessary to push it down until the required weight for milling can be applied.

  • Review using a non-rotating rubber stabilizer in the assembly to keep the drill bit/mill stabilized and centralized during the operation. A shock sub to reduce vibrations and a jar can be added in the BHA at appropriate positions.

  • Start rotation and circulation at least 3 feet above the top of the bridge plug.

  • Start with low RPM and WOB, then gradually increase these parameters as needed. This cautious, attentive approach is vital, as excessive weight can break the bridge plug into chunks and complicate the milling process.

  • Managing reactive torque while milling a bridge plug is crucial for safe and efficient well-intervention operations. If reactive torque is observed, reduce RPM and WOB to reduce the torque, then increase gradually while maintaining stable torque.

2. Trapped pressure

  • Review the well configuration and conditions. If a perforated zone is below the bridge plug, trapped pressure cannot be ruled out.

  • Review milling a smaller hole through the plug to assess and control the release of pressure trapped below the plug.

  • In any case, prepare to handle higher pressure in the well. Keep kill mud weight for the anticipated pressure, test pressure control equipment, and discuss plans with the crew to identify indications, precautions, roles, and responsibilities for mitigating and controlling trapped-pressure conditions. 

3. Debris management and well clean-up

  • Milling cast iron bridge plugs generates significant debris.

  • Use one or more junk subs and string magnets above the bit in the milling assembly.

  • Use a ditch magnet on the shaker area to collect the metal cuttings that come to the surface with circulation fluid.

  • Maintain optimal mud weight and viscosity. A viscosity of 30–60 centipoise and a yield point of 60+ is recommended.

  • Use an adequate fluid flow rate based on well configuration. Pump regular sweeps and closely monitor returns for debris and cuttings.

  • Keep flow velocity high to maintain turbulent flow for cleaning while milling and avoid cuttings accumulation.

  • Plan well clean-up after milling operations are completed. Design a clean-out assembly with components such as a scraper, brushes, string magnets, etc., and pump a cleaning fluid train to ensure the well is thoroughly cleaned of milled debris.

  • If the forward operations require tieback, plan to polish the top of the liner and PBR after milling the bridge plug.

Reference: My-Spread Forum | Experience of Drilling Out Cast Iron Bridge Plugs