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 in place firmly. 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
a. Review the plug specifications, material composition, and manufacturer recommendations for milling the bridge plug. Depending on the manufacturer's recommendations, a tri-cone bit or a flat-bottom mill of the appropriate size and grade should be selected.
b. 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.
c. 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.
d. Depending on the well configuration, strategies for milling the plug at the top of the liner and circulating out the debris should be reviewed. The plug may move down when WOB is applied, so it might be required to push it down until the required weight for milling can be applied.
e. 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.
f. Start rotation and circulation at least 3 feet above the top of the bridge plug.
g. It is important to start with low RPM and WOB and gradually increase these parameters as needed. This cautious and attentive approach is crucial, as excessive weight can break the bridge plug into chunks, complicating the milling process.
h. 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 and increase gradually while maintaining stable torque.
2. Trapped pressure
a. Review the well configuration and conditions. If a perforated zone is below the bridge plug, the possibility of trapped pressure cannot be ruled out.
b. Milling a smaller hole through the plug to assess and control the release of the pressure trapped below the plug should be reviewed.
c. In any case, prepare to handle higher pressure in the well. Keep kill mud weight of anticipated pressure, test pressure control equipment, and discuss plans with the crew to look for indications, precautions, roles, and responsibilities for mitigating and controlling the trapped pressure conditions.
3. Debris management and well clean-up
a. Milling cast iron bridge plugs generates significant debris.
b. Use one or more junk subs and string magnets above the bit in the milling assembly.
c. Use a ditch magnet on the shaker area to collect the metal cuttings that come to the surface with circulation fluid.
d. Maintain optimal mud weight and viscosity. The viscosity of 30 – 60 centipoise and 60+ Yield Point is recommended.
e. Use an adequate fluid flow rate based on well configuration. Pump regular sweeps and closely monitor returns for debris and cuttings.
f. Keep the flow velocity high to have a turbulent flow for cleaning while milling and avoid the accumulation of the cuttings.
g. Well clean-up should be planned 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 completely cleaned of milled debris.
h. If the forward operations require tieback, polishing the top of the liner and PBR should be planned.
Reference: My-Spread Forum | Experience of Drilling Out Cast Iron Bridge Plugs