Expected trapped pressure below surface plugs creates risks

During the 1980s, an operator plugged and abandoned its wells in compliance with the standard of that time, but when the BSEE updated the federal regulations, the operator had to reenter previously abandoned wells and perform operations to comply with the stricter updated regulations. These wells had surface plugs that needed to be milled to access the wellbores. The use of a pulling unit is the conventional method used for these abandonment operations, but the risks of trapped pressure below the surface plugs and the uncertainty in the ability to maintain well control during milling operations posed great concerns to the operators.

Unprecedented use of large-OD CT provides control

Upon evaluation, Coil Tubing Services, a Schlumberger company, selected large-OD (>3.0-in [>7.6-mm]) CT to mill the 300-ft [91.44-m] cement plugs located approximately at 300 ft [91.44 m] deep. The surface plugs were set on 7-in, 9 5/8-in, and 11-in casings. Unlike the sizes typically used in the Gulf of Mexico, large-OD CT sizes are capable of handling high pump rates (more than 3 bbl/min), which large-OD downhole motors require to deliver high torque. Using CoilTOOLS tools and solutions in conjunction with the PRESSURE & FLUID MANAGEMENT SYSTEM technology, Coil Tubing Services was able to work on live wells and mill the surface plugs despite the expected trapped pressure.

Because of lift capacity ratings on the crane for the second location, an injector head skidding system was required, which would be capable of holding the weight of the well control equipment, injector head, and downhole tools and would improve the rig-up and rig-down mobilization from one well to the next.

Successful campaign leads to even larger CT string implementation

The capabilities of the selected CT equipment led to several operational advantages, including effective response to live well situations, faster operational mobilization from well to well, and the efficient removal of solids from the wellbore back to surface. The capability of the CT equipment to work on live wells improved the safety and performance of the multiwell projects due to the faster rig-up and rig-down operations when compared with operations completed by pulling units.

The entire operation for all nine previously abandoned wells lasted 145 days—approximately 16 days per well. In comparison with a 30-day intervention performed with pulling unit, this CT intervention saved the operator more than USD 5 million in time spent on location. During operations, no pressure control issues or HSE incidents occurred. The success of the campaign led the operator to award Coil Tubing Services a two-well campaign, implementing an even larger CT string.