Two subsea exploration wells located in the Gulf of Mexico at water depths of approximately 1,800 ft had remained temporarily abandoned while waiting on further decision since 1997–1998. When the current operator decided to permanently abandon those wells, the procedure called for a reentry by drilling out cement plugs set below the mudline inside the 9⅝-in. outside diameter (OD), 8.535-in. minimum inside diameter (ID), casing and permanently abandoning the wells by isolating the annular space that communicated with the open hole and extended to the mudline. Operations had to be completed rigless using an intervention riser system (IRS) with a 6.31-in. drift ID.

A system incorporating capabilities normally associated with costly rigs was developed. It included a 2⅜-in. OD coiled tubing (CT) deployed from a multiservice vessel. To overcome challenges associated with milling cement using undergauge mills and possible pressurized gas trapped beneath the plugs, a two-step approach leveraging a 4.5-in. OD pilot mill bottomhole assembly (BHA) followed by a 5.9-in. OD mill and 8.3-in. OD underreamer BHA was implemented. A biopolymer-based milling fluid was selected to increase the solids carrying capacity and facilitate hole cleaning at low annular velocities (limited by friction losses through the CT workstring). A fluid management system provided a closed-loop solution for controlling fluid properties while removing solids from the system. The annular spaces were isolated by using a combination of decentered casing perforations, inflatable cement retainers, and latex cement systems. The cement was optimized to reduce recipe variations for different placement depths and bottomhole static temperatures.

All cement plugs were successfully drilled out and the milling fluid provided good hole cleaning performance without any stuck pipe events. All fluids and tools performed as expected. The 9⅝-in. × 13⅜-in., 16-in. × 13⅜-in., and 20-in. × 16-in. annuli were isolated, and permanent abandonments were completed in accordance with the applicable regulations.

The project required multiple service lines to work synergistically. Careful planning and seamless communication between all parties involved were key to ensuring optimal efficiency and safety while reducing the overall operational expenditures. The collaboration instilled between all the project stakeholders delivered incident-free operations at the lowest total operational cost, which came below the authorization for expenditures of each well as compared to rig-based abandonments.

Although reentry well abandonments are not new to the industry, there is no history of such services performed inside large casings with CT from a multiservice vessel through an IRS with minimum ID restriction. This study details the engineered solutions and methodologies that enabled successful operations. It also discusses the project parameters, examines the operations, and proposes the lessons learned to serve as a benchmark for future such projects.

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