Drilling technologies continue to be driven by—and developed in spite of—the fluctuations in the price of crude oil. In an effort to further optimize the drilling process, especially for deep-water exploration, a system has been developed that allows casing to be run during drilling of a well. The Casing/Drilling System ("CDS") will save rig time by altering the traditional drilling process. This system has been designed primarily for multi-well offshore platforms, multi-well operations on land, deep-water operations, and for situations requiring operators to drill through and place casing across problem formations quickly. This technology uses a combination of traditional components—bits, mud motors, MWD/LWD tools—as well as some newly developed components that allow drilling of most hole sizes from any drilling unit, with the ability to recover the drilling assembly using wireline or drill pipe.
This paper will describe the physical components and operational aspects of the casing drilling system. Actual well data will demonstrate the value of the system and the lessons learned during initial field use.
The improvements initially sought were in reducing the amount of time that the borehole was left open prior to running casing, mitigating the problems caused by wash-outs and sloughing shales while running casing, and reducing the total tripping time in drilling and then running casing. These improvements would help avoid the instability that occurs when "time-sensitive" formations have been exposed during drilling. The improvements would also save time spent waiting. Saving wait time is especially important on expensive deep-water drilling units operating where there may be severe tidal changes between recovery of the last borehole assembly (BHA) and running casing below the seabed.
Ideally, current technologies should be utilized as much as possible with no modifications made to a drilling unit. The CDS system described in this paper has advantages over other first generation CDS systems because its use has minimum impact on a rig and rig-operations.
Because casing can be viewed as any tubular utilized in the drilling industry, albeit with lower stress limits, casing appears as a natural vehicle for use in the drilling process once its limitations are recognized. Initially the following characteristics for the casing were considered. The casing would be 13-3/8-in., 68 lb/ft, NS-CC casing with nominal ID of 12.415 in., a drift ID of 12.259 in., and, tolerant of a maximum make-up torque of 16,000 ft-lb. This casing was envisioned to provide service to ±1640 ft (500m) depth.
The drive mechanism for drilling has to be recoverable through casing. This constraint further requires that the drill bit for the subsequent hole-section be coupled to an under-reamer. Additionally, because there are torque limitations when casing is rotated, a mud motor must provide the power to drive the bit and under-reamer. The characteristics envisioned for a candidate BHA components are as follows. (See Fig 1.) The mud motor could have an OD of 9-5/8-in. or 8-in., with a maximum torque of 10,000 ft-lb, and a maximum flow of 1200 GPM. Maximum weight on bit (WOB) while operating should be no more than 55,000 lbs. to avoid buckling the casing. As noted, the bit must be pass through the casing after the shoe is drilled out. Thus, a mud motor that can manage a maximum-OD 12.25-in. bit is required.
Desired characteristics for the under-reamer are that it be built into the BHA, that it use PDC blades, and that it be able to open a standard hole to 17.50 in.