The enlarge-while-drilling (EWD) tool has been used extensively to reduce drilling costs and problems encountered with slenderhole casing plans.

Drilling deep wells involves one main challenge: cost. One approach to make these wells more financially attractive is to set a larger production casing, which enables high production rates and decreases the time to recoup investment. Another cost-saving initiative is to use smaller surface casing, which allows for a smaller rig, smaller blowout preventer (BOP) equipment and lowers the tubular costs.

The larger production casing and smaller surface casing can be accomplished by using a slenderhole casing plan, whereby a hole is drilled larger than the drift diameter of the last casing cemented in the well. The subsequent casing or liner outside diameter is about 1 in. less than the drift diameter of the last casing.

History of hole-opening tools

In the early days, the drilling industry used a pump-pressure-activated, flip-arm underreamer to enlarge previously drilled holes. The flip-arm underreamer was run with limited weight on bit (WOB) and limited rotary speed in order to accommodate the fragile nature of the flip-arm cutting structure. Underreaming with this tool resulted in higher costs as a result of rig time and frequent fishing of the lost underreamer arms in the hole.

The bicenter bit was introduced in the early 1950s to circumvent some of the underreamer's limitations, and it became popular in the mid-1980s with the increase in deepwater drilling activities. The bicenter tool was the first tool to underream and drill the pilot hole simultaneously, and it significantly reduced the risk of losing junk in the hole as compared to an underreamer. However the limitations of the bicenter bit include poor directional control, irregular size and spiraled hole due to the bicenter bit's eccentric design.
The EWD, introduced by Tri-Max Industries in 1997, was the first symmetrical tool that enlarged the hole as you drilled. The tool drills the cement and shoe track, then it can be activated to underream the hole all in the same trip. It is a simple, weight- or pressure-activated, symmetrical tool; it has no weight, RPM, pressure, volume or temperature limitations.

Tool design

The EWD is a short (3 ft to 5 ft), strong tool with three symmetrical cutter blocks. The cutters are made of one block of steel and are inserted from the inside of the EWD housing (Figure 1). The wedge-shaped design of the cutter blocks prevents them from being lost in the hole. To keep the EWD in the tripping position, shear pins are set into the drive shaft. These shear pins can be run between 3,000 lb and 80,000 1b.

Located below the cutters are three interchangeable nozzles, which increase rate of penetration (ROP) due to improved hole cleaning at the bit face by reducing the regrinding of cuttings and bit balling.
In 2002, the EWD is in its fourth generation, with many improvements to its original design. When the tool was first introduced to the industry, it was run directly above the bit, either below a motor or on the straight rotary. Today, it is run further up the string with the rotary steerable system. The EWD's lock, incorporated since the second-generation design for extended reach applications, was further utilized to keep the tool in the drilling position even with weight below it. As a safety feature, the lock is set to shear at a 10,000-lb to 40,000-1b overpull, depending on the size of the tool and the operator's request. The internal parts of the EWD have telescopic mandrels that create a metal-to-metal seal to prevent flow behind the cutter blocks. This seal is the reason the tool has no pressure or temperature limitations.

The latest addition to the EWD is the pressure sensor, which indicates whether the tool is locked in the drilling position. If the EWD were to move from the drilling position for any reason, there would be a 500-psi to 700-psi pressure drop at the surface. All drilling personnel would notice this immediately and
re-lock the EWD before drilling forward.

Expandable stabilizer

The EWD can be used along with an expandable stabilizer, which features the same design as the EWD except with tungsten carbide stabilizer pads instead of the tapered PDC cutter blocks. The expandable stabilizer is used for directional control and can be sized to build, hold or drop hole angle.

Successes

The EWD is particularly suitable for directional applications. In one run with a major oil company, the tool entered a sidetrack at 31 degrees through a milled-out window. After EWD activation, the well was drilled dropping the angle to vertical, turned

azimuth 180 degrees, then rebuilt the angle to 55 degrees. A full-gauge expandable stabilizer was used to help drop angle with minimal sliding. A 1/2-in. undergauge expandable stabilizer was used to rebuild angle.

With another major company, one EWD drilled more than 9,500 ft (2,898 m) in one run with a rotary steerable system.

The cost savings and time savings that can be generated using the EWD are great, particularly when used with a successful slenderhole casing plan.

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