The cost of drillout trips in the Gulf of Mexico from a deepwater drillship can easily approach $70,000 a day. A new eccentric drillout tool, which employs a field-exchangeable pilot bit solves this problem.

Christensen was the first company to use eccentric bits in the 1960s in West Texas. The tools evolved by adding PDC bicenter bits in 1984. Certain limitations of bicenter bits led to the introduction of ream-while-drilling (RWD) products in the mid-1990s. The limitations included lack of interchangeable pilot bit and bottomhole assembly flexibility, uncontrollable deviation tendencies, inconsistent borehole quality, destructive wear pattern and bit balling.
The RWD products developed out of a need to reduce the cost of under reaming trips in the Gulf of Mexico. Three concepts evolved: RWD, SRWD (steerable ream while drilling) and STRWD (slimhole technology ream while drilling).
The RWD tool includes several patented innovations, including the hole-opening blades and the pilot stabilization pad, a nonaggressive pad that acts to stabilize the assembly in the pilot hole (Figure 1). The hole-opening blades allow a more equivalent depth of cut for the four blades. The RWD is the only two-piece eccentric tool on the market.
Gulf of Mexico drillout trips
Untilrecently it was not possible to drill out cement plugs without damaging the RWD and the casing's inside diameter. Typical problems encountered may include damaged cutters close to the pass-through area, worn casing, other parts of the tool that contact the casing may be damaged or cutters in the center of the pilot bit may rotate backward while drilling the float equipment.
Laboratory-scale tests were performed in the Hughes Christensen surface rig facility in The Woodlands, Texas. A 97/8in. SWRD tool was used to drill through 9in. casing cemented into an Indiana limestone test block. The test results showed mild vibration between 100 and 120 rpm, however, no cutter damage resulted. The tool drilled out below the casing and transitioned to the hole opening mode within 12in. of drill size. The final caliper was measured as 9½in. and 7in. vs. 97/8in. and 6½in. The borehole and casing wall were in excellent condition.
A small amount of haze on the gauge cutters was observed, but with no significant damage. A few steel shavings were found from the casing, however the tool drilled out below the casing and gradually opened up the hole. Inspection of the inside diameter casing wall revealed minor scarring, not exceeding 1/16-1/32in.
Drillout SWRD
SRWD technology was developed into a commercial product from conception to prototype and testing in 6 months (Figure 2). The key elements are a pass-through bearing surface that allows rotation in the casing, sharp polished cutters and a pilot stabilization pad (PSP).
The pass-through bearing guardian ovoids consist of nonaggressive tungsten carbide inserts that act as a bearing surface. They are placed on the first blade upon pass-through. They reduce the aggressiveness of the gauge cutters protecting the casing. The pass-through bearing limits lateral motion and absorbs impact, which protects the cutters.
The guardian inserts also are nonaggressive tungsten carbide inserts placed on blades with pass-through points. They are oriented to maximize the bearing surface in the casing and limit lateral motion. The trailing guardian inserts are on the trailing blade, which provide additional contact with the casing wall and reduce lateral motion.
The Spectrum cutters (DIAX 45) provide increased diamond table thickness with large chamfer for improved durability. This reduces the gauge cutter aggressiveness, protecting casing and cutters while rotating inside the casing. Used on the gauge and near pass-through blades, they are recommended on the gauge of the pilot bit.
The RADXC cutters have an engineered planar interface, which reduces internal stress. They are suitable where impact and downhole vibration is expected and are used on the remaining cutters.
The PSP offsets the resultant force created by the bearing surface and minimizes bending. The PSP limits the depth of cut on the pilot in the casing and provides one of the pass-through contact points. It improves the transition from pass-through to drill size (Figures 3 and 4).
Testing
Nine prototype RWD assembly tests were conducted at The Woodlands facility and the Beta test site in Tulsa, Okla. Each of the tests were designed to improve the level of understanding of how the RWD behaves in constrained mode in the casing.
The goal of the field test at the beta site was to closely duplicate field conditions. A 17½in. hole was drilled to a depth of 310ft. A Weatherford Gemeco cementing plug was cemented into the test hole. A 12¼in. drillout SRWD510.6250 and 8in. Mach 1 motor with 1.75 adjustable kick off setting was used to test the directional motor and drillout tool.
The procedure used was to drill through the cement and stop above the plug without string rotation. The bit and SRWD assembly were inspected for wear prior to drilling out the float shoe. The drill float and shoe equipment were drilled without string rotation to 315ft. It took 45 minutes to drill the float equipment. The entire assembly was inspected after the run. Minimal wear on the pass-through bearings and no cutter damage was observed. Two cutters on the PDC pilot bit were damaged due to aggressive backrake on the gauge cutter. The casing caliper showed no unusual wear.
South Texas field test
Pioneer Rig 4 performed a field run in Hidalgo County, Texas. The 97/8in. drillout SWRD worked as intended and drilled the cement, landing collar and float shoe in 2½ hours with 4,000 to 8,000 lb weight on bit, at 60 rpm and 480 gpm mud flow. Even wear was observed on the Guardian inserts and bearings, and a drillout trip was eliminated.
Based on the development work, a series of recommended operating practices were developed. The pilot bit recommendations include PDC bits with nonaggressive gauge cutters, which tend toward a higher blade count. When roller cone bits are used, bit offset space stabilizers are recommended because they provide better protection for the bit and limit side cutting.
With a steerable motor assembly, a slow-speed, high-torque motor is recommended. The drillstring should not be rotated, and the mud pumps should be brought up to normal operating speed once the cement is tagged. The weight on bit should be between 2,000 and 8,000 lb. A minimum weight that will produce an acceptable penetration rate should be used. While drilling the float collar and shoe, the drill rate will slow and should resemble the performance of other drill bits, depending on the pilot bit used (PDC or tricone). More weight may be applied with caution if required.
Drilling out on rotary takes a different tack. When the cement is tagged, the pumps should be brought up to maximum flow and the rotary speed held in the 60 to 70 rpm range. Weight on bit should be between 2,000 and 4,000 lb. A minimum weight that will produce an acceptable rate should be run. If excessive vibration is encountered while drilling cement, rotary speed should be varied until a speed is found in which the condition ceases. While drilling the float collar and shoe, the drill rate may fall off, therefore more weight may be cautiously applied to produce an acceptable rate.
When tripping in and out of the hole,
the operator must pay special attention when passing through tight spots, the casing shoe or any other problem areas. The drillout SRWD should be slowly pulled back through the casing shoe to ensure no damage is done to the shoe or tool on the trip out. While the drillout SRWD tool is designed to rotate in casing, it is recommended that testing of mud motors or measurement-while-drilling tools be conducted at the surface before picking up the tool.
The drillout SRWD product eliminates a drillout run while minimizing casing damage. It can be used with a flexible bottomhole assembly, and the driller can use a PDC or roller cone pilot bit. It drills and enlarges the hole in the formation. Many successful bit runs by major oil companies have been completed
to date.