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Reaming-with-liner gains new ground with high-drillability PDC casing bit.
Seventy-five feet of fish in the hole made drilling precarious in an unstable subsalt rubble zone in the Gulf of Mexico (GoM). The 22 m (72 ft) of bottomhole assembly (BHA) MWD tools lost in the exploration well decisively blocked the drilling progress. Rather than incurring the time, cost, and risk of sidetracking, the operator proposed to wash and ream a 9 7/8 -in. liner around the fish.
In addition to inclusions and tar associated with the rubble zone, planning for the reaming-with-liner (RWL) application had to consider the possibility that the bit could come in contact with the fish while drilling.
The casing bit had to be tough enough to drill in the rubble and survive possible contact with the fish while still allowing easy drill-out after the liner was set. To achieve this balance, a recently developed bit design with PDC cutting elements mounted on aluminum nose and blade supports was applied for the first time in the GoM.
Reaming through difficult conditions
Multiple trips were made to condition and ream the well’s 12/4 -in. open hole below an 11 3/4 -in. liner. Lost mud returns and difficult reaming and pack-offs were experienced while washing and reaming to reach bottom.
Once the hole was conditioned, the 9 7/8 -in. liner was prepared, and the RWL system was rigged up. The RWL assembly was fitted with a 9 5/8 -in. x 10 5/8 -in. Weatherford Defyer DPA 4413 drillable casing bit with PDC cutting structure.
The system was run in the hole, washed, and reamed to the 9 7/8 -in. liner setting depth. While reaming, torque as high as 40,000 ft-lb (54,232Nm) and weight on bit (WOB) up to 100,000 lb (45,359 kg) were recorded. The higher WOB seen was thought to result from the large amount of drag from wellbore contact.
Installation of the 9 7/8 -in. liner went smoothly, especially considering that the wellbore instability required extensive reaming. Interchangeable nozzles on the bit allowed for hydraulics optimization, abetting bit face cleaning. Lost circulation conditions experienced at several points were quickly mitigated.
The liner was successfully washed and reamed through the rubble zone to reach the setting point at 6,992 m (22,940 ft) measured depth. Once the liner was set and cemented, the 183-m (600-ft) cemented shoe track and the 4413 bit were drilled out with an 8/2 -in. tricone bit to formation integrity test depth and pulled out of hole in good condition.
The operation eliminated the potential of multiple wiper trips and potential sidetrack, enabling running and cementing the liner in a single trip.
In anticipation of future deepwater applications, the operator is exploring the design of several custom drillable casing bits to fit tight tolerance well geometries.
This first use in the GoM follows successful applications of the bit in Malaysia, Australia, Oman, Russia, Colombia, and Argentina. Off the coast of Australia, nonproductive time was a problem due to reactive shales, lost circulation, and borehole instability over a 795-m (2,600-ft) interval below the 20-in. conductor. The casing bit was deployed in a drilling-with-casing (DWC) application to reach total depth at a high ROP.
The system mitigated all top-hole-formation-related issues at an average ROP of 86.9 m/hr (285.1 ft/hr). The deployment was a record setter for interval length using DWC methods and saved the operator approximately 24 hours of rig time, or about US $600,000.
Field trials of the bit were conducted in Malaysia with an operator with extensive DWC experience using steel alloy casing bits. In these applications, the PDC drill-out bits had exhibited severe wear after drilling through the steel casing bit and the following interval of medium soft formations embedded with hard stringers.
The bit was rotated 6.56 hours for an average on-bottom ROP of 48.4 m/hr (160 ft/hr) for the entire interval, including the hard stringers. The high ROP indicates the PDC cutting structure remained sharp, even after drilling out the 20-in. shoe track and a previously cemented 20-in. casing bit. In two subsequent wells, ROP improved further and averaged 52% higher than earlier steel alloy casing bits.
The Asia Pacific region was an early adopter of the casing drilling technology. The area’s softer rock strength and high rig rates continue to make it a cost-effective technology. But in areas with harder rock, casing bit utilizations were limited by design constraints.
Casing bits have to be durable enough to withstand drilling forces yet easily drillable once cemented in the hole. Two basic designs evolved. One is a steel-bodied variation of a standard bit that is durable but tough to drill out. The single-piece bodies are easy to manufacture but leave a significant amount of steel in the drill-out path, which makes for a slower drilling and milling process.
Style two is a relatively complex aluminum design. Once drilling is completed, the casing bit is converted to drillable form by dropping and pressuring up on a ball, causing the center aluminum piston to protrude and displace the cutting structure into the wellbore annulus and out of the drill path. The design is easy to drill out but limited to medium formations.
A harder design
The drillable bit technology helps extend casing drilling to harder formations over longer intervals with much less drill-out, milling, and cleanout requirements. It is a simpler design, that is not displaced and has no moving parts. A ball drop is not required. Instead, the aluminum nose and blade supports remain in place to be drilled out with the next bit (casing or standard). The result is a stronger, durable casing bit that is still highly drillable.
The design features a full PDC cutting structure brazed to steel rails (one rail for each blade of the bit). The rails retain the PDC cutters during drilling while minimizing the amount of steel required. By mechanically locking the steel rails and PDC cutters to aluminum blades, the design achieves the rigid support structure needed for drilling in hard formations without hindering the drill-out process. The bit is able to drill in formations with up to 25,000 psi unconfined compressive strengths.
The cutting structure is comparable with conventional PDC bits but reduces steel in the drill-out path by up to 80%. This steel reduction allows the casing bit to be drilled out with conventional PDC or roller cone bits, eliminating a dedicated drill-out trip and without reducing ROP in the next hole section. The technology has an average drill-out time of less than 20 minutes compared to steel alloy drill-out times of 125 minutes.
Incorporating the new bit into casing drilling operations helps extend the interval length and enables the operator to drill out with the planned BHA for the subsequent hole section. In addition, reducing the number of trips lowers pipehanding requirements to make the rig floor a safer working environment.