New Bit Designs

Baker Oil Tools

New window milling and casing exit technology developed

Application of whipstocks and casing mills, to kick-off out of an existing wellbore, has been performed for many years. This technique typically involves setting a whipstock at a pre-determined depth with a window mill attached. The window mill is then unlatched from the whipstock and pushed ahead to mill the window.

Often, in hard or abrasive formations, the mill's metal-cutting elements will quickly wear outside the casing, requiring a trip for a formation-type bit to drill a sufficiently long rat hole to accommodate the next BHA. This operation is even more critical now, because extended rat holes are often required to fit long, rotary steerable assemblies out of the casing before formation drilling can begin. This extra trip, depending on window depth, can take up to two days of rig time and significantly increase well costs. Ideally, window milling and rat hole drilling should be performed in one run, to improve operational efficiency and cut costs.

The PathMAKER formation mill requires only one trip instead of two trips to mill a window, saving operators time and money.

Solution. The PathMAKER formation mill results from ongoing development work between Baker Oil Tools (BOT) and sister firm, Hughes Christensen (HCC), that utilizes technology from HCC's Genesis PDC line. The new formation window mill uses a special polycrystalline diamond compact (PDC) compound, to effectively cut both metal and formation.

The mill's new enhancements improve milling and drilling performance. A specially engineered, spiral-set cutter alignment reduces vibration, while large water courses keep the PDC cutters cool for increased endurance. Milling a window in certain hard formations can now be done in one trip, saving time and money. Finally, this formation mill can adapt to any BOT whipstock system, or it can be run as stand-alone technology. It can also provide extra drilling strength, when needed, over conventional carbide mills in many hard formations.

Field performance. The formation mill has proven itself in several demanding casing exit applications around the world. In a recent Rocky Mountains project, the operator required a window to be cut in 7-in, 32 lb/ft casing at 11,682 ft, to perform sidetracking operations in the Dinwoody formation. The Dinwoody is dolomitic sandstone, with compressive strengths reaching 30 kpsi. It typically is drilled with a 6XX IADC-series, roller cone TCI bit.

Traditionally, normal casing exits in this formation have taken up to three trips to complete. However, this device enabled the operator to mill a 12-ft window and drill 10 ft of formation on the first run in just 10 hr, saving more than two days of rig time. The tool was then re-run on the same well, milling another 12-ft window and drilling 5 ft of rat hole in just 10 hr.

Other significant field performance includes:

• North Sea. The mill saved two days of rig time. It milled a 16-ft window and drilled 18 ft of rat hole out of 9-5/8-in, 47 lb/ft casing, at 10,455 ft, MD. This occurred in a hard, abrasive Maureen sandstone in one run, in just 15 hr.
• US. A 12-1/4-in., OD mill, directional milling assembly and whipstock were used to mill a window and 120 ft of rat hole out of 13-5/8-in, 88.2 lb/ft, Q-125 casing. This was done at 11,900 ft, MD, in salt, in one run building from vertical to >4° inclination. On this deepwater well, a rotary steerable assembly was to be used to drill the section in one trip. This MWD system typically required at least 3° inclination to high-side sense direction at kick-off. It eliminated an intermediate build trip, saving the operator significant time and money.