Hybrid system takes drilling in new direction

Today's reservoirs are increasingly defined by challenging, tortuous wells that require more precise directional drilling. Operators seek innovative solutions that deliver improved drilling efficiency and directional control that also remain cost-effective. Traditionally, there have been two alternatives for drilling horizontal and deviated wells – conventional mud motors and rotary steerable systems (RSS).

Mud motors are a reliable, proven technology that is economical to use with little risk. But mud motors often cannot optimize drilling efficiency and lose the ability to provide extended-reach drilling (ERD) capability like rotary steerable tools do. Rotary steerables offer precise directional control, thus improving drilling efficiency and enhancing ROP, but they are expensive to operate and costly to replace if lost in-hole.

A new technology is bridging the gap between the two, providing the industry with a third economically attractive option designed to improve ROP and reduce sliding in low buildup-rate directional drilling situations. Weatherford's MotarySteerable directional control system combines the hydraulic horsepower of a positive displacement mud motor (PDM) and the directional control of an RSS, providing continuous 3-D directional control while rotating and creating high-quality, smoother boreholes.

The design features a mud motor with a bent sub and specialized MWD tools using a new steering technique called targeted bit speed (TBS) that enhances drilling efficiency for a variety of drilling scenarios. These include vertical control, horizontal S-shaped wells with long tangents, and J-shaped well designs.

TBS facilitates steering by modulating the fluid flow within the drillstring, creating slight oscillations in the flow rate through the mud motor. This allows high-frequency variations in drilling parameters to control the bit speed to help steer the well in any targeted azimuth. TBS also overcomes sliding, one of the key limitations of mud motors, which requires that the drillstring rotation be stopped while the bit is rotating downhole. This can lead to drilling inefficiency and reduced drillstring rotation, especially in highly deviated wells. When TBS is employed, drillstring rotation can be near continuous thereby reducing wellbore frictional forces and helping with ERD applications.

TBS facilitates steering by modulating the fluid flow within the drill-string, creating slight oscillations in the flow rate through the mud motor. This allows high-frequency variations in drilling parameters to control the bit speed to help steer the well in any targeted azimuth. (Images courtesy of Weatherford International Ltd.)

Low buildup-rate wells

The technology is designed for any application that uses a mud motor, including vertical control, long horizontal laterals, and tangential situations where a hole is drilled and held at an angle. In S-shaped wells, the tool can be used to build up angle to a desired inclination, drill a tangent section, hold the angle, and redirect the bottomhole assembly (BHA) to maintain verticality.

But the system is particularly effective as a cost-efficient alternative to RSS for optimizing horizontal drilling when buildup rates are low – 0° to 3°/30 m (100 ft). If a higher build rate is required, the system is flexible in that it can switch from continuous rotation to sliding or conventional drilling until the desired build rate is achieved without making changes to the BHA.

Developed in 2008 and 2009, the system was designed for use in hole sizes ranging from 6 in. to 12 1/4 in.,

including the 7 7/8 -in. size, which is common in North America but infrequently serviced by RSS.

Following successful field trials in 2010 in the Uintah basin, Barnett, and Eagle Ford shale plays, the technology was introduced commercially to the US shale plays in 2011 and has since been deployed globally. It has been used in a variety of applications in Canada, Russia, Mexico, and Eastern Europe, drilling more than 76,200 m (250,000 ft) in close to 50 wells.

The design features a mud motor with a bent sub and specialized MWD tools using a new steering technique called targeted bit speed (TBS) that enhances drilling efficiency for a variety of drilling scenarios. These include vertical control, horizontal, S-shaped wells with long tangents and J-shaped well designs.

Suited for shale

The system is particularly well suited for low-cost applications in the US unconventional markets for vertical control and horizontal drilling.

In a long-lateral oil well in the North Dakota's Bakken shale play, a 4 3/4-in. system was deployed in a 6-in., 91.23° hole with a maximum dogleg of 5.95°/30.5 m (5.95°/100 ft) as an alternative to an RSS to reduce sliding time and produce a smother wellbore. Drilling commenced at 2,870 m (9,415 ft) and continued to 5,984 m (19,632 ft), more than 3,000 m (10,000 ft). The system allowed the operator to gain precise steering capability and decrease sliding time from 30% to 8%, resulting in a reduction of seven days of rig time. The smoother wellbore enabled the operator to easily run casing, saving more than US $520,000 in rig time and other associated costs.

Challenging formations, well designs

TBS technology also is well suited for hard formations such as limestone and dolomite that require the horsepower of a PDM and for remote, logistically challenging locations where the cost of bringing in and servicing complex and

costly RSS is prohibitive. The company's system was used to successfully drill a straight hole in an extremely hard formation in Poland with 12?-in. bits. The well was drilled from approximately 1,700 m (5,577 ft) to 3,200 m (10,498 ft), and because of formation characteristics, it took 45 days to drill approximately 1,500 m (4,921 ft).

Most of the well was kept below a 2° inclination, with the final section being allowed to drift up to 3.5° once the tool was removed from the BHA prior to reaching total depth (TD). Both PDC bits and less expensive tri-cone bits were used in the operation. However, the tri-cone bits proved to be only about 50% as effective as the PDC bits, which provided better directional control.

The technology also was deployed in a trial program for a major operator in the Buzuluk region of west-central Russia in the southern Ural Mountains. Although rich in oil and gas reserves, the area is very remote and presents extreme operational challenges for bringing in equipment. In this case, the system was used to drill four S-shaped wells to reach the reservoir targets.

Seven S-shaped and J-shaped wells, all 8½ in., were drilled with the system in Mexico. These types of well designs are typically the most difficult to drill with conventional BHA assemblies because the same BHA must be used to drill the vertical, build the curve, hold the tangent, and then drop the well back to vertical while drilling to TD.

TBS technology allowed these wells to be drilled using conventional BHAs with near-contiguous rotation of the drillpipe. Most of the wells exceeded 95% rotation of the drillpipe, with several of them building and holding tangents of more than 60°. High-angle tangents tend to be very difficult to hold direction without significant corrections as the BHAs often tend to build, drop, or turn depending on formation and BHA dependencies.

Successful applications of TBS technology resulting in enhanced ROP, reduced slide time versus rotating time, and fewer days on location in various regions have opened opportunities for further global expansion of the Motary-Steerable system, with deployments planned in Colombia, the Middle East, and China in the near future.