Drilling Direction

Directional casing drilling

A number of different casing drilling systems have been used both onshore and offshore. However, there are no known cases where an offshore directional well has been drilled with the expectation that the directional BHA would be pulled with a wireline and replaced while drilling the section.

The mature North Sea field application requires a fully retrievable casing drilling system to replace the directional assembly in the event of a component failure, to provide flexibility in BHA selection, and to recover the drilling assembly once the casing point is reached. The system must provide circulation, pipe movement, and full well control while running/retrieving the BHA.

These requirements are met with Tesco’s wireline retrievable directional casing drilling system. A conventional RSS directional assembly is run below the casing shoe to drill a pilot hole that simultaneously is underreamed to a diameter large enough to accept the casing. Use of this system on a conventional offshore rig may require modification to the surface equipment to allow effective deployment of the wireline system to run and retrieve the drilling assembly.

The surface system selected for the Norway well requires a wireline unit and crown sheave, wireline BOP system, split traveling block, and casing drive tool. All this equipment requires certification to Norway regulatory specifications. The portable casing drive system used for connecting the top drive to the casing without needing to make a threaded connection is used routinely for offshore casing running jobs and already was certified.

On the other hand, providing the wireline unit is somewhat problematic in that only a single wireline unit has been built that is appropriate for pulling the directional assemblies in typical Norway well profiles. This prototype traction winch was used for the testing described below, but a new unit was designed and built in Norway to facilitate obtaining the required certification. This unit has the capability to pull up to 40,000 lbf with a 7/8-in. braided cable while meeting the required design factors.

The casing drilling directional BHA includes a RSS and MWD run between the pilot bit and underreamer. The rotary steerable tools are run immediately above the pilot bit so a standard Schlumberger PowerDrive RSS can be used without modification. That would not be possible if the rotary steerable tools were run in the larger hole provided above the underreamer.

The BHA is released and pulled out of the casing in a single trip for vertical and low angle wells by using a releasing and pulling tool run on wireline. These tools must include the capability of being pumped into the well because of the high inclination in the Norway wells. Tool manipulations during setting and retrieval are performed with a combination of wireline tension and hydraulic operations that can be performed effectively in a horizontal well.

Test site

The cost of offshore operations and the inefficiencies (learning curve) associated with the introduction of a new drilling technology drive most first-time applications to being tested at land operations. Unfortunately for this purpose, most land wells are vertical. Furthermore, in using commercial wells for testing new technology, the well must be drilled to completion. This could be an expensive proposition if unexpected problems are encountered with the new technology.

These limitations led to the decision to test both the 7 5/8-in. and 10 3/4-in. systems at a Schlumberger facility. The facility has a large land drilling rig on rails that can access 11 slots and drill wells with displacements up to 5,000 ft.

7 5/8-in. drill test

The 7 5/8-in. casing directional drilling test was performed in July 2005. The tests were conducted from an existing horizontal well built at about 3.0 deg/100 ft at a TVD of 2,200 ft. It was cased with 13 3/8-in. casing and included 600 ft of horizontal section. The well and BHA set/retrieval tests were planned at 0, 45, and 90 degrees inside the existing casing before drilling began. After these tests were completed, 850 ft of new hole was drilled horizontally to test the complete system under actual drilling conditions.

The directional BHA incorporated a 4 3/4-in. RSS and PDC bit to drill a 6 1/2-in. pilot hole that was underreamed to 9 7/8-in. The directional plan provided for first turning the well to the right at 1.0 deg/100 ft and then to the left at 3.0 deg/100 ft while maintaining the borehole horizontal to test the directional performance of the BHA. The drilling assemblies were run and retrieved with a pump-down wireline (5/8-in. braided cable) system.

More detail of the 7-5/8-in. testing was provided by Borland, but the major conclusions were:

• The rotary steerable system in the pilot hole section of the BHA, coupled with the straight hole motor run above the underreamer, provided effective directional performance
• BHA vibrations were observed, but did not impact the performance of the drilling system
• Real-time monitoring of vibration and surface parameters allowed the BHA shocks to be reduced
• Testing in an actual drilling situation provides insight into tool design and performance that is essential before using the tools in a critical, expensive offshore operation.

10 3/4-in. drill test

The 10 3/4-in. test was in November 2005 at the test facility in a well with 13 3/8-in. casing set vertically at 2,003 ft. After establishing the cement plug at the 13 3/8-in. shoe for sidetracking, the well was sidetracked with casing and directionally drilled with first a low build rate of 0.5 deg/100 ft and then a higher 3.0 deg/100 ft.

The BHA design was similar to those in the 7 5/8-in. test in July. An RSS and MWD were used for directional control in the pilot section of the BHA. A straight motor was placed above the underreamer to supply downhole RPMs. Downhole shock counts were transmitted up hole in real-time from the MWD tool. Shock counts were also recorded downhole in the rotary steerable system. Additionally, three drilling research sensor packages were placed in the BHA; one above the underreamer and two below it, between the MWD and RSS. Downhole recorded measurements included annular pressure, lateral, axial and torsional acceleration, axial speed, torque, and weight-on-bit.

Lessons learned

• The two drilling tests at the Schlumberger facility have provided a cost-effective dress rehearsal for casing directional drilling in the Eldfisk field offshore Norway
• Casing drilling technology with retrievable BHAs, can be transferred to different hole sizes, in this case to 10 3/4-in., 7 5/8-in., and 7 3/4-in.
• Improvements to the setting and retrieval tools have resolved problems in high angle casing directional drilling operations
• Traction winch operations with a split block and derrick sheave system allow efficient and safe BHA handling on wireline for longer and larger BHA’s
• A rotary steerable system in the pilot section of the BHA coupled with a straight motor run above the underreamer is effective in directional performance
• External BHA vibration problems still exist but their effects have been reduced with an increased robustness in downhole tools and real-time monitoring of shocks
• Real-time monitoring of downhole shocks and vibrations and surface torque, weight and delta pressure allows for an actual reduction in these shocks on the BHA.

Plans for Eldfisk

Two Eldfisk oil-producing directional wells are planned for 2006 using RSS with the casing drilling system. Both wells will start from a 13 3/8-in. surface casing set at +/- 1,200 ft. Plans include drilling with 10 3/4-in. to approximately 4,850 ft TVD (5,100 ft MD) and 7 3/4-in. to approximately 9,600 ft TVD (10,800 ft MD). These two wells will be drilled back to back and will share a common wellhead. These field trials are expected to demonstrate a learning curve similar to that observed in the Lobo field. Further implementation of this technology will be depend on the field trial outcome; however, plans are being made for casing directional drilling to become the preferred drilling method for future Eldfisk work. In the short term, additional Eldfisk drilling will be done through recovered slots and will require the 10 3/4-in. casing to be drilled out of a pre-milled window off of a 13 3/8-in. whipstock.

Sumber : 

http://www.offshore-mag.com/articles/print/volume-66/issue-9/drilling-completion/preparations-for-offshore-casing-directional-drilling.html