Basic Knowledge of Casing while Drilling (CwD)‎‎

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Casing while drilling (CwD) has been around for many years and it is one of proven technologies that can save both time and money. CwD is a process where a well is simultaneously drilled and cased; the casing is used for the drill string, and is rotated to the drill and cemented into the well at TD. One of the main benefits of this process is that it greatly cuts down on the tripping time needed to pull out the bottom hole assembly (BHA) and run the case- if not removing this need entirely. Therefore, the flat time is reduced, and the process is made more economically viable.

As shown in Figure 2 below, which is an example of Casing while Drilling utilized in one of oilfields in Oman for drilling surface section; this process can save up to 37.5% of time spent on a well based on historical data.

Types of Casing while Drilling Systems

Three main types of CwD, which is determined by the configuration and operation of the drill, are as follows;

• Non-Retrievable Casing While Drilling System

• Retrievable BHA Casing While Drilling System

• Drilling with Liner Systems

Non-Retrievable Casing While Drilling System

The non-retrievable system is the simplest type of CwD. In this case, the system is made up of a drillable bit or drill shoe, a casing string, and a casing drive system. The drill shoe is fitted securely to the bottom of the casing string; the latter is rotated by a power swivel which is hooked up to the drive system. This system only offers a limited number of options- it can only drill in a straight hole, and to a pre-determined depth.

Multiple drill shoes are available, which vary according to hardness and strength. Figure 4 is an example of a drill shoe manufactured by Weatherford.

In order to provide proper rotational movement, and to pump through casing, water brushing is normally used in the drilling process. However, this is not a viable option when drilling using CwD, since the make up and breakout of water brushing means that the drilling threads are more likely to become damaged. The casing drive system used for CwD is specifically built based on the casing spear principle integrated with a cup-type packer seal. This then internally slots into the new joint of the casing. It is then able to connect to, pump, and rotate the casing properly. Once the casing joint is drilled down, the casing drive system rapidly disengages by releasing the casing spear. However, this spear requires a large gripping surface to properly distribute the load and prevent pipe deformation. The internal catch is suitable for large casing up to 13-3/8&#;, whereas an external catch is necessary for casing which is smaller than this.

Retrievable BHA Casing While Drilling System

The retrievable casing while drilling BHA system strikes a balance between conventional drilling tools and CwD. The main advantage of this system is that it can be steered, and used with both conventional measured while drilling (MWD) and logging while drilling (LWD) tools.

Most BHA systems are connected to the bottom of the casing string, and drill a pilot hole. This hole can then be enlarged using one of three methods: 1) a reaming casing shoe, 2) a near casing shoe underreamer, or 3) a near bit underreamer.

Advantages and disadvantages of all three hole enlargement methods are shown below;

Reaming Casing Shoe

Advantages

• Maintain good directional control and LWD reading
• No risk of reamer not collapse when retrieving

Disadvantages

• Rat hole equal to BHA length. This will leave the longest rat hole among three methods.
• Limited reamer RPM equals to casing RPM

Near Casing Shoe Underreamer

Advantages

• Shorter rat hole
• Enable to improve reamer performance with PDM

Disadvantages

• Risk of underreamer stuck

Near Bit Underreamer

Advantages

• Shortest rat hole
• Enable to improve reamer performance with PDM

Disadvantages

• Risk of underreamer stuck
• Effect on LWD reading and directional control performance

The pilot BHA connects with the main casing, using Drill-Lock-Assembly (DLA) to set in the casing profile nipple (CPN). Once it has reached the TD, the BHA can then be retrieved using a drill pipe or a wireline; which method is used will depend on the weight and angle of the BHA.

The BHA system can cause a rathole of the same length as the BHA itself; for that reason, there are two techniques used to minimize the rat hole. Firstly, the DLA may be released at the TD, and reamed down with a casing reamer shoe up to the full bit length- this method can damage the BHA, though. An alternative to this method is to place the underreamer behind the bit; when the BHA reaches the TD, the DLA is then released, and the casing is able to be run all the way to the bottom. However, by positioning the underreamer behind the bit effects when using an LWD reading can impact upon directional control performance, particularly when using a rotary steering system (RSS). RSS is a popular choice when using CwD, given that it performs better than mud motors or positive displacement motors (PDMs). Drilling with mud motor is particularly difficult to use in conjunction with CwD, because it requires a larger contact area with the wellbore in order to effectively control the tool face.

With this system, cementing is usually done after BHA retrieval. Using a pump down float, which is dropped into the casing and pumped to lock in at the CPN, the cementing can be quickly and easily performed normally.

Drilling with Liner Systems

Drilling with Liner (DwL) works in much the same way as the previous two systems, except it does not involve the use of a casing drive system. The liner hanger setting tool is connected to the drill pipe, and then attaches to the power swivel at surface. There are three sub-types of this system: non-retrievable, wireline retrievable and drill pipe retrievable.

Once the drill has reached the TD, the non-retrievable DWL is able to set the liner hanger, and then complete the cementing job. With a retrievable DWL, the BHA needs to be retrieved once the liner hanger has been set, before a liner wiper plug latching system or cement retainers are run with the liner top packer and seal assembly to set in the polished bore receptacle (PBR) atop of liner. When the seal assembly is attached to the liner, the cementing can then be carried out normally.

The advantages and disadvantages of each system are listed below

Non retrievable Drilling with Liner (DwL)

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Advantages 

• Minimal rat hole
• No downhole tools to retrieve
• No rig modifications
• Not as expensive as others
• Quick cementing upon reaching section TD
• Relatively simple operations

Disadvantages 

• Limitations in drilling shoe size used
• No directional and MWD capabilities
• Only cased hole logs can be acquired
• Possible lack of availability of technology
• Torque limitations

Wireline retrievable Drilling with Liner (DwL)

Advantages 

• Directional and LWD capabilities
• Higher torque applications than non-retrievable DWL
• No rig modifications

Disadvantages

• Higher operating cost
• Multiple trips are required to retrieve BHA
• Possible lack of availability of technology
• Rathole length equals to BHA length
• Risk of BHA being irretrievable
• Unable to cement immediately upon reaching TD

Drillpipe retrievable Drilling with Liner (DwL)

Advantages 

• Directional and LWD capabilities
• Downhole tools can be retrieved in one trip after setting liner
• Highest torque applications
• No rig modifications
• Stable drilling system with low vibrations

Disadvantages 

• Higher operating cost
• Possible lack of availability of technology
• Rathole length equals to BHA length
• Risk of BHA being irretrievable
• Unable to cement immediately upon reaching TD

References

Australian Drilling Industry Training Co (ed.) () The drilling manual: The manual of methods, applications, and management. 2nd edn. Boca Raton, FL: CRC Press.

Bourgoyne, A.T. () Applied drilling engineering. Richardson, TX: Society of Petroleum Engineers.

Cantu, C. () Technology adoption: No risk means no gain. Available at: http://www.drillingcontractor.org/technology-adoption-no-risk-means-no-gain- (Accessed: 11 December ).

Schlumberger Limited . () TDDirect Casing-Drilling and liner-drilling technology. Available at: http://www.slb.com/services/drilling/drilling_services_systems/casing_drilling.aspx (Accessed: 11 December ).

Mitchell, R.F., Miska, S.Z. and Aadnoy, B.S. () Fundamentals of drilling engineering. Richardson, TX: Society of Petroleum Engineers.

PennWell Corporation () Casing drilling marks a century of progress. Available at: http://www.offshore-mag.com/articles/print/volume-72/issue-10/drilling-and-completion/casing-drilling-marks-a-century-of-progress.html (Accessed: 11 December ).

Sánchez, F. J., Said, H., Turki, M., & Cruz, M. (, June 1). Casing While Drilling (CwD): A New Approach To Drilling Fiqa Formation in he Sultanate of Oman&#;A Success Story. Society of Petroleum Engineers. doi:10./-PA

Tesco Ltd () Tesco Drill-Lock-Assembly. Available at: http://www.drillingcontractor.org/dcpi//dc-julaug04/July4-Tesco.pdf (Accessed: 11 December ).

Weatherford Ltd () Drilling with Casing services. Available at: http://www.weatherford.com/en/products-services/well-construction/tubular-running-services/total-depth-services/drilling-with-casing-services (Accessed: 11 December ).

A tension cut and pull subsea wellhead retrieval system has reduced the time required to abandon subsea wells, according to the service provider Baker Oil Tools.

For instance, James A. Sonnier, Baker Oil Tools marketing manager for subsea tools, estimates TotalFinaElf E&P USA Inc. with the subsea wellhead retrieval system saved 10-12 hr of rig time on a recent plug and abandonment job in more than 4,000 ft of water in the East Breaks area of the Gulf of Mexico.

The job, carried out from the Glomar Celtic Sea semisubmersible drilling rig, cut 133/8-in casing, pulled the hanger, set a hydraulic spear, and recovered the casing in one trip. In a second trip, the job included engaging the wellhead and cutting two strings of conductor casing (20 and 36 in.) in less than 11/2 hr.

In another example, he says a major operator working from a floating vessel in the North Sea cut 20 and 30-in. strings of conductor casing in less than 2 hr and recovered the wellhead from the seafloor in a single trip.

Sonnier estimates this job would have required two or more trips with conventional cutting and recovery tools.

Baker's retrieval system consists of a four-component fishing tool designed to keep the workstring in tension during cutting. Sonnier says the ability to cut in tension creates less pipe fatigue on the workstring while also providing a more efficient cut.

Another uniqueness to the system is the procedure sequence that cuts the casing before extraction of the casing hanger seals, according to Sonnier.

Well retrieval system

Sonnier describes wellhead recovery systems before the development of the multifunction fishing tools as very time-consuming and thus, cost intensive.

He indicates the jobs, typically, required three or more drillpipe trips for removing each intermediate casing string.

For instance, each intermediate casing string required a trip to recover the casing hanger seals from the wellhead, a second trip to cut the casing, and a third trip to remove the casing and casing hanger from the well.

Following recovery of each intermediate casing string, the next step entailed cutting the conductor strings just below the mudline and recovering the wellhead.

Sonnier says during the past decade, advances in cut and pull technology have consolidated cutting and recovery of the wellhead into a one-trip procedure, but the jobs still required multiple trips for each intermediate casing string.

Rig time consumed by these trips is a function of water depth. Sonnier indicates that 8-10 hr round trips plus additional time for cutting are common.

He says conventional cutting equipment often requires 7-10 hr for cutting large-diameter, 20-36 in., thick-walled, 11/2-in. conductor casing that ties into subsea wellheads.

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