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.
Figure 1- Casing while Drilling Operation (Courtesy of Weatherford)
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.
Figure 2 – A comparison between conventional drilling and casing while drilling of one field in Oman (SPE 136107-PA ,SPE Journal Paper – 2012).
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.
Figure 3 – Non-Retrievable Casing While Drilling System
Multiple drill shoes are available, which vary according to hardness and strength. Figure 4 is an example of a drill shoe manufactured by Weatherford.
Figure 4 – Weatherford Drill Shoe (Courtesy of 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.
Figure 5 – Retrievable BHA Casing While Drilling System
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.
Figure 6 – Drill-Lock-Assembly
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.
Figure 7 – Drilling with Liner Systems
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)
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.) (2013) The drilling manual: The manual of methods, applications, and management. 2nd edn. Boca Raton, FL: CRC Press.
Bourgoyne, A.T. (1986) Applied drilling engineering. Richardson, TX: Society of Petroleum Engineers.
Cantu, C. (2012) Technology adoption: No risk means no gain. Available at: http://www.drillingcontractor.org/technology-adoption-no-risk-means-no-gain-13073 (Accessed: 11 December 2016).
Schlumberger Limited . (2016) TDDirect Casing-Drilling and liner-drilling technology. Available at: http://www.slb.com/services/drilling/drilling_services_systems/casing_drilling.aspx (Accessed: 11 December 2016).
Mitchell, R.F., Miska, S.Z. and Aadnoy, B.S. (2012) Fundamentals of drilling engineering. Richardson, TX: Society of Petroleum Engineers.
PennWell Corporation (2012) 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 2016).
Sánchez, F. J., Said, H., Turki, M., & Cruz, M. (2012, 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:10.2118/136107-PA
Tesco Ltd (2014) Tesco Drill-Lock-Assembly. Available at: http://www.drillingcontractor.org/dcpi/2004/dc-julaug04/July4-Tesco.pdf (Accessed: 11 December 2016).
Weatherford Ltd (2013) 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 2016).