Casing while drilling provides immediate benefits, saving both time and money by altering the steps needed for the drilling process. On top of this, the CwD system also provides a whole host of additional benefits. The benefits of casing while drilling can be summarized below;
Save Time and Cost
As mentioned in the introduction part, Basic Knowledge of Casing while Drilling (CwD), CwD is able to save operation time by cutting down flat time and reducing operational risk. When compared to conventional drilling, CwD can provide a time saving of between up to 37.5% of time spent on a well based on historical data from a field in Oman (136107-PA SPE Journal Paper – 2012).
Figure 1 – A comparison between conventional drilling and casing while drilling of one field in Oman (136107-PA SPE Journal Paper – 2012).
Get Casing to Total Depth
Because of wellbore degradation, conventional drilling is usually unable to run casing all the way to the bottom of a hole. This degradation is caused by a number of factors, including wall damage, filter cake build up, and open hole time left for long time. However, a non-retrievable system allows casing string to drill all the way through ledges and other obstacles, fit through tight sections, and reach the TD successfully.
Reduce Formation Exposure Time
Introducing the casing during the drilling process also removes tripping times for BHA. This doesn’t just save on costs, but it also helps to reduce the formation exposure time that is usually an issue until cementing. This exposure time can cause mud degradation, salt creeping, and clay swelling, but these problems are eliminated by CwD. What’s more, since there is no need for an additional casing run after the initial drilling, there is less risk of the casing getting stuck.
Overcome Challenging Drilling Zones
CwD can also be used to deal with potentially challenging drilling zones, such as depleted formations, salt sections, and fractured zones. This is because CwD allows one to set and case off a zone instantly once it has been drilled. Conventional drilling, on the other hand, takes longer to deal with these areas, and can even complicate the problem, meaning a significant amount of both time and money is spent trying to recover the well, not taking into account the severe formation damage which can also be caused.
Improve Hole Cleaning
When CwD is used, it creates a small, uniform annulus between the casing and the borehore. This is beneficial because it means the drill is able to circulate and clear cuttings easily, without the need for a high slow rate. It is important to note, though, that drilling fluid properties and pipe movement need to be planned out carefully in advance to ensure that the hole remains clean.
Improve Hole Quality
By using casing while drilling, one can see that the hole quality is significantly higher than with conventional drilling. This is due to a process known as the plastering effect (Figure 2), in which cuttings and filter cake are crushed up and pressed into the formation by the rotation of the smooth pipe. This creates a low permeability wall cake. On the other hand, pipe rotation is not smooth in conventional drilling processes; a wall cake is produced, but it easily comes off the formation, and is therefore less effective.
Figure 2 – Wellbore stability improvement by Casing while Drilling (right) compared to conventional drilling (left) Courtesy of Schlumberger
Reduce Mud Loss
Thanks to the mud cake produced by cementing, drilling fluid is kept within the formation, thus sealing off the wellbore and preventing circulation problems. CwD therefore makes it possible to continue drilling in a more controllable fashion. CwD requires a lower flaw rate than a conventional drilling at the same annular velocity because the clearance between wellbore and casing is much smaller than wellbore and drill pipe. Therefore, hole cleaning can be improved. Conventional drilling requires that the drill be stopped to solve lost circulation problems, and often needs the setting of cement plugs, which can cause a great deal of hours non-productive time (NPT) which can soon add up to a great expense.
Wellbore Strengthening
Not only does this help to limit mud loss, but it also improves upon formation fracture resistance, a process known as wellbore strengthening. The cuttings and filter cake are crushed into the formation, sealing up tiny cracks and giving some additional integrity to the area around the wellbore, while also giving a wider mud weight window.
Improve Formation damage
Casing while drilling has a plastering effect, which is beneficial because it creates a mud cake layer that is impermeable, preventing the productive zone from being blocked up by both liquid and solid materials. Damaged zones are much smaller with CwD than conventional drilling, which is demonstrated in Figure 13. This means that the well is more productive, and damage is kept to a minimum. However, this point is still being contested by some, as the results of research may be seen to be inconclusive.
Reduce Operational Problems
Some operational issues such as stuck pipe and well control can be minimized by Casing while Drilling. The reasons are explained below;
- Stuck Pipe
A stuck pipe mechanism can be diagnosed into three major categories: differential, mechanical and solid induced stuck. In theory, a larger casing area should be more likely to suffer from a differential stick, but in practice, it is actually less likely. This is due to the effects of plastering, which creates a fine, impermeable layer that acts as a barrier, and keeps differential pressure to a minimum.
Mechanical sticking usually comes about due to micro-dogleg, undergauge holes, or mobile formation. CwD eliminates the first two causes by starting and ending the drill process with the casing. In terms of mobile formation, CwD reduces exposure times to limit the risk of this happening.
As CwD results in superior hole cleaning, the pack-off which is caused by cutting accumulation is drastically lowered. In addition, the plastering effect provides additional structural support. This means that CwD helps to eliminate the problem of stuck pipes, and is therefore useful for wells with a stuck pipe problem.
- Well Control
Conventional drilling often causes well control situations which need to be dealt with accordingly. This is due to tripping; therefore, since CwD eliminates tripping BHA, this risk is reduced. With retrievable BHA systems, the large by-pass between the BHA and the casing eliminates swabbing, and keeps the casing string firmly on the bottom, further reducing the risk of well control.
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).
Karimi, M., Moellendick, T. E., & Holt, C. (2011, January 1). Plastering Effect of Casing Drilling; a Qualitative Analysis of Pipe Size Contribution. Society of Petroleum Engineers. doi:10.2118/147102-MS
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).
