6 - Casing While Drilling (CwD)...Is It The Solution For Problematic Formations?
Updated: Aug 20, 2018
The Oil & Gas Industry is being continuously challenged by unlocking new hydrocarbon reserves.
The no-more easy oil had pushed companies to develop various techniques and technologies in order to overcome the elevated challenges. One of these technologies is Casing While Drilling (CwD).
What is Casing While Drilling?
It is a concept that was first developed and applied in the late 90's and early 2000'.
It implies that the well is instantly cased as it is being drilled. This aims to seal off the newly drilled formation, and prevent extended period of mud exposure.
This technique is executed by either:
rotating the casing and using it as your drill-string, or
drilling wit a Retrievable Motor BHA connected at the bottom of the casing string
When Do We Use Casing While Drilling?
This technique is mostly used when a problematic formation is faced and drilling risks are increased. Examples of problems that CwD intends to solve:
formation instability (sloughing shale, unconsolidated sand)
Total losses (fractured limestone)
Formation swelling (reactive shale)
Another motive to implementing CwD will be to reduce rig time. As you are cutting the time for tripping in and out of the hole with your BHA, your total rig time can be significantly reduced.
What Are The Implementation Challenges Of Casing While Drilling?
As an unconventional drilling technique, CwD comes with a new set of challenges and risks. These challenges should be properly identified and risks should be properly managed.
We suggest that some of the challenges to be considered during the planning phase are:
Well Control: As your annular space is reduced, a kick will travel faster in the annulus. Therefore; preventative well control measures should be in place, and frequent kick-drills should be performed to mitigate the impact of a kick in case it occurs.
Mud Pump Downtime: Due to the restricted annular space, pump pressure is usually higher, and might result in frequent mud pump downtime (changing piston on liner)
Mud Losses: Due to the higher Equivalent Circulating Density (ECD) in the annulus, accidentally fracking the formation becomes more likely to happen.
Insufficient WOB: especially at shallow depth as there might not be enough casing weight to be applied on the bit.
Directional Drilling Limitations: This applies when using the casing as your drill string as you can only drill vertically.
Premature Casing Wear: This applies when using the casing as your drill string. Therefore, hard-band centralized need to be used to mitigate the risk.
Differential Sticking: This applies when using a retrievable BHA where the casing string is not rotating while drilling the section (since the mud motor is performing the drilling).
Increased Casing Cost: due to higher grade and torque requirements.
Additional Procurement & Logistics: as more equipment is needed such as torque rings, top drive system, drill shoe, hard-band centralizers, retrievable BHA, etc.
Jarring Limitation: as the only way to release a stuck is by applying tension or mechanically jerking the casing string.
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