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AS SEEN IN JANUARY 2012

Drilling with casing curbs costs and lowers risk
Approaching trouble zones with drilling-with-casing technology keeps many problems behind the pipe.
conventional casing running scenarios is diminished because the casing is always on or near the bottom of the hole. In the event of wellbore collapse, the trouble zones remain behind the pipe, and remedial cementingoperations can be carried out to seal off the annular space behind the casing, negate the need to redrill the collapsed zone, and eliminate risk of not being able to run casing through those zones. Water-sensitive claystone formations will deteriorate due to incompatible drilling fluid, which can cause the wellbore to shrink over time during tripping and casing running operations. The eventualrestriction in the wellbore can prevent casing from reaching the desired depth. Drilling with casing eliminates this risk. As soon as the terminal depth is reached, the cementing operation can commence almost immediately, leaving no time for the wellbore to react to water. By eliminating tripping and subsequently minimizing the risks of lost circulation or wellbore instability events, drilling with casingis typically 30% faster than conventional drilling practices. In addition, sticking problems associated with conventional casing running operations are greatly minimized. The system’s ability to efficiently
Ming Zo Tan, Weatherford

rilling with casing is a technique that is increasingly garnering attention from drillers looking for a novel way to simultaneously improve drilling efficiency andmitigate hazards. Using this technique, production casing is used in place of drillpipe to drill the wellbore, enabling the operator to case the well while drilling. This step removes the need to trip drillpipe and bottomhole assemblies (BHAs), which is a necessity in conventional drilling scenarios. Tripping drillpipe raises the risks of wellbore instability by generating surge and swabpressures. Surge pressure can break down weak formations and lead to a lost circulation scenario. Swab-reduced pressure can cause an influx of formation fluids or gas into the borehole, reducing overbalance pressure and potentially resulting in a well control situation. Conventional methods used to mitigate wellbore instability – the use of drilling mud additives or pumping cement plugs and gunk plugs –can be time-consuming and expensive and may not be fully effective.

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Keeping problems behind the pipe Drilling with casing avoids the need for these mitigation measures. The fundamental components of the system include a drillable casing bit, a float collar, a centralizer or stabilizer, a top drive, and a drive system to transmit torque from the top drive to the casing drill bit. As thedrill bit progresses to its target depth, the casing string moves with it, isolating, controlling and curing problematic zones by means of higher equivalent circulating mud density and plastering of cuttings onto the wellbore due to smaller annular space. The risk of dropping casing as in
Drilling with casing improves drilling efficiency to lower well construction costs. Because the hole is drilledwith the casing or liner, drill pipe trips are reduced along with associated nonproductive time. (Images courtesy of Weatherford International)

Januar y 2012 | EPmag.com

COST AND RISK MANAGEMENT

drillable casing bit to its series that achieves the proper balance between durability in harder formations and drilling efficiency. The company’s Defyer DPA drillable casing bit incorporates PDCcutting elements mounted on steel alloy blades, providing a cutting structure that is comparable to conventional PDC bits. The technology is constructed with 80% less steel in the drillout path compared its conventional counterpart, which imparts additional damage protection. The new drillable casing bit technology has been deployed South China Sea globally in formations of varying Drilling with...