Drilling with casing is a technique that is increasingly garnering attention from drillers looking for a novel way to simultaneously improve drilling efficiency and mitigate 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 swab pressures. 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.

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 the drill 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 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 cementing operations 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 eventual restriction 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 casing is 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 drill – to rotate, circulate, and push the casing string – ensures success in setting the casing at the target depth the first time.

Reduced numbers of trips make the rig floor a safer working environment. Risks associated with running in and out of hole with pipe and equipment are reduced, and fewer workers are required on the rig floor. In addition, the system simplifies wellbore construction by minimizing the pipework and other inventory that are required on location.

Drilling with casing improves drilling efficiency to lower well construction costs. Because the hole is drilled with the casing or liner, drill pipe trips are reduced along with associated nonproductive time. (Images courtesy of Weatherford International)

South China Sea

Drilling with casing is applicable to a range of drilling scenarios, but early adoption of the technology has been highest around the Asia-Pacific region as a cost-effective technique for mitigating risk in relatively soft formations.

One offshore operator in the South China Sea deployed Weatherford’s drilling-with-casing (DwC) system to stop lost circulation from a loose gravel and sediment layer at an approximate depth of 136 m (449 ft) in a developmental well. The service was deployed using a 13 3/ 8 -in. casing and a 17-in. diameter casing bit specifically designed for very soft formations. The system penetrated the loose gravel layer, and the casing was set in place to isolate the lost circulation zone.

The company’s system was subsequently deployed to mitigate lost circulation problems in another 12 wells in the field, seven additional 13 3/ 8 -in. wells and five 20-in. vertical wells. The operator estimated that the DwC service saved 200 hrs of rig time on these 13 wells – equivalent to a savings of US $1.6 million – compared to drilling by conventional methods.

Expanding technology applications

For operators charged with overcoming drilling hazards in hard, abrasive formations, additional development work for the drillable casing bits is required. The technology needs to deliver the required drilling performance in a safe and cost-efficient manner while also ensuring that the PDC cutting structure of the bit does not sustain significant damage.

In comparison to existing steel alloy drillable casing bits, the Defyer DPA drillable casing bit is constructed with 80% less steel in the drillout path, enabling drillout with conventional PDC bit technology without imparting damage to the drillout bit.

To respond to this need, the company added a new 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 PDC cutting 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 globally in formations of varying type and compressive strength and has achieved an average recorded drillout time of less than 20 minutes. One of the first deployments was for an operator developing a well off the coast of Australia. Weatherford was asked to develop a drilling solution that would reduce NPT by overcoming top hole formation-related issues in the form of reactive shales, lost circulation, and borehole instability over a 2,600-ft (795-m) interval below the 20-in. conductor. The selected solution also had to help the operator avoid the need for a conventional bottomhole assembly trip from total depth (TD).

Following a detailed analysis of the top-hole formations and offset drilling data, a DwC system was recommended, which employed 13 3/ 8 -in., 68-lb/ft casing; a casing bit to help ensure that TD was reached at a high rate of penetration (ROP); a 500-ton casing running system; flush-mounted slips; torque rings; and large-bore float collars.

The operator deployed this drilling option and successfully mitigated all top hole formation-related issues while achieving an average ROP of 285.1 ft/hr (86.9 m/hr). The deployment was a record setter for interval length with DwC and saved the operator approximately 24 hours of rig time – equivalent to $600,000.

Drilling hazards posed by lost circulation and wellbore instability will likely become more prevalent as the industry continues to develop more technically complex and deeper reservoirs. Drilling with casing is increasingly proving to be a cost-effective and safe alternative to conventional hazard mitigation techniques and one that is applicable to a wide range of reservoirs.