Drilling techniques that go beyond conventional methods to reach total depth (TD) with lower costs and reduced nonproductive time (NPT) are essential as operators strive to boost recovery potential in deeper, more complex and aging oil fields. Drilling with casing (DwC) or liner, which enables simultaneous drilling, running, setting and cementing of casing at TD, has gained increasing industry acceptance over the past decade to meet the economic demands of reducing downhole complexity, drilling through unstable hole intervals and mitigating hazards.

Today DwC is commonly used in a wide range of wellbore types worldwide. By drilling the hole and setting casing in a single trip, operators can save time and mitigate a number of the problems normally encountered while drilling, tripping and running casing. The technique also simplifies well architecture by reducing surface casing size or eliminating contingency casing and liner strings and improves cementing and long-term well integrity.

In an ongoing project with integrated oil company OMV Petrom, Weatherford’s European-Caspian Division has performed several single-run DwC operations on multiple well sections, significantly reducing time and achieving a time-savings record to reach TD. The centerpiece of the nonretrievable DwC system is an advanced drillable casing bit. The uniquely engineered polycrystalline diamond compact (PDC) bit is highly durable, which enables it to achieve high ROP for fast and easy drillout without damaging the drillout bit.

The operator’s objective in this case was to reduce rig time and NPT, primarily by eliminating dedicated wiper trips in low-budget wells and to safely reach TD without HSE issues. The wells are located across several aging onshore oil fields, including the Babeni, Vata, Bustuchin, Slateoara and Preajba fields in Romania as well as Austria’s Vienna Basin, which are characterized by soft-to-medium clay/sand formations.

Since the technology was introduced to European-Caspian land drilling in early 2013, the operator has successfully drilled more than 43 well sections using the Weatherford nonretrievable DwC system in conjunction with the Defyer DPA Series drillable casing bit. Weatherford has deployed DwC systems in 9 5⁄8-in. and 13 3⁄8-in. vertical well intervals ranging from 350 m to 900 m (984 ft to 2,953 ft) in length. In one case, where the casing was not run to surface, a 7-in. drilling-with-liner application was deployed.

OMV Petrom selected Weatherford’s integrated DwC approach after an initial test verified the system’s capability to deliver a wide range of operational benefits, including significantly improving drilling efficiency, ease and time of drillout, which resulted in substantial savings in cost and time.

Casing set at TD in single runs
The system, which has drilled all the well sections in a single run, enhances operational safety with simplified and streamlined procedures, less manual handling of tubulars, and minimal rig floor equipment. Drilling efficiency is maximized by eliminating conventional drillstring tripping and the associated trip margin required to mitigate the swabbing effects caused by tripping out of the hole. It also reduces sticking problems and keeps trouble zones behind the pipe. It is designed to overcome drilling hazards caused by depleted zones, pressure transitions and wellbore instability; to eliminate lost circulation; and to reduce mud costs.

In undertaking the multiwell project, Weatherford applied an engineered approach that relies on upfront planning. Prior to deploying any equipment, offset well data were analyzed to determine formation drillability and performance to advise the operator of the suitable casing bit choice and establish drilling parameters for executing the operation in the required time per nonretrievable procedures.

The integrated solution also includes a poppet-valve float collar, installed one joint of casing above the casing bit, which helps to prevent U-tubing during the primary cementing operation. The string also includes one or two Weatherford SpiraGlider centralizers on the first casing joint followed by one every third joint to the surface.

The fit-for-purpose Defyer drillable casing bits are designed for drilling with casing in soft, medium and medium-hard formations and are rated to 204 C (400 F). They feature a proprietary thin steel-alloy strip that is brazed with PDC cutters and mounted to the aluminum blade using a unique interlock profile. Multiple models are available, with various blade counts and cutter sizes to suit the targeted distance and rock hardness.

A key design feature of the DPA series is a selectable blade count and a cutter size that can be configured to suit the formation type and application. The blades are made of a special steel alloy, and the PDC cutters are mounted on the face of the tool to maximize drilling efficiency and durability.

Because about 80% of the core drillout area is composed of drillable aluminum, the Defyer bit is extremely fast and easy to drill out. This enables operators to continue drilling to TD with the planned bottomhole assembly using any standard oilfield drillbit, including a PDC drillbit, without compromising drillout-bit durability.

This improvement over conventional casing bits eliminates the need to make dedicated drillout trips and/or to pull out of the hole due to bit damage. A damaged or partially damaged bit also can negatively impact ROP. Interchangeable nozzles composed of either copper or erosion-resistant ceramics are available to ensure optimal hydraulics, bit cleaning and drilling performance. The nozzles, which come in a range of sizes, can be removed and replaced easily at the drillsite.

Achieving time-savings record
After completing each DwC operation, the operator had the option of drilling out the casing bit using either roller cone bits or PDC bits. For the Caspian operation, the operator chose a light-set aggressive PDC-drillable casing bit suited for the soft-to-medium formations. It took between 30 and 60 minutes to drill each shoe track, including 10 to 15 minutes to drill out the Defyer casing bit.

To date, each section has been drilled in between 11 and 53 hours, depending on length, with an average on-bottom ROP of 33 m/hr (108 ft/hr). Notably, the team drilled a 355-m (1,165-ft) measured depth section in 10.6 hours with an on-bottom ROP of 67.5 m/hr (221 ft/hr) and a 354-m (1,161-ft) section in 11.7 hours with an on-bottom ROP of 59.5 m/hr (195 ft/hr).

One 344-m (1,129-ft) section, which the operator anticipated would take 28 hours, was drilled in 14.75 hours with an at-bottom ROP of 28.8 m/hr (94 ft/hr) and zero NPT. In reducing the drilling time by 47%, the DwC system produced considerable cost savings.

The operator is planning to deploy the integrated DwC system in two additional well sections. With more than 1,500 global deployments in land and offshore operations, the nonretrievable DwC system has achieved a success rate of more than 98%.

The Defyer DPA Series drillable casing bit can be drilled out with any standard oilfield drillbit. (Source: Weatherford)

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