What Is Casing Drilling Technology?

Casing drilling is an advanced drilling technology that uses casing instead of drill pipe to transmit torque and weight on bit (WOB). It replaces drill bits inside the casing via a wireline system, completely eliminating the repeated tripping operations required in conventional drilling. This technology was first successfully tested by Canada’s Tesco Corporation in 1996, and by 2000, more than 20 development wells had been completed. Although the concept was proposed as early as the 1950s, it was not practically applied until the 1990s due to limitations in technology and equipment at that time.

With the rapid development of new materials, electronic technology and drilling equipment, casing drilling has gradually matured and been widely used in global petroleum engineering, becoming one of the mainstream directions for efficient, low-cost and high-safety drilling.

1. What is Casing Drilling Technology?

The core logic of casing drilling is to use casing instead of drill pipe to apply torque and WOB to the drill bit, enabling bit rotation and drilling.

• The casing is rotated by a top drive system to directly transmit power.
• The drill bit is mounted on the front end of a dedicated downhole tool assembly, which is locked at the end of the casing string.
• The tool assembly is connected to a surface winch via wireline, allowing quick retrieval and replacement of the drill bit.
• The drilling process is equivalent to running casing: casing is run into the well section by section and is generally not pulled out.
• Cementing can be performed immediately after drilling is completed, realizing synchronous drilling and completion operations.

A complete replaceable-bit casing drilling system consists of three main components:

• Surface running/pulling tools
• Downhole locking tool string
• Landing casing

When a bit change is required, the downhole locking mechanism is simply released, the tool assembly is quickly pulled out via wireline, a new bit is installed, and the assembly is then run back in and locked at the casing end—all without pulling the casing string.

2. Technical Features of Casing Drilling

• Synchronous drilling and casing running: Integrated operation of drilling and completion.
• Rapid BHA retrieval: The bottom-hole assembly (BHA) can be quickly pulled out via wireline.
• Continuous casing to bottom: Casing extends from surface to bottomhole throughout the drilling process.
• One-way casing running: Casing is run in a single direction and is generally not tripped out.
• Compatibility with conventional operations: Compatible with directional drilling, cementing, logging, coring, well testing and other standard processes.
• Wireline-based bit change: Bit replacement relies on wireline instead of drill pipe tripping.
• Modified standard casing: Standard oilfield casing is used, with threads and couplings upgraded for torque resistance.
• Wellbore strengthening effect: The narrow annulus and casing rotation promote cuttings adhesion to the wellbore wall, forming a "wall-building effect" that enhances wellbore strength.
• Expandable bit design: Matching blade-expandable bits can open up after drilling to provide a passage for the next section bit, further reducing tripping frequency.

3. Core Advantages of Casing Drilling

• Significantly reduce well construction cycle: The integrated design of drill string and casing eliminates frequent tripping and tool changes, enabling synchronous drilling and completion. According to Tesco’s calculations, a 10,000 ft well can save approximately 30% of drilling time.
• Greatly improve wellbore stability: The casing remains in the wellbore at all times, providing real-time support to the wellbore wall and reducing risks of collapse, lost circulation and stuck pipe. It also eliminates swabbing and pressure surges caused by drill pipe tripping, improving well control safety.
• Lower comprehensive drilling costs: Eliminates costs associated with drill pipe and drill collar procurement, transportation, inspection and maintenance. Reduces labor, equipment occupancy and material consumption. Lighter rigs also lower moving and operating costs.
• Improve cuttings transport and hydraulic efficiency: Mud circulation can be maintained continuously during wireline bit changes, preventing cuttings accumulation and kicks. The larger inner diameter of casing reduces hydraulic losses, while the smaller annular area increases upward return velocity, improving wellbore cleaning.
• Simplify rig structure and reduce equipment investment: Eliminates the need for the monkey board and pipe rack. The derrick height can be reduced and the substructure weight lightened, resulting in smaller, lighter rigs with faster moving, less manpower and lower energy consumption.

4. Operational Considerations

Hole Deviation Control

Without drill collars and centralizers, the casing string is prone to bending under pressure, leading to hole deviation.

• Strictly control WOB within the reasonable range of 10–30 kN.
• Keep rotary speed low, generally within 60–120 r/min, to stabilize the casing string and control deviation.
• Preferentially use PDC bits for better performance.
• Ensure the derrick base is installed straight to maintain vertical wellhead.
• Strengthen intermediate surveying to monitor hole deviation and true vertical depth (TVD) in real time.

Casing Protection

Since the casing string is permanently left in the well, effective protection is critical:

• Use casing-specific thread compound to ensure reliable sealing and connection strength.
• Select casing with internal and external anti-corrosion coatings.
• Adopt low rotary speed and low WOB to minimize outer wall wear.
• Appropriately increase the bit nozzle size to reduce pump pressure inside the casing and minimize erosion of the inner wall by drilling fluid.

5. Core Comparison: Casing Drilling vs. Conventional Drilling