Coiled tubing (CT) drilling is the ever-evolving concept that combines CT with directional drilling using a mud motor to create a system that allows operators to reach target depths. Relatively speaking, CT drilling is a modern technique within the industry for drilling wells compared to the conventional method of using drillpipe. Although CT drilling is not the first choice for drilling wells, there are a number of cases where it is the best option.

The most suitable applications for CT drilling are reentry drilling, or sidetracking, from existing wellbores. The ability to run through the existing production tubing with ease provides an edge over alternative methods. Another occasion for choosing CT drilling is for wells where downhole pressure is a concern, for example, in underbalanced drilling. Finally, as a cost-saving application, CT drilling may be paired with a conventional rotary drilling rig, allowing precise entry into a desired zone followed by the completion of the well.

Apart from niche applications where CT drilling is the most suitable technology for the job, there are a number of benefits to CT drilling. The main advantage over conventional drilling is the ease in which CT drilling may work in underbalanced conditions. The second is reduced drilling times since operators do not need to stop to make and break the connections of jointed pipe. Furthermore, communication from downhole to surface improves, the environmental footprint decreases and efficiencies can be seen in transportation as well as the personnel required to complete a job.

Increased efficiencies

The rise and fall in popularity of CT over the last couple decades can be attributed to these advantages and the challenges that accompany them as the demand for increased efficiencies and greater depths become more pronounced. Some of the major disadvantages of using CT drilling include the cost of consumables and the inability to rotate, which leads to a number of drilling obstacles. Operators rely on the expertise of service companies to provide solutions to these problems.

CT Energy Services specializes in developing downhole tools for drilling and completions. Specifically, it is the company’s expertise in friction reduction devices through the use of vibration that have the ability to overcome the disadvantages or obstacles of CT drilling.

The inability to rotate the pipe accounts for the greatest obstacle when using CT technology. The resulting disadvantages are reduced ROP and the inability to reach target depths. CT’s Toe Tapper uses the existing pressure in the CT or drillstring to induce a negative pressure pulse when operated. This negative pressure pulse causes what is known as a water hammer effect in the tubing, generating movement in the tubing and reducing friction. The pressure pulse also improves helical buckling by creating a dynamic stiffness in the tubing.

What makes the negative pressure pulse so dynamic is the fact that it uses existing tubing pressures to enhance the pressure pulse. By venting the excess pressure, a larger pulse amplitude is generated, making the tool’s performance integrated with the operating environment.

Performance study

A pulse amplitude study was performed to quantify the gain in performance due to the negative pressure-pulse approach. What was determined was that when drilling motor operating pressures were introduced, the performance of the tool increased by 40%. What this means for CT drilling is that for the same pressure loss as conventional friction reduction devices, the Toe Tapper negative pressure pulse uses the system pressure more efficiently and improves tool performance.

The effects on the motor performance are negligible— only a small volume of fluid needs to be vented to achieve effective negative pressure pulse amplitude. Bit speeds in the motor are only affected by about 5%, while torque is still maintained.

Since the motor differential exists during most of the operation, the Toe Tapper performance is truly realized. Evidence of this can be seen in the trendline for milling times during completions operations in CT. Typically, mill times increase when operating farther and farther out in a well. With the Toe Tapper in the field operators have experienced average milling times that maintained a more level trend. This is due to the increase in localized hydraulic forces generated when the motor differential is increased. A parallel can be drawn, and this gain in performance also can be applied to CT drilling.

When comparing the cost of consumables between jointed pipe and CT, it is the longevity of jointed pipe that puts it in the forefront. Operators might recut or resurface connections or simply swap out a single joint, but CT is a consumable product. Spool-off tubing has a finite number of times it may be tripped in and out of the hole before the properties of the pipe are compromised and the entire string must be discontinued or sold to less demanding applications. Limiting the number of times a spool is unraveled to complete a job is where a tool like the Toe Tapper becomes significant.

Two features of the Toe Tapper remedy the need to trip out of the hole during CT drilling. First is the negative pulse that vents fluid from the tool up the wellbore, and second is the low frequency of vibrations. Both components allow cuttings to circulate up the wellbore more efficiently. Hole cleaning is often the main reason for a reduced ROP, and wiper trips are traditionally the solution to this problem. The combination of the patented venting system of the Toe Tapper, which allows an additional area of turbulent flow to assist in the movement of the cuttings, and the low frequency of the tool, which produces more of a sweeping movement of the tubing to dynamize cuttings, reduce the need to perform a wiper trip. Therefore, the cost of CT as an expendable is minimized.

The Toe Tapper was developed from CT Energy’s Ratler system used in drilling applications. The design utilized knowledge in wear patterns and flow dynamics specific to negative pulse generation. This yielded a design capable of operating in the harshest environments; has been used with high lost circulation material content, substantial solids concentrations, acids and nitrogen; and is capable of having cement pumped through it.