Coiled tubing drilling (CTD) has received a great deal of attention over the last 20 years from operators looking for efficient methods to access mature fields and extract maximum yields from existing reserves.

One of the benefits of this technique is the ability to drill continuously in underbalanced conditions. This protects the integrity of the well formation while increasing drilling rate and is particularly useful for sensitive reservoirs. In addition to the benefits associated with managing surface pressure, CTD also can offer faster mobilization speed, reduced rig time and fewer preparation requirements, all of which can lead to significantly reduced operational costs.

As the technology develops, it is increasingly possible to directionally drill larger hole diameters with CT. Fields previously thought to be inaccessible because they didn’t make economic sense can now be exploited, making low-margin drilling operations a viable proposition for operators.

The benefits of directional CTD (DCTD) when combined with the continual improvement in technology and increasing number of successful commercial operations result in wider market acceptance and uptake of the technique across a range of applications.

AnTech Ltd. recognized that an economical solution to accessing mature fields lay in the speed and underbalanced capabilities of DCTD. The POLARIS tool is a 5-in. outer diameter (OD) bottomhole assembly (BHA) targeted at drilling directional, S-curve and horizontal wells and capable of drilling hole sizes up to 8.5 in.

The tool can drill multiple wells from a single pad thanks to the integration of a solid-state gyro. The gyro sensor is located close to the bit, resulting in quick directional feedback in a BHA package. The BHA is less than 12.2 m (40 ft) overall between bit and coil connector.

The capabilities of the gyro-steered BHA were demonstrated during a five-well program in the Niobrara Formation along the Colorado-Kansas border in early 2012, using mud for the build sections and air for the horizontal sections.

DCTD also can be beneficial in a number of circumstances, including:

  • Projects with multilateral branches from the main wellbore (i.e. several wells from a single pad) where it is essential to use the gyro orienting capability to avoid intersecting with other wells;
  • Areas where a minimal environmental footprint is critical, such as commercial, residential or farming regions; and
  • Sensitive reservoirs that would experience excessive skin damage if drilled overbalanced.

Despite the advantages of the gyro system, it does require circulation to stop to take a survey, which is not consistent with maintaining a stable flow regime and can cost valuable time. For operations where continuous circulation is required and therefore stopping to take gyro surveys is not possible, POLARIS can be configured with magnetic steering capability.

AnTech’s magnetically steered directional measurement unit provides constant feedback of data at high speed, enabling the driller to precisely control the tool’s trajectory and immediately react to changing well conditions without any delay.

Case study: Villerperdue Field in France

In 2014 the magnetically steered POLARIS BHA became the first CTD BHA of its size to be deployed in Europe. AnTech was awarded an underbalanced CTD project for Lundin Petroleum.

The main objective of an operation in Lundin’s Villerperdue oil field in France was to increase production and reduce the water cut of the existing well by drilling a new lateral from an existing wellbore, something only possible with the underbalanced CTD technique.

It was a requirement of the project that the wellbore be kept within tight vertical depth limits, ensuring the well was fully contained during drilling and reducing the environmental impact. Further project objectives required:

  • Performing all work with zero HSE incidents;
  • Initiating an openhole sidetrack inside the porous reservoir boundaries of 4-m (13-ft) thickness;
  • Drilling a drain at least 300 m (984 ft) long; and
  • Capturing and recording technical learning and best practices.

To ensure that the work could be completed successfully, considerable pre-well analysis and modeling was carried out by AnTech using its proprietary ATLAS software to predict tubing forces (such as weight on bit), downhole pressures and hole cleaning.

For any CTD job to be viable, the following two fundamental conditions must be met:

  1. The CT must be capable of transferring sufficient weight to the bit for an effective ROP to be maintained; and
  2. While maintaining a sufficiently energetic flow regime in the annulus to clean the hole, the downhole pressures must be low enough to prevent formation damage, and the internal CT pressure at surface must be low enough to avoid premature CT fatigue.

The project was completed in collaboration with two other partners to provide the complete service package for the operator. The drilling fluids used in the operation were a combination of liquid nitrogen and diesel, which were deliberately selected to protect the formation.

AnTech successfully drilled to target a 438-m (1,437-ft) lateral within a 4-m window without any HSE incidents. The project clearly demonstrated that AnTech’s tool is suitable for drilling directionally while maintaining sufficient two-phase flow for effective hole cleaning.

Comparing operations

The significant operational differences between the projects in Kansas and France demonstrate the flexibility of the POLARIS BHA and its capability to be used for a range of applications.

The shallow gas wells drilled in the Niobrara Formation in Kansas used the gyro-steered configuration to enable the build section of the well to commence directly out of the predrilled surface casing. This allowed a build to horizontal in less than 305 m (1,000 ft) true vertical depth without difficulty, which is a target that would have been difficult to achieve using a magnetically steered system due to the interference of the casing at kickoff.

The shallow horizontal wells drilled in this campaign also demonstrated the large range of hole sizes that can be achieved with POLARIS without sacrificing steerability for an 8.5-in. build section that was drilled before the 6.25-in. horizontal section.

In comparison, the openhole sidetrack performed in France used the magnetically steered configuration to allow continuous circulation, thus maintaining a constant underbalanced downhole condition in a formation known for showing high losses. Magnetic measurement eliminates the need to stop the mud pumps to take a survey.

This real-time feedback provides the high level of accuracy required to reduce the risk of dropping below the narrow target formation when performing an openhole sidetrack in an already horizontal wellbore.

As BHA technology capable of such diverse applications becomes increasingly available, the oil and gas industry will continue to embrace DCTD as a reliable and economically viable technique for unlocking the potential of mature fields.