HDD installation methodology; cleaning the bore - Part 1

This is the first blog in our deliver better outcomes series.

Horizontal Directional Drilling is a common and popular method for installing underground infrastructure. Like all civil engineering methodologies; a scientific approach will reduce risk and increase certainty for the contractor and asset owner.

An HDD installation requires the contractor to progress through a number of stages (pilot, reaming and pipeline pullback), completing each step effectively to ensure the following stage can be completed. The steps are interrelated; challenges or poor procedures on one stage influence the following phase, and then may require alterations to the methodology to maximise success.

First, let’s look at project design, fluid design and the pilot phase.
Part 2 dives into reaming and pipe pull.

Project design

The design has a strong influence on the overall risk and cost of the project. A design which takes into account borehole length, depth, inclination angle, hole geometry, geology and rig capability is essential for a successful HDD installation.

The design can be improved substantially by having access to accurate relevant geotechnical data. This assists the HDD contractor with tooling selection, fluid properties and drilling parameters - which will produce cuttings that can be efficiently removed from the borehole, with reduced risk of hydro-fracture. Hydro fracture or “frac out” is when drilling fluids escape from the bore hole to the surface.

During the design phase a qualified drilling engineer can provide valuable input, potentially reducing risk, and providing a design which is easier to and simpler to install. This can include using virtual hydraulics software to conduct a hydrofracture analysis; which can be used to benefit the overall design, potentially lengthening shots, and providing greater security in waterway and sensitive area crossings. 

Fluid design

The value that an effective drilling fluid brings to the job cannot be overestimated - as drilling fluid has a number of roles to play. These include; transmission of hydraulic energy to the bit, lubrication and cooling, stabilisation of the borehole and transporting cuttings out of the bore hole. 

One of the key roles fluids play is maintaining cuttings in suspension, during rod changes and when the pumps are turned off. Inadequate cutting suspension leads to cutting bed development, annular pack and an increased risk of hydro fracture or stuck pipe. Here the key is optimising the fluids static gel structure formation, or ability to rapidly gel and suspend cuttings, when flow ceases. The fluid's ability to rapidly gel when flow ceases is determined using a rotational viscometer.

An effective fluid will be designed to take into account challenges provided by the design; 

• a long installation length requiring extended cuttings transport time

• significant grade changes “dog-legs”

• changes in bore geometry resulting in flow velocity fluctuations 

• the size and concentration of the cuttings produced by the tooling and fluid jet impact force.

To effectively transport cuttings; the HDD contractor must balance the drilling advance rate, pump flow rate and the fluid’s dynamic Yield Point and low shear rate viscosity determined using a rotational viscometer. 

Finally, drilling fluid allows the contractor to stabilise the borehole, by applying pressure to the borehole walls, and by creating a filter cake controlling the flow of water across the borehole wall from the annular region. This control of water across the borehole wall is the primary tool for maintaining borehole stability. The biggest cause of instability is water crossing into the formation and destabilising the formation. Here the key is the fluid’s ability to form a low permeability filter cake, which is evaluated using an API filter press.

The larger and more critical the bore - the more sophisticated the fluid design and management needs to be, in both planning and executing the installation. In large scale designs - early engagement with a qualified fluid engineer will provide valuable insight into drilling fluid design, and drilling parameters required, to provide efficient hole cleaning function.

The Pilot Phase

The pilot phase of the HDD process is often overlooked in importance, but is critical to the success of the following reaming and pipe pull stages. Errors or bad practice, during drilling of the pilot hole, have significant impacts on the successful reaming and pipeline pullback phases.

The pilot phase will be drilled using the most appropriate tooling for the formation. Access to geotechnical data allows an informed choice to be made, over the optimal tooling configuration. In hard ground a mud motor might be selected, or a rock bit for weaker rock formations, or a standard blade in softer soil and sands. 

Drilling mud is pumped at prescribed volumes, to remove the cuttings from the bore, as the formation is cut by the drill bit. During this time the newly established bore is at the most risk. The relatively narrow annulus created means that the down hole pressure is at its highest of any of the following stages of the HDD process. 

If this fluid circulating pressure exceeds the formation fracture pressure, then micro fractures will occur - allowing fluid to escape into the formation, and potentially to the surface, causing  damage to infrastructure and environmental damage. Hole cleaning function is heavily influenced by annular velocity, but flow rates are also proportional to pressure - so the pump rate must be planned and managed accurately during the pilot phase. Where insufficient fluid pump rates are implemented, the cutting concentration will be increased, and poor transportation will result. This can quickly lead to annular pack off in the small pilot hole, and induce hydro-fracture.

It is key to note; any hydro-fracture event, induced on the pilot phase of the HDD process, has significant implication for the remainder of the pilot hole, and subsequent reaming phase. As fluid carrying the cuttings passes a zone of the borehole where hydro-fracture has previously occurred, some flow will be lost into the fracture. This results in a drop in annular velocity and build up of cuttings at this location. Eventually, full annular pack off occurs at these zones, resulting in full mud flow into the fracture which often presents at surface. This also increases the chance of a stuck pipe. 

The correctly designed drilling fluid, that has been optimised for the specific borehole design, rig capabilities, tooling and methodology, will result in good cuttings transportation from the borehole during the pilot phase, and significantly increase the likelihood of a successful HDD installation. 

Whilst a hydro fracture is typically caused by damage to the formation in the pilot phase, it may only become evident during the ream (a lower pressure process but with higher volumes passing the damaged area). A hydro fracture is most effectively healed during the pilot phase, where lost circulation materials can be applied most accurately to the damaged area. It is normally possible to treat a hydro fracture discovered during the pilot phase, however specialised technical support is required to manage the process.

Once the pilot bore has been successfully installed, with circulation of drilling fluids back to the rig, the reaming phase begins. 

Part 2

In HDD installation methodology; cleaning the bore - Part 2 we dive into reaming and pipe pull. Our team are available if you have any questions around project design, fluid design and the pilot phase. We work closely with councils, engineers and contractors on all stages of the design and installation process. Give our Australia team a call on 03 9068 5688, or our New Zealand team on 07 849 2366.