How Drilling Fluid Helps Prevent Borehole Collapse During Horizontal Directional Drilling (HDD)

Horizontal Directional Drilling (HDD) is widely used to install pipelines beneath rivers, roads, railway lines and other obstacles without excavating an open trench across the entire route. During construction, engineers drill an underground opening, enlarge it to accommodate the pipeline and eventually pull the pipe through the completed borehole.

For this process to work, the borehole must remain stable enough to support drilling, reaming and pipeline installation. However, underground conditions can change as drilling progresses, particularly when the route passes through loose sand, gravel, clay or fractured rock.

One of the challenges engineers must manage is borehole collapse, which occurs when surrounding soil or rock moves into the drilled opening. This can increase drilling resistance, restrict circulation, trap equipment or make pipeline installation more difficult.

This is where drilling fluid plays an important role. Beyond cooling drilling tools and transporting excavated material, a properly formulated drilling-fluid system helps support the borehole, manage fluid loss and maintain suitable conditions underground.

Understanding how drilling fluid works, what affects its performance and how engineers manage borehole instability is important for contractors planning HDD projects, particularly large-diameter pipeline crossings.

During Horizontal Directional Drilling (HDD), engineers first create a smaller underground opening known as a pilot hole. The opening is then enlarged through reaming to provide sufficient clearance for the pipeline, and throughout these stages, the surrounding geological formation must remain stable enough to maintain the required borehole dimensions.

Borehole collapse occurs when soil or rock surrounding the drilled opening becomes unstable and moves into the borehole. This may happen gradually, with loose material accumulating inside the opening, or more suddenly when part of the surrounding formation loses stability.

Minor ground movement may increase drilling resistance or interfere with fluid circulation, while more serious collapse can obstruct the borehole, trap drilling equipment or prevent the pipeline from being installed.

Not every underground obstruction indicates borehole collapse, however. Accumulated drill cuttings, unexpected rock formations and mechanical problems can produce similar symptoms, which is why engineers examine drilling performance, geological conditions and fluid circulation before deciding how to respond.

HDD drilling fluid performs several functions that help maintain suitable conditions within the borehole. Its effectiveness depends on the geological formation, the properties of the fluid and how circulation is managed during construction.

Supporting the Borehole Wall

As drilling removes soil or rock, the surrounding formation may require support to prevent loose material from entering the opening. Drilling fluid helps maintain pressure within the borehole, while Bentonite-based systems can form a filter cake along suitable formations.

This filter cake helps limit the movement of liquid into the surrounding ground and can contribute to borehole stability. The level of support required depends on the formation, groundwater conditions and pressure within the borehole.

Transporting Excavated Material

Every stage of drilling produces cuttings that must be removed from the borehole. Drilling fluid carries these particles towards the surface, where they can be separated from the circulating fluid.

If the fluid does not transport cuttings effectively, material may accumulate underground, increasing resistance and making it more difficult for drilling tools to move through the opening. Although cuttings accumulation is different from borehole collapse, both can interfere with drilling and pipeline installation.

Managing Fluid Loss

Some geological formations allow drilling fluid to escape into the surrounding ground. Sand, gravel and fractured rock can create pathways through which fluid leaves the intended circulation route.

When fluid loss becomes excessive, maintaining suitable circulation and borehole conditions becomes more difficult. Filtration control is therefore an important consideration when preparing drilling fluids for permeable formations.

Maintaining Suitable Circulation Pressure

Drilling fluid must circulate at a pressure sufficient to transport excavated material and support the drilling operation. However, excessive pressure can create additional problems, including hydraulic fracturing and unintended fluid returns.

Engineers must therefore balance circulation requirements with the pressure limits of the surrounding formation. The appropriate operating conditions depend on the crossing depth, geological characteristics and borehole geometry.

Although drilling fluid plays an important role in borehole stability, several other factors influence how the surrounding ground behaves during construction.

Loose Sand and Gravel Formations

Loose sand and gravel can be difficult to maintain as an unsupported underground opening because the individual particles may have limited cohesion. As drilling removes material, surrounding particles can move into the borehole, particularly where groundwater is present.

A suitable drilling-fluid system can help support the formation and limit fluid penetration, but the drilling method and borehole design must also account for the ground conditions.

Changes in Geological Conditions

An HDD crossing may begin in sand, pass through gravel and encounter clay or rock further along the route. Each formation responds differently to drilling, and the fluid properties required in one section may not be suitable for another.

For example, clay can affect viscosity and adhere to drilling equipment, while fractured rock may create difficulties with fluid circulation. Engineers monitor these changes and adjust drilling parameters or fluid formulations when necessary.

Groundwater Movement

Groundwater can influence the stability of loose or permeable formations. Water movement may contribute to the displacement of soil particles, while differences between groundwater pressure and borehole pressure can affect the surrounding ground.

Geotechnical investigation helps engineers understand these conditions before drilling begins, allowing them to plan the borehole geometry and fluid system around the expected groundwater environment.

Borehole Design and Reaming

The size and geometry of the borehole also influence stability. Large-diameter pipelines require wider underground openings, meaning more material must be removed during reaming.

As the borehole becomes larger, effective cuttings transport and fluid circulation become increasingly important. Engineers must therefore coordinate borehole design, reaming procedures and drilling-fluid performance to maintain suitable conditions for pipeline pullback.

The effectiveness of a drilling-fluid system depends on more than the amount of fluid being pumped into the borehole. Its properties determine how well it transports cuttings, controls filtration and responds to changing geological conditions.

Viscosity describes a fluid's resistance to flow and influences its ability to carry excavated material. If viscosity is too low for the conditions, cuttings may settle within the borehole, while excessive viscosity can increase pumping resistance and interfere with circulation.

Gel strength describes the ability of a drilling fluid to develop structure when circulation slows or stops. Suitable gel strength helps keep cuttings suspended during interruptions, reducing the likelihood of material settling inside the borehole.

Filtration control influences how much liquid enters permeable formations. A properly formulated Bentonite-based fluid can develop a filter cake along the borehole wall, helping limit liquid movement into the surrounding ground.

Fluid density and circulation pressure must also be considered because they influence the pressure acting on the formation. The drilling-fluid system must provide suitable operating conditions without exceeding the limits of the surrounding ground.

These properties are connected, which means adjusting one part of the formulation can affect how the fluid behaves during drilling. Regular testing helps engineers determine whether the system continues to meet the requirements of the crossing.

A drilling-fluid formulation that performs well in one geological formation may need to be adjusted when conditions change. The objective is to maintain suitable circulation, cuttings transport and borehole support throughout construction.

Bentonite is commonly used as the foundation of water-based HDD drilling fluids because it helps develop viscosity and filtration-control properties. At PCE Nigeria, our BRSBENT SQ activated Bentonite provides a base material for these formulations, while BRSCMC can support additional filtration control where permeable formations allow excessive liquid to enter the surrounding ground.

However, filtration control is only one part of drilling-fluid performance. When drilling through rock, excavated particles may require greater suspension capacity to prevent them from settling inside the borehole. Additives such as BRSMMH can support fluid stability and suspension under suitable conditions.

Clay formations present different challenges because excavated clay can increase viscosity and adhere to drilling tools. In these situations, BRSVR can help manage clay-related viscosity problems, while BRSXTG may be used where additional viscosity and suspension performance are required, including certain high-salinity conditions.

These materials form part of our Brighter Star drilling-fluid range. Their suitability depends on the geology, water chemistry and operating requirements of the project, with each product serving a particular function within the fluid system.

For contractors, the important consideration is how the selected materials work together and whether the resulting formulation can maintain the properties required throughout drilling and reaming.

Because HDD operations take place underground, engineers cannot always observe borehole instability directly. Instead, they monitor drilling performance, fluid circulation and returning cuttings for changes that may indicate developing problems.

Some common warning signs include:

• Unexpected increases in drilling torque: Greater resistance may indicate changing ground conditions, accumulated cuttings or an obstruction.
• Higher pulling or pushing resistance: Difficulty moving drilling equipment may suggest instability or other mechanical problems.
• Reduced drilling-fluid returns: Changes in circulation may indicate fluid loss into surrounding formations or restrictions within the borehole.
• Unusual circulation pressure: Unexpected pressure changes can indicate restrictions or changes in underground conditions.
• Repeated difficulties during reaming: Abnormal equipment loads or obstructions may require further investigation.
• Changes in returning cuttings: Differences in the quantity or characteristics of excavated material can provide information about the formations being drilled.

These warning signs do not necessarily mean that the borehole has collapsed. Engineers must assess the available information to identify the cause before determining the appropriate response.

Managing borehole stability begins before drilling equipment arrives on site. Geological investigation, borehole design and drilling-fluid planning all influence how the underground opening will behave during construction.

Conduct Geotechnical Investigations

Geotechnical investigations help identify expected soil and rock formations, groundwater conditions and areas where borehole stability may be difficult to maintain.

This information supports decisions about bore path design, drilling equipment, fluid formulation and construction methods. For long-distance crossings, understanding geological variations is particularly important because conditions can change significantly along the route.

Prepare a Suitable Drilling-Fluid Programme

The drilling-fluid programme should be developed around the expected geology, crossing length, borehole dimensions and circulation requirements.

Engineers need to consider how the fluid will transport cuttings, control filtration and maintain suitable properties as drilling progresses. Water quality, mixing procedures and the availability of additives can also influence performance.

A properly prepared fluid programme should include arrangements for testing and adjusting fluid properties when the operating conditions change.

Monitor Drilling Conditions

During construction, engineers monitor drilling torque, pulling loads, circulation pressure, fluid properties and returning cuttings.

Unexpected changes may require adjustments to drilling parameters, fluid formulation or the construction method. Identifying developing problems early can help reduce the likelihood of more serious interruptions.

Coordinate Drilling and Reaming Operations

Borehole stability must be maintained throughout pilot-hole drilling, reaming and pipeline installation. The equipment and drilling methods selected for each stage should therefore be compatible with the planned borehole geometry and geological conditions.

For large-diameter installations, reaming procedures and drilling-fluid circulation must be coordinated because enlarging the borehole produces additional material that needs to be removed.

Long-distance HDD crossings place considerable demands on drilling-fluid circulation because excavated material must be transported through an extended underground route. These requirements become more significant when the pipeline diameter increases and the borehole must be enlarged through multiple stages.

On the AKK River Niger crossing, we installed a 40-inch gas pipeline along a 1,565-metre underground route through mixed sand, gravel and rock formations. Our construction approach included dual-rig intersecting technology, synchronous reaming and a specially developed drilling-fluid formulation suited to the conditions of the crossing.

The OB3 River Niger crossing involved a larger 48-inch gas pipeline installed along an approximately 2,000-metre underground route. We also used a specialised drilling-fluid system alongside dual-rig drilling and reaming methods for this installation.

For crossings of this scale, drilling-fluid performance is closely connected to equipment operation, cuttings removal and borehole preparation. Maintaining suitable circulation throughout construction helps support the different stages required before the pipeline can be installed.

If borehole collapse or significant instability is suspected, the drilling team must first assess the available information to understand what may be happening underground.

This can involve reviewing drilling loads, circulation pressure, returning cuttings and the geological conditions encountered along the bore path. Depending on the problem, engineers may need to remove accumulated material, condition the borehole or adjust drilling parameters.

More serious instability may require further investigation or changes to the construction method. Continuing drilling without understanding the cause of an obstruction can increase equipment loads and worsen underground conditions.

For this reason, HDD construction planning should include monitoring procedures and contingency measures for responding to unexpected geological conditions.

Can drilling fluid prevent borehole collapse?

A properly formulated drilling-fluid system can help maintain borehole stability by supporting suitable pressure conditions, transporting cuttings and controlling filtration. However, it cannot eliminate every risk associated with unstable geological formations.

What drilling fluid is best for HDD borehole stability?

The appropriate drilling fluid depends on the geological conditions, groundwater, borehole dimensions and circulation requirements. Bentonite-based systems are commonly used, with specialist additives introduced where additional properties are required.

Why is Bentonite used in HDD drilling fluid?

Bentonite develops viscosity when mixed with water and can help form a filter cake along the borehole wall. These properties support cuttings transport, filtration control and suitable borehole conditions.

Can poor drilling-fluid circulation cause borehole problems?

Yes. Inadequate circulation can allow excavated material to accumulate inside the borehole, increasing resistance and potentially interfering with drilling, reaming and pipeline installation.

What causes an HDD borehole to collapse?

Borehole collapse can result from unstable ground formations, groundwater movement, unsuitable drilling-fluid performance or construction methods that do not adequately account for underground conditions.

How do engineers identify borehole instability?

Engineers monitor drilling torque, pulling resistance, fluid returns, circulation pressure and the characteristics of returning cuttings. Unexpected changes can indicate conditions that require further investigation.

Drilling fluid plays an important role in maintaining suitable borehole conditions during HDD construction. Its ability to transport cuttings, control filtration and support circulation influences how effectively drilling and reaming operations can be carried out.

However, fluid performance must be considered alongside geological conditions, borehole geometry, equipment selection and construction methods. A suitable formulation can help manage several causes of instability, but it must be selected and monitored according to the requirements of the crossing.

At PCE Nigeria, we provide HDD engineering and construction services alongside Brighter Star drilling-fluid products, including activated Bentonite and specialist additives for different geological conditions.

Planning an HDD project or looking for drilling-fluid materials suited to challenging ground conditions?

Explore our drilling-fluid products or contact our technical team to discuss your project requirements.

Project and product enquiries: wanyang@pcenigeria.com