HDD Drilling Fluid Testing: Sand Content vs Total Solids

During Horizontal Directional Drilling (HDD), drilling fluid plays an important role in maintaining suitable underground conditions. It helps transport excavated soil and rock out of the borehole, supports borehole stability and assists drilling and pipeline installation.

However, as drilling progresses, the fluid continuously comes into contact with the surrounding ground. Sand, soil, rock cuttings and other materials enter the circulating system, changing its composition and performance.

If these changes are not properly monitored, they can affect fluid circulation, increase equipment wear and create difficulties during drilling and reaming.

This is why HDD drilling-fluid testing is an important part of construction. Engineers need to understand the condition of the circulating fluid and determine whether it can continue performing its intended functions. Two useful measurements are sand content and total solids. Although both relate to solid material present in drilling fluid, they measure different things.

Understanding the difference helps HDD contractors assess fluid condition, manage solids-removal equipment and make informed decisions throughout construction.

Sand content is a measurement of the proportion of relatively coarse particles present in a drilling-fluid sample. The test typically uses a 200-mesh sieve with openings of approximately 75 micrometres. Particles retained by the sieve are collected and measured, with the result generally reported as a percentage by volume of the original drilling-fluid sample.

Although the test is called sand content, the retained material is not necessarily made entirely of sand. It can include rock cuttings or other particles larger than the specified sieve opening. The test therefore measures a particular particle-size fraction rather than identifying the mineral composition of every particle.

During Horizontal Directional Drilling (HDD), sand-content readings can change as the drill passes through different geological formations or as solids-removal equipment processes the returning fluid. Monitoring these changes helps engineers assess the condition of the circulating system.

Total solids refers to the solid material present in a drilling-fluid system, including materials intentionally added during preparation and solids introduced during drilling.

For example, a bentonite-based drilling fluid contains fine clay particles that contribute to its viscosity, suspension capacity and filtration properties. As drilling progresses, the fluid also collects excavated soil, sand and rock particles from the borehole. Both the intentionally added materials and the excavated solids contribute to the total solids present in the fluid. However, these materials do not all serve the same purpose.

Bentonite is intentionally added to develop useful drilling-fluid properties.

Drilled solids are introduced during excavation and must be managed to prevent excessive accumulation within the circulating system.

Total-solids assessment considers a broader range of material than sand-content testing. Depending on the fluid formulation and testing method, engineers may use retort analysis or other appropriate laboratory and field methods to assess the solids present. The measurement method must be clearly defined because dissolved salts, fluid composition and other factors can influence how results are calculated and interpreted.

A sand-content test alone cannot determine total solids because fine particles may pass through the test sieve while remaining suspended in the drilling fluid.

Sand content and total solids are related measurements, but they provide different information about the condition of drilling fluid.

These measurements should not be treated as interchangeable. For example, a bentonite-based drilling fluid may contain a substantial amount of fine clay while producing a relatively low sand-content reading.

Similarly, a fluid containing increasing quantities of coarse excavated material may show a rising sand-content result even when changes in its other properties are less noticeable.

This is why engineers evaluate drilling-fluid measurements together rather than relying on a single test to describe the complete system.

During pilot-hole drilling and reaming, excavated material enters the circulating drilling fluid. The fluid transports this material towards the surface, where solids-removal equipment can separate cuttings before the fluid is reused. If excessive coarse particles remain in circulation, they can create several problems.

Increased Equipment Wear

Sand and abrasive rock particles can contribute to wear in pumps, piping and other components of the drilling-fluid circulation system. Excessive abrasive material may increase maintenance requirements and affect equipment performance.

Reduced Solids-Control Efficiency

Increasing sand-content readings may indicate that coarse particles are entering the system faster than they are being removed. This can require engineers to review the performance of screens, separation equipment and fluid circulation arrangements.

Changes in Fluid Performance

Accumulated drilled solids can affect fluid density, viscosity and other operating characteristics. These changes may influence circulation performance and the ability of the fluid to transport additional cuttings.

Greater Challenges During Reaming

Reaming enlarges the borehole and can generate substantial quantities of excavated material. For large-diameter HDD installations, effective solids removal becomes particularly important as the volume of material increases.

Sand-content testing provides useful information about coarse particles in the circulating fluid. However, it should be interpreted alongside other measurements and drilling conditions.

Sand content is commonly measured using a dedicated sand-content testing kit. The equipment generally includes a graduated measuring tube, funnel and specified sieve.

The testing process follows several basic stages:

1. Collect a representative sample: Obtain drilling fluid from the designated sampling point using the approved procedure.
2. Prepare the sample: Measure and dilute the sample according to the testing-kit instructions.
3. Pass the mixture through the sieve: Allow fine material and fluid to pass through while retaining particles larger than the specified sieve opening.
4. Wash and collect the retained material: Transfer the retained particles into the graduated measuring tube using the prescribed method.
5. Record the result: Read and report the sand content as a percentage by volume.

Consistent sampling and testing procedures are essential. For example, a sample taken from a poorly mixed section of a tank may not accurately represent the fluid circulating through the drilling system. The sampling location should also be recorded because fluid collected before solids-removal equipment may produce different results from fluid collected after treatment. Engineers should follow the applicable testing standard, equipment instructions and project quality-control requirements.

Determining total solids generally requires a different approach from sand-content testing. A sand-content sieve only captures particles above a specified size. Fine bentonite and other smaller particles may pass through the sieve even though they remain part of the drilling-fluid system.

Total-solids assessment must therefore consider a wider range of material. Depending on the fluid system and purpose of testing, engineers may use methods such as retort analysis or appropriate calculations supported by laboratory measurements.

Retort testing separates a measured fluid sample into components through controlled heating and condensation, allowing the liquid and non-volatile portions to be assessed. However, the interpretation depends on the drilling-fluid formulation and the assumptions used in the test. Fluid-density measurements may also provide useful supporting information. An increase in density can indicate a change in the fluid's composition, but it does not independently establish how much sand, bentonite or other material is present. For this reason, engineers should avoid treating density as a direct substitute for total-solids analysis.

The value of drilling-fluid testing becomes clearer when results are monitored throughout an HDD operation. Consider two examples.

Example 1: Sand Content Increases During Reaming

During reaming, a drilling team observes that sand-content readings are increasing. This may indicate that more coarse excavated material is entering the circulating fluid or that the solids-removal equipment is not separating the material as effectively as expected.

Before adjusting the system, the team needs to consider the geology being drilled, circulation conditions, sampling location and performance of the separation equipment. The increasing reading identifies a condition that requires attention, but it does not establish the cause by itself.

Example 2: Fluid Density Increases but Sand Content Remains Low

In another situation, fluid-density readings increase while sand-content measurements remain relatively low. This may indicate that the fluid contains more fine suspended material or has experienced another change in composition.

However, further testing is required to determine the cause. The change cannot automatically be attributed to excessive sand. These examples demonstrate why drilling-fluid test results should be interpreted together with the actual drilling conditions.

A single measurement rarely provides enough information to describe the entire circulating-fluid system.

The amount and type of solids present in drilling fluid influence how the mixture behaves during circulation. Bentonite is intentionally introduced into many HDD drilling-fluid systems because it helps develop viscosity, transport cuttings and control filtration. Excavated material, however, enters the fluid as a result of drilling.

As drilled solids accumulate, they can alter density, viscosity and circulation characteristics. Coarse or abrasive material may contribute to equipment wear, while excessive fine solids can also affect rheological properties. The effects depend on particle size, concentration, mineral composition and the drilling-fluid formulation.

This makes it important to distinguish between useful additives and unwanted drilled solids. Effective fluid management involves maintaining the required drilling-fluid properties while controlling the accumulation of excavated material.

Long-distance HDD projects create demanding conditions for drilling-fluid circulation and solids management. As crossing length increases, excavated material must travel through a longer underground route before returning to the surface.

During reaming, the amount of material removed can increase significantly as the borehole is enlarged to accommodate the pipeline. These requirements become particularly important for large-diameter pipeline installations.

Our work at PCE Nigeria on the AKK and OB3 River Niger crossings provides examples of the scale involved in HDD construction.

AKK River Niger Crossing

The AKK River Niger crossing involved installing a 40-inch gas pipeline along a 1,565-metre underground route. The crossing encountered mixed sand, gravel and rock formations, creating different requirements for drilling, cuttings removal and fluid circulation.

We used dual-rig intersecting technology, synchronous reaming and a specially developed drilling-fluid formulation. The HDD installation was completed on 26 July 2025.

OB3 River Niger Crossing

The OB3 River Niger crossing involved a larger 48-inch gas pipeline installed along a 2,000-metre underground route. The crossing also encountered sand, gravel and rock formations.

We used dual-rig intersecting technology, synchronous reaming and a specially developed drilling-fluid system as part of our construction approach, completing the installation on 28 April 2026.

Both crossings required careful coordination of drilling, reaming, fluid circulation and solids management throughout construction.

Reliable drilling-fluid performance begins with selecting materials suited to the geological conditions and technical requirements of the crossing. However, material selection must be supported by appropriate preparation, testing and monitoring.

At PCE Nigeria, our Brighter Star drilling-fluid range includes activated bentonite and specialist additives used in different HDD applications.

Bentonite-based systems provide viscosity and filtration-control properties, while products such as BRSCMC can support additional filtration control where required. Other additives, including BRSMMH, may be used to improve suspension and cuttings transport, while BRSVR helps manage excessive viscosity associated with clay-rich formations.

Our BRSBENT SQ activated bentonite serves as a primary material in drilling-fluid formulations where suitable viscosity development and filtration control are required.

The appropriate products and formulations depend on ground conditions, water chemistry, drilling requirements and compatibility within the complete fluid system. Even with suitable materials, effective drilling-fluid management still requires testing, solids-removal equipment and continuous monitoring during construction.

Is sand content the same as total solids in drilling fluid?

No. Sand-content testing measures particles retained by a specified sieve, while total solids refers to the broader solid material present in the drilling fluid under the applicable testing method.

What sieve is used for drilling-fluid sand-content testing?

A commonly used sand-content test uses a 200-mesh sieve with openings of approximately 75 micrometres. The test reports the retained particles as a percentage by volume under the specified procedure.

Can fluid density be used to measure sand content?

No. Fluid density measures mass per unit volume. It cannot independently determine sand content or identify the composition of suspended solids.

Why does sand content increase during HDD?

Sand-content readings may increase when more coarse excavated material enters the circulating system or when solids-removal equipment does not remove the material effectively. The cause should be assessed using other drilling and fluid-monitoring information.

Does bentonite count towards total solids?

Yes. Bentonite contains fine solid clay particles that contribute to the properties of many water-based drilling-fluid systems. These particles are included in the overall solid material present in the fluid.

How often should HDD drilling fluid be tested?

Testing frequency depends on geological conditions, drilling stage, circulation requirements and the approved fluid-monitoring programme. Engineers should follow the project's testing procedures rather than assume one interval is suitable for every operation.

Sand content and total solids provide different but useful information about the condition of HDD drilling fluid. Sand-content testing helps engineers monitor a specified fraction of coarse particles, while total-solids assessment considers a broader range of material present in the fluid.

Neither measurement should be interpreted in isolation. Their value comes from combining reliable testing with an understanding of the geological conditions, drilling-fluid formulation, circulation system and construction requirements.

For HDD contractors, this supports informed decisions about solids removal, fluid treatment and maintaining suitable operating conditions throughout drilling and pipeline installation.

At PCE Nigeria, we supply Brighter Star drilling-fluid products, including activated bentonite and specialist additives, alongside technical support for HDD projects involving different geological and construction conditions.

Need drilling-fluid materials or technical guidance for an upcoming HDD project?

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

Product and project enquiries: wanyang@pcenigeria.com