Bentonite Drilling Mud: Why Higher Viscosity Does Not Always Mean Better Fluid-Loss Control

Adding more bentonite can make a drilling mud thicker without delivering the improvement in fluid-loss control that the contractor expected. When this happens, the next step is to examine the fluid’s preparation and filtration behaviour before making another concentration adjustment.

Viscosity and filtration answer different questions. One concerns how the fluid flows; the other concerns how readily its liquid phase passes through a permeable medium under specified conditions. Selecting bentonite drilling mud requires evidence of both.

What Does a Higher Funnel Reading Actually Tell You?

Marsh funnel time provides a convenient field comparison of flow behaviour. However, the reading is influenced by fluid density and gelation as well as the material suspended in the fluid. It is not a direct measurement of filtration performance. ^[1]

If used mud gives a longer funnel time than fresh mud, investigate what changed. Record density, solids accumulation, water additions and treatments. The practical question is whether the change supports cuttings transport and circulation within the equipment’s operating requirements.

Look at What the Fluid Leaves Behind

During filtration, liquid passes through the test medium while solids build a filter cake. Assess the collected filtrate and the remaining cake together. A substantial cake can still be porous and provide poor filtration control; thickness alone does not establish its effectiveness. ^[2]

For each comparison, record:

• Filtrate volume, test duration, pressure and temperature.
• Filter medium, filtration area and any required reporting correction.
• Cake thickness and observations of texture and uniformity.
• Sample preparation, hydration time and whether the fluid is fresh or used.

Equipment details can affect comparison. For example, Fann specifies that readings from its half-area filter press require conversion to the standard filtration area. Two reported volumes should therefore be compared only after confirming their test and reporting basis. ^[3]

Before Adding More Powder, Check the Preparation

Hard mixing water can interfere with bentonite performance. CETCO’s drilling-fluid guidance describes poorly performing bentonite fluids prepared in untreated hard water that produce thick, porous filter cakes. This makes water quality a useful starting point when filtration results fall below expectations. ^[2]

Review the water source, hardness, pH and conductivity, then document powder addition, mixing conditions and hydration time. Base any conditioning trial on the measured water chemistry rather than applying a fixed treatment to every supply.

For preparation-related checks, see our discussion of water quality and bentonite hydration.

Choose the Adjustment by the Property You Need

Filtration-control additives differ in their contribution to viscosity. Bentonite Performance Minerals describes its POLYSELECT POWER PAC-L as reducing filtration with minimal viscosity impact, while its PAC-R product reduces filtration and also increases viscosity. ^[4]

These product-specific examples help explain why additive selection should begin with the required adjustment. Check the candidate product’s technical data, compatibility and resulting fluid properties in the intended bentonite system.

Keep Filtration Loss Separate from Lost Circulation

Drilling fluid escaping into conductive fractures presents a different problem from filtrate passing through a permeable test medium. Fracture-related losses require assessment of the loss pathway and an appropriate response, which may include lost-circulation materials. ^[5]

Use a Small Trial to Decide the Next Treatment

A practical comparison can help separate preparation issues from formulation needs. Keep the water, equipment, measurement times and test methods consistent, and change one factor at a time.

1. Establish the baseline. Prepare the selected bentonite at a documented concentration. Measure flow behaviour, density, filtration and cake characteristics.
2. Investigate preparation. Where water analysis or mixing observations justify it, compare a controlled change in water treatment or preparation procedure.
3. Compare formulation adjustments. Evaluate a bentonite concentration change separately from a compatible filtration-control additive.
4. Assess the complete result. Compare filtration improvement with changes in rheology, mixing practicality and equipment requirements.

Agree on the evaluation criteria before testing. This gives the buyer and supplier a clearer basis for deciding whether a candidate grade or treatment offers a useful improvement.

Bring Your Current Results into the Material Inquiry

HERMANO supplies bentonite for water-based drilling fluids. For sample selection, share your drilling method, formation conditions, water analysis, current bentonite concentration and viscosity and filtration results with their test methods. Include the additives already in use and the issue you want to correct so that the trial can focus on the required performance.

Technical References

1. CETCO: Marsh Funnel and Cup. Factors influencing funnel readings and application-specific viscosity selection.
2. CETCO: Testing Times for HDD. Filtration testing, filter-cake observations and mixing-water effects.
3. Fann: Half Area Filter Press. Filtration area and conversion of reported filtrate volumes.
4. Bentonite Performance Minerals: POLYSELECT POWER PAC-L and POLYSELECT POWER PAC-R. Product-specific filtration and viscosity functions.
5. Albattat and Hoteit: A Semi-Analytical Approach to Model Drilling Fluid Leakage Into Fractured Formation. Research on fracture-related drilling-fluid losses.