Understanding the Correlation and Limitations of Turbidity (NTU), Total Suspended Solids (TSS), and Maximum Particle Size in Completion Fluid Filtration
Abstract
In oilfield operations, particularly during well completion and workover activities, fluid cleanliness is paramount. While Turbidity (NTU) and Total Suspended Solids (TSS) are commonly used indicators to assess fluid quality, these metrics alone may not provide a complete picture of a fluid’s potential to damage the formation. Furthermore, maximum particle size in the fluid—often overlooked—plays a crucial role in formation plugging. This paper explains the limitations of relying solely on NTU and TSS, emphasizes the importance of controlling particle size through high-efficiency filtration, and outlines best practices to ensure well integrity and maximize production.
Introduction
The primary objective of completion and workover operations is to enable, restore, or optimize the safe and efficient production or injection of fluids from or into a well, while preserving reservoir integrity and maximizing well performance over its productive life. Fluids used during these operations must be free of harmful solids that can plug the formation, reduce permeability, or alter wettability. Industry practice has traditionally relied on NTU readings to estimate cleanliness, and in some cases TSS measurements. However, neither NTU nor TSS captures particle size distribution, which is often the direct cause of reservoir plugging.
Key Metrics in Filtration: NTU, TSS, and Particle Size
Turbidity (NTU)
- A rapid, optical method for estimating the presence of suspended particles based on light scattering. Simple low cost hand hold device can measure accurate the turbidity.
- Sensitive to particle size, shape, and composition, but cannot indicate actual mass or count.
Total Suspended Solids (TSS)
- Measures the mass of retained solids per unit volume (mg/L) through filtration.
- Directly quantifies loading but gives no indication of individual particle size.
Maximum Particle Size
- The largest particle present in the fluid.
- A single oversized particle—even in trace amounts—can plug pore throats or gravel pack openings, especially in low-permeability formations.
The Danger of Large Particles in Visually Clear Fluids
It is a common misconception that clear fluids (low NTU) are always safe. A fluid may appear clean but contain large, isolated particles that evade optical detection and cause irreversible damage to the formation.
- Example: A 100 µm particle in a 1 NTU fluid will not significantly affect light scattering but can block pore throats in a 100 mD formation.
- The relationship between pore throat size and maximum allowable particle size must guide filtration selection. A general rule of thumb:
- For 1000 mD formations: max particle size ≤ 100 µm
- For 100 mD: ≤ 40 µm
- For <10 mD: ≤ 10 µm or less
Field Data: TSS and NTU Are Inconsistent Predictors of Solids Risk
A study in West Africa on various brine and base waters showed:
- A strong correlation (R² = 0.996) between NTU and TSS in base waters
- Moderate correlation (R² = 0.765) in brines
- Weak correlation (R² = 0.186) in brine + additive mixtures
This proves that neither metric alone reliably reflects dangerous particle presence.
Recommendations for Field Practice
- Use all three metrics: Monitor NTU, TSS, and particle size.
- Determine max allowable particle size from permeability data or core samples.
- Always use Beta 5000-rated filters for final polishing stages.
- Employ particle size analysis tools, such as laser diffraction or microscopy, especially in offshore completions.
Conclusion
Completion fluids must be genuinely clean—not just clear to the eye or within acceptable weight limits, but free of particles that can cause irreversible formation damage. While turbidity (NTU) and total suspended solids (TSS) provide helpful indicators of fluid cleanliness, they are insufficient when used alone. These measurements do not account for maximum particle size, which is a critical factor in protecting the permeability of the reservoir.
To safeguard well integrity and ensure efficient production, filtration strategies must prioritize high-efficiency, size-specific particle removal. This requires the use of Beta-rated filter cartridges, specifically rated at Beta 5000 or higher, which guarantee removal efficiency of 99.98% for the target particle size. The appropriate micron rating—typically 2, 5, or 10 microns—should be selected based on the formation’s permeability to prevent pore-throat plugging and minimize formation damage.
It is essential that such filters are sourced from trustworthy manufacturers who can guarantee consistent filtration performance. Only then can the fluid quality be assured before it enters the wellbore.
Although NTU meters do not directly measure particle size or count, they remain an invaluable operational tool. When properly calibrated and used after filtration, an NTU reading can provide a quick indication of system performance. A sudden increase in NTU levels typically signals a filtration issue—whether due to bypassing, filter failure, or upstream contamination.
In short, proper filtration using high-efficiency, absolute-rated filters, combined with NTU-based monitoring, is not merely good practice—it is critical insurance against formation damage, reduced production, and costly remediation.
