Not All Uniformity Index Is Created Equal

By Seismos Team

The Same Metric, Two Different Measurements

Uniformity Index has become a standard metric for evaluating cluster efficiency. But not all UI values are calculated the same way. Two fundamentally different approaches exist — and understanding what each one actually measures is critical for interpreting results correctly.

The distinction is not about which method is better. It is about what question each method answers.

Imaging-Based Uniformity Index

Post-frac downhole imaging captures high-resolution measurements of perforation geometry after the treatment. A camera or ultrasonic tool is run into the wellbore on wireline, and the final diameter of each perforation is measured precisely.

To calculate UI from this data, an initial perforation diameter is assumed based on the perforating charge design. The growth of each perforation — the difference between the assumed starting size and the measured final size — is calculated for every cluster. The variation in growth across clusters produces the Uniformity Index.

What it tells you: How similarly perforations eroded during the stimulation. A UI close to 1 means clusters experienced comparable erosion. A lower UI means erosion varied significantly between clusters.

What it requires: A downhole tool run after the frac (typically wireline-conveyed), and an assumed starting hole size based on charge specifications.

Acoustic Friction-Based Uniformity Index

Acoustic measurement calculates UI from perforation friction data captured during active pumping. Surface-mounted sensors measure acoustic signals that allow the system to isolate perforation friction from pipe friction in real time.

From the measured friction and flow rate, the system calculates an effective starting perforation diameter — derived from the data, not assumed from the charge design. As pumping continues, additional friction measurements track how perforations evolve. The system calculates slurry volume through each cluster based on the resulting friction profile. The variation in volume distribution produces the Uniformity Index.

What it tells you: How evenly slurry was distributed between clusters during the treatment. A UI close to 1 means fluid was distributed proportionally across clusters. A lower UI means some clusters received significantly more flow than others.

What it requires: Surface-based acoustic sensors installed before the stage. No downhole hardware, no assumed starting geometry.

Why the Distinction Matters

Perforation erosion and fluid distribution are related — but they are not the same thing. A perforation that erodes more may have taken more fluid. But erosion is also influenced by proppant concentration, fluid chemistry, formation hardness, and time. Two clusters with identical final diameters may have received different cumulative volumes. Clusters with different erosion profiles may have received similar volumes at different points in the stage.

This means a UI calculated from post-frac geometry and a UI calculated from real-time flow distribution can produce different values for the same stage — and both can be correct. They are measuring different physical quantities.

For operators comparing data across technologies or evaluating completion performance, understanding which UI is being reported is essential. A direct numerical comparison between the two can be misleading if the underlying measurement basis is not accounted for.

Different Questions, Different Use Cases

Post-frac imaging provides a definitive geometric record of what happened to perforations. It is particularly valuable for validating perforating system performance, evaluating charge designs, and understanding erosion patterns in specific formations. It is a diagnostic tool — it tells you what happened after the fact.

Acoustic friction-based measurement provides flow distribution data during the treatment. It is particularly valuable for real-time completions optimization, intra-stage intervention decisions, and closed-loop fracturing workflows where the system needs to act on cluster-level data while pumping is still underway.

The industry benefits from both. The key is understanding which measurement each UI value represents — and not treating them as interchangeable numbers.