Why Horner Plots Aren’t Appropriate for Negative or Inflow Testing

August 30, 2021

Negative testing, also known as inflow testing, offers oil and gas companies a reliable way to test well barrier integrity and remain in compliance with the Code of Federal Regulations (CFR) Title 30 § 250.721(g). During a test, displacement occurs through the use of a lower density completion fluid — thus lowering pressure — and the well barrier is monitored for leaks.

For a long time, Horner analysis has served as the industry standard for the study of negative test results, with the Horner plot being the de facto choice for recording such results. There’s now evidence to suggest, however, that this has been a mistake. The truth is Horner analysis and Horner plots aren’t just discouraged in this testing scenario — they really shouldn’t be used at all. Here’s why.

Horner Plots Take a Lot for Granted

Horner plots are most often used to test buildup pressure. Results are recorded onto a graph with a sloped line, where the line, according to Crane’s Petrophysical Handbook, represents “a cross plot of buildup (or drawdown) pressure (Pi) on the Y-axis versus a dimensionless time coefficient (HTi), usually called Horner Time, on a logarithmic X-axis.”

Horner analysis utilizes the same radial flow equation used to record pressure build up inside shut-in wells. Later, it was discovered this equation had enough in common with thermal diffusion equations for it to predict static formation temperature. When the connection was discovered that inflow tests created thermal effects and subsequently caused flowback, Horner analysis appeared to be a logical choice for interpreting test results.

The issue with this approach, unfortunately, stems from details that are seemingly ignored and assumptions that must be made. Use of Horner analysis — and the Horner plot — requires one to disregard temperature change, heat generation, and heat transfer between fluids. It also assumes system and formation temperature will remain the same with a change in pressure, formation temperature is not impacted by distance from the wellbore, and temperature is consistent at the wellbore. None of these are true.

As a result, even the tiniest tweaks to variables can result in a Horner plot trend line that is misinterpreted. This led James Peyton to conclude in a 2021 paper that Horner analysis “should not be used for analyzing flowback or pressure buildup during an inflow test.” Fortunately, a more accurate testing method exists.

Be Absolutely Sure With SureTec

Some methods of validating inflow tests, such as Horner plots, are either unsound scientifically or are prone to misinterpretation. This problem is eliminated with SureTec.

SureTec records tests with an unmatched level of precision and consistency. And because test results are digitally compared to the required pass-fail criteria, an objective result is produced.

Increased accuracy in testing is not the only benefit of going digital with SureTec. Our SureView platform enables viewing of testing data locally or remotely.

And if you need help with any of our solutions, IPT is ready to help. Experts at our Real-Time Operations Center are on hand 24/7 to provide support and answer any questions.

Contact IPT today to learn how digital solutions like SureTec and SureView can make negative testing more accurate and help you access your data from anywhere.

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