Portable XRF Calibrations in Well Site Geochemistry

Exploration and production (E&P) of unconventional oil and gas requires advanced and innovative technologies. Horizontal drilling and fracking are examples of innovations that opened the door to unconventional hydrocarbon resources. Well site geochemistry is another new technique that can provide invaluable and near real-time data, with practical applications from precise rock identification to mineralogy and mineral modeling, chemostratigraphy, geo-steering, reservoir characterization (permeability, porosity, fracture population, brittleness) and even productivity (TOC inference). This technique is not unknown in E&P and is currently used in chemostratigraphy and mud logging.

The data for well site geochemistry is commonly provided by using laboratory instruments, an often expensive and time-consuming process. A supplemental technique is portable X-ray fluorescence (PXRF), which uses small, portable and rugged handheld or benchtop analyzers. These analyzers can provide timely data on a variety of samples, from drill cuttings to cores and outcrops, without the need for a highly skilled workforce.

3 Calibration Methods

When using PXRF, a common question is, “Do I need to calibrate the analyzer?” There are three calibration methods that are used in PXRF: Empirical, Fundamental Parameter (FP), and Compton Normalization.

Both FP and Compton Normalization calibrations are performed at the factory.

Empirical calibration needs known samples (e.g.. standards) and uses the correlation between the readings obtained from these samples and their known values in order to quantify elements in unknown samples. This calibration can provide precise and accurate assay data for a narrow range of sample types and matrices. Unreliable data can be created, however, if the sample type or matrix differs from the ones that were used for calibration.
Compton Normalization only works with a sufficient scatter (Compton Peak) in a spectrum related to the sample matrix. As the average atomic mass of the sample increases, the Compton peak decreases. Therefore, samples with a high percent of heavy elements cannot be analyzed with Compton Normalization. As a result, this calibration is used for very specific sample types.
FP calibration is an algorithmic approach based on the predictable detector response to known parameters, such as sample geometry, source energy, distance and angle between the detector and the sample, as well as other factors. FP calibration is preferred when a wide range of samples and matrices are analyzed. In such sample sets, FP can provide very precise data with good accuracy. Accuracy in the analysis can be increased by a cal-factor adjustment, which can be done internally in handheld analyzers and externally in FXL analyzers. The cal-factor adjustment uses a few known samples to adjust calibration by changing the slope and y-intercept of the factory calibration.

Read the application note, Factory-set FP Calibration Provides Precise Results in Well Site Geochemistry for data and analysis demonstrating the use of a portable XRF analyzer to deliver reliable, repeatable and accurate data on elements from Mg to U using FP factory calibration.