A Step Change in Neutron-Induced Gamma Ray Spectroscopy: Using a High-Resolution LaBr3:Ce Detector in an Integrated LWD Tool
Authors:   SLB, Haranger Fabien , Allioli Francoise ,   SLB, Berheide Markus ,   SLB, Craddock Paul ,   SLB, Øpsen Daniel Finnvik ,   Neptune Energy, Grau James ,   Consultant, Horstmann Mathias ,   SLB, Maggs David ,   SLB, Mauborgne Marie-Laure ,   SLB, Pallain Alexis ,   SLB, Radtke Richard J. ,   SLB, Rodriguez Rubi ,   SLB, Rose David ,   SLB, Rouanet Benjamin ,   SLB, Sergeyeva Viktoriya ,   SLB, Stoller Christian ,   Consultant
Published: November 2024

A consistent approach to evaluating mineralogy and petrophysical attributes through a field’s life cycle is essential to ensure proper assessment of the reservoir potential. Accurate mineralogy provides a deep understanding of the sedimentological and depositional environment and allows the identification of weaker or more friable formations that can pose challenges to drilling and completion operations. Advanced logging-while-drilling (LWD) measurements are taken to provide an accurate evaluation of complex clastic and carbonate lithologies. This is done when standard services are inconclusive or ambiguous and deploying wireline is not viable because of highly deviated boreholes. Neutron-induced gamma ray spectroscopy is a cornerstone for evaluation in these complex reservoirs as it provides the essential rock matrix information for an accurate petrophysical interpretation. An upgrade of our LWD tool equipped with a pulsed-neutron generator provides the radical improvement needed to get closer to wireline-quality lithology results. This major advance is made possible by replacing the sodium iodide (NaI:Tl)-based spectroscopy detector with an advanced ruggedized lanthanum bromide (LaBr3:Ce) detector combined with refined spectral acquisition. LWD-specific challenges affecting the measurement, such as drilling conditions and spectral tool background contribution, are addressed. This combination significantly improves spectral quality, precision, and accuracy. Neutron-induced gamma rays of the different elements have unique signatures that can be used to detect and quantify the presence of these elements in the formation. A major advantage of the spectroscopy measurement lies in the fact that the interpretation does not require user input. The measurement is self-consistent, improving answer quality while also reducing interpretation errors or biases. Precise identification of the elemental constituents opens the door for an accurate lithology determination and a holistic approach to processing and interpreting with the assistance of trained neural networks. This maximizes the interpretation efficiency and the reliability of the answer products. The downhole spectral processing delivers high-definition spectroscopy while drilling and provides substantial operational gains and essential input for real-time decisions. The new spectroscopy measurement is currently being field tested with several operators worldwide in a variety of lithologies from clastics to carbonates. The results confirm the predicted major improvement in the accuracy and precision of the measured elemental concentrations and the resulting enhancements in the lithology determination. In some cases, comparison with the results from a large-diameter LaBr3:Ce-based wireline spectroscopy is available, and good agreement has been found. Furthermore, obtained LWD logs were benchmarked against coreDNA and XRD/XRF measurements. The measurement technology is presented, and field test results are discussed, providing a first look at the exciting new technology. This technology promises to provide high-quality lithology answers in LWD, leveraging the collocation of the spectroscopy measurement with the neutron porosity, sigma, neutron gamma density, and resistivity measurements.