De-Risking Shallow Hazard Assessment: Novel Approaches Using Innovative Pulsed Neutron Logging
Authors: Lerdsuwankij Thiti , Cavalleri Chiara , Wanwises Juntra 
Published: February 2024

Pulsed neutron logging can be used for evaluating shallow gas hazards before initiating new infill drilling projects. The evaluation can be performed in existing platform through completed wells; however, the use of conventional pulsed neutron tools in old wells with multiple casing and tubing often yields to inconclusive results, leading to project delays and safety concerns. The use of an innovative pulsed neutron spectroscopy logging tool helps address these challenges. It resolves inconclusive anomalies detected by conventional neutron logs, providing confident results using new measurements, including fast neutron cross section (FNXS) and high precision spectroscopy elemental yields. This paper discusses interpretation challenges, and solutions in shallow hazard evaluation, and presents successful examples showcasing the effectiveness of the innovative pulsed neutron tool application.

Featuring a stronger minitron output improving the statistical significance of acquired data, and the addition of a deep specialized detector, facilitating data acquisition in multi-string configurations, the innovative technology offered several advantages when deployed in wells where conventional tools’ results were ambiguous. The measurement of fast neutron cross section (FNXS) assists in differentiating real formation gas from tight formations. Additionally, spectroscopy data obtained through hybrid logging mode provides insights into complex materials in the annulus behind the casing, identifying fluids, cement, or solid barite using elements such as H, O, Ca, Ba, and S. This information is crucial for interpreting borehole trap gas and explaining trapping mechanisms.

Relevant examples demonstrating the efficacy of the innovative pulsed neutron logging tool are presented. Example 1 illustrates a case where a new measurement called fast neutron cross-section assists in confirming the presence of gas through multi-string completion. Example 2 showcases a well with triple casing, where conventional logging detected two small anomalies but couldn't clarify the fluid content. By employing the new tool, a clear water zone was identified at the top anomaly, while the bottom anomaly exhibited a distinct gas response. Example 3 presents results obtained in an air-filled tubing with three-string completion, highlighting the interpretation of formation gas and multiple borehole-trapped gases behind the annulus below a mixed cement-barite layer. Example 4 demonstrates another result obtained in an air-filled tubing within two and three-string completion, showing the mechanism of borehole-trapped gas below solid barite in the annulus. Example 5 shows the applicability of innovative technology to confidently exclude the presence of gas trapped in one or more annuli in 3 concentric casings plus tubing for a few wells in a plug and abandon (P&A) project, ensuring safe operations.

These examples highlight the capabilities of the innovative pulsed neutron logging tool, incorporating new measurements such as FNXS and spectroscopy elemental yields, from improved hardware. These advancements in interpretation methodology enable more accurate and informed decision-making in drilling operations or P&A, reducing project delays, and enhancing safety.