De-Risking Shallow Gas Hazards: Case Studies on the Effectiveness of an Innovative Pulsed Neutron Logging Tool
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. However, the use of conventional pulsed neutron tools in old wells of existing platform and through multiple casing and tubing, often yields to inconclusive results, leading to project delays and safety concerns. To address these challenges, an innovative pulsed neutron logging tool was introduced. It resolves inconclusive anomalies detected by conventional neutron logging, providing confident results with the fast neutron cross section (FNXS) and supporting spectroscopy elemental yields. This paper discusses interpretation challenges, limitations, and solutions in shallow hazard evaluation, and presents successful examples showcasing the effectiveness of the innovative pulsed neutron tool.

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 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 clear gas response. Example 2 demonstrates the tool's ability to differentiate between formation gas (a drilling hazard) and gas trapped in the borehole (not a drilling hazard) in a shallow depth anomaly case. Detailed analysis revealed borehole gas trapped beneath the cement layer above the tubing centralizers within the annulus between the 7″ casing and 3.5″ tubing. Example 3 presents results obtained in an air-filled tubing with three-string completion, showcasing the interpretation of formation gas and multiple boreholes trapped gases behind the annulus below a mixed cement-barite layer. Example 4 shows the applicability of the innovative technology to confidently exclude the presence of gas trapped in one or more annulus in 3 concentric casings plus tubing for few wells in a plug and abandon (P&A) project and ensure 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.