Pulsed-Neutron Logging-While-Drilling Tool Enables Direct Carbon Measurement

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 228060, “A Novel Pulsed-Neutron LWD Geochemical Logging Tool With Sigma and Direct Carbon Measurements,” by Gregory Schmid, SPE, Jiaxin Wang, and Jeffrey Crawford, Halliburton, et al. The paper has not been peer-reviewed. _ A novel 6.75-in. logging-while-drilling (LWD) geochemical tool has been developed for accurate lithology, mineralogy, well-placement, and geosteering applications in complex reservoirs. This LWD tool uses capture and inelastic gamma rays induced by a pulsed-neutron generator to provide real-time formation elemental and mineralogical concentrations and sigma and direct carbon measurements. Tool Description General Design. The LWD tool is available in a 6.75-in. nominal size. A deuterium/tritium pulsed-neutron generator produces fast (14.1‑MeV) neutrons. The neutrons interact with the formation to generate gamma rays that are scattered back into the tool and recorded by a scintillation detector. The collar around the detector is surrounded by a thermal neutron shield containing boron, reducing the thermal neutron interactions with the tool body. Stabilizers are placed on the top and bottom of the tool body to maintain tool position in the borehole. Neutron Generator. An LWD-rated, ruggedized pulsed-neutron generator operates with a customized timing scheme to emit bursts of high-energy neutrons into the formation. During the neutron pulse, fast neutrons collide with nuclei, exciting them and causing the immediate emission of high-energy inelastic gamma rays. After the pulse ends, the remaining neutrons lose energy and become thermalized. These slow (thermal) neutrons are then captured by nuclei, resulting in the emission of characteristic capture gamma rays. By measuring these capture gammas in the interval between pulses, the tool can identify and quantify elements sensitive to neutron capture. After a series of pulses, the generator will remain off for a prolonged period, and the sigma (thermal neutron cross section) of the formation is measured. Electronic System. The electronic system is designed to control the neutron-tube yield and provide timing synchronization on the detector side for spectroscopy measurements. In addition to stability and precision at high count rate, the other important aspects considered in the system design are fast settling time and automation. Three subsystems are involved: the neutron-generator subsystem, generator-controller subsystem, and gamma-detector subsystem. These subsystems are detailed in the complete paper. Data Acquisition and Processing The pulsed‑neutron generator’s burst‑on/burst‑off sequence is repeated 200 times in a 20-ms time frame followed by a 5-ms idle time where sigma decay and background windows are located. The entire 25-ms time frame is repeated 40 times per second. Time-gated energy spectra are acquired during the burst, capture, and background time windows in 512 channels for an integration period of 1 second. In addition, during this 1-second integration period, a time-decay spectrum is acquired during the sigma-decay window. The energy and time spectra from the 1-second integration constitute a raw spectral sample that is passed to spectral‑fitting processing.