Abstract

Neutron porosity logging is one of the most fundamental techniques used to perform the estimation of reservoir hydrocarbon reserves. Together with resistivity and gamma-gamma density measurements, these three constitute the "triple combo" well logging suite that is used to log almost all wells. In the case of classical neutron porosity measurement, two neutron detectors are used to measure the flux of thermal neutrons created by the fast neutrons emitted by a chemical neutron source in the process of their interaction with the formation. The obtained flux ratio depends on the hydrogen concentration and enables the determination of porosity.

Currently, almost all available neutron porosity logging-while-drilling (LWD) tools use He-3 detectors to detect neutrons downhole due to their mechanical robustness and the absence of the limitations to operate at high temperatures. Unfortunately, the lack of sufficient quantities of the He-3 isotope caused by the depletion of its stockpile accumulated during the Cold War makes this material unavailable to well logging industry for the next 3 to 5 years. Among all other available neutron detection technologies, only Li-6 scintillation detectors do not have limitations on neutron detection efficiency that would prevent them from consideration for LWD applications.

The key component of Li-6 scintillation detector is the scintillation material containing Li-6 isotope. To be used as detectors for neutron porosity LWD tools based on pulsed neutron generators (PNG), such material should be able to operate at high temperature and enable large neutron detector constructions. In this paper we present new Li-6 scintillation nanostructured glass-ceramics that perform substantially better than all available Li-6 scintillation materials. It is this performance improvement provided by nanostructured nature of obtained material which enables its use in the neutron detectors of PNG-based neutron porosity LWD tools.

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