A conceptual C/O tool design with coincidence counting

The pulsed neutron carbon/oxygen (C/O) measurement is often the preferred tool in cased hole logging to distinguish hydrocarbon from water in freshwater or low salinity formation environments. However, the existence of production tubing and pipe requires that the diameter of C/O instruments not exceed a few inches. Modern C/O tools, even when equipped with dense crystals, always encounters the challenge of low signal-to-noise ratio and poor spectral resolution. A new C/O logging instrument design with coaxial detectors and a coincidence counting technique is investigated. The detector section of the instrument consists of a well-type scintillation detector and a cylindrical detector, which is fitted inside the well-type detector. The coincidence counting technique relies on the annihilation photons from pair production interactions to record the coincidence events between the well-type and cylindrical detectors. The method works extremely well in C/O measurement in which high energy inelastic carbon and oxygen gamma rays are produced and measured and pair production is the dominant interaction for photons of about 2 MeV and above. Laboratory experiments show that the new instrument design significantly improves the measurement accuracy over the traditional C/O logging devices using single crystal detectors. Two conceptual designs utilizing both BGO and NaI detectors have been tested in the laboratory. A diagonal summing technique and other methods are being studied and developed to analyze the coincidence data. Monte Carlo simulation has been used to optimize the instrument design. The modeled data is in good agreement with experimental results.