Application of position sensitive detector in nuclear well logging tools

Synopsis

Nuclear well-logging tools play an essential role in helping petroleum engineers and geoscientists to understand properties of formations and give a quantitative evaluation of the hydrocarbon reservoir. As the exploration of oil and gas shifts to thinner and more heterogeneous reservoirs, there is a high demand for new tools with better vertical resolution. This thesis proposes using a position-sensitive detector (PSD) to replace the conventional detectors in nuclear well-logging tools to improve their vertical resolution.

With the Monte Carlo method, the performance of PSD and its applications in a density logging tool and a natural gamma-ray tool were studied. The results show that with a PSD, the vertical resolutions of both tools are significantly improved. Notably, the density tool with a PSD can evaluate the density correctly when the thickness of a layer is only 5 cm, which is dramatically better than the 16 cm for a conventional density tool. Besides, because the proposed density tool has distinct depth of investigations (DOI) at different positions of the PSD, the tool can provide a two-dimensional density image which indicates the structure of the formation. As to a natural gamma-ray well-logging tool with a PSD, its vertical resolution depends on the position resolution of the PSD, which is different from the conventional tool whose vertical resolution is determined by the crystal's length. Based on this change, the statistical fluctuation of the gamma-ray measurement can be reduced by increasing the crystal's length of the PSD without affecting its vertical resolution.

In short, this work confirms the feasibility of using the PSD in nuclear well-logging tools in theory and indicates that logging tools with PSD have a promising potential application in determining the properties of thin-bedded strata.

Keywords: Density well-logging tool; Natural gamma-ray well-logging tool; Monte Carlo simulation; vertical resolution; position-sensitive detector.