Unconventional oil and gas resources, such as tight oil and gas, have become indispensably succeeding energy sources in nowadays. At the stage of exploration, gas saturation is essential for the evaluation of tight formation, which can provide the key parameters for reserves calculation and development plans making. Conventional logging technologies including acoustic logging and resistivity logging have played a role in gas formation identification and evaluation. Besides, inelastic and capture gamma energy spectrum or time spectrum from pulsed neutron logging tools with NaI, BGO, LaCl3, or LaBr3 detectors are used to realize the quantitative evaluation of gas saturation. With the development of nuclear technology, the new detector, called CLYC (Cs2LiYCl6:Ce), can simultaneously measure the signals of gamma ray and thermal neutron, providing a new mean for gas saturation evaluation use pulsed neutron logging technique.
The CLYC scintillation crystal with a density of 3.31g/cm3 has an energy resolution in the order of 4%-5% (0.662MeV), and its light output efficiency of gamma ray and neutron are 20000 photons/MeV and 70500 photons/MeV. Meanwhile, its excellent temperature characteristics in the range from -30℃ to 180℃ can fit the downhole environment. Consisting of the D-T neutron source and CLYC detector, the pulsed neutron logging system is designed in this paper, in which the burst gate is 0 to 40 microseconds and the capture gate is 50 to 100 microseconds. To evaluate gas saturation, this system combines the inelastic gamma ray and thermal neutron recorded from the burst gate and the capture gate.
The new pulsed neutron logging tool consists of two LaBr3 detectors and a CLYC detector, and the spacing of the CLYC detector is 75cm. In addition to the conventional C/O and Sigma measurement functions, the new instrument can also realize the quantitative evaluation of gas saturation by the CLYC detector. The inelastic gamma, capture gamma, and thermal neutron distribution in long-detector are simulated by the Monte Carlo method under the condition of tight gas saturated formation with porosity from 3% to 20%. Based on the spatial flux distribution characteristic of inelastic gamma and thermal neutron, the new parameter (RGTH) is defined as the ratio of inelastic gamma counts to thermal neutron counts from the CLYC detector to calculate gas saturation. The results imply that RGTH is positively correlated with porosity and negatively correlated with gas saturation, and the gas and water dynamic range is about 36% under the condition of a sandstone formation with 10% porosity. Different lithology has different RGTH benchmark values. RGTH is not affected by the yield of the neutron source and water salinity, and the subtract coefficient can be accurately determined by the time spectrum of the thermal neutron to acquire the pure inelastic gamma. A tight lime-bearing sandstone formation with 5% porosity has been set by MCNP to check validity, the absolute error of gas saturation calculated by RGTH is less than 5%.