In our previous research which has published in 62th SPWLA symposium, a numerical simulation method was proposed which about the double particle detector CLYC (Cs2LiYCl6:CE) to get the response of thermal neutron and gamma ray. It is feasible to use CLYC detector in pulsed neutron well logging in theory by simulating relationship between the ratio of inelastic gamma ray and thermal neutron from extra spacing and gas saturation.
The well completion method of multi-string combination and borehole size will influence the gas saturation interpretation accuracy in pulsed neutron logging, which limited by the information from single extra far CLYC detector. Besides, the lower counts of extra far detector will enhance the gas saturation evaluation lower limit of porosity. Therefore, a new pulsed neutron system, which consisted of double CLYC detectors and a D-T neutron source, has been proposed for solving the problem about the lower evaluation accuracy and borehole environment influences. The new pulsed neutron system can provide multiple information for formation evaluation while simplifying the structure of the instrument.
The CLYC well logging response which including the fast neutron, thermal neutron and gamma ray, is simulated under the different situation of lithology, porosity, water salinity, gas saturation and borehole size. The field distribution of inelastic gamma ray and thermal neutron will mainly influenced by the hydrogen index and the spacing can reflect the counts contribution of borehole and formation. Based on the numerical simulation result and theoretical analysis, a novel gas saturation evaluation parameter called RIT21 (the ratio of inelastic gamma ray and thermal neutron from far spacing minus the ratio of inelastic gamma ray and thermal neutron from near spacing) has been proposed.
The parameter RIT21 is positively related to the porosity and limestone content, negatively related to the gas saturation. Its evaluation sensitivity can reach 124% under 8% porosity conditions, which is eight times of the conventional RCP12. Finally, a simulation model is proposed to verify the availability under the condition of 8% porosity limestone content formation which absolute error of gas saturation is less than 5%. In summary, using a capability that can simultaneously detect the neutron and gamma ray of the CLYC detector, this novel pulsed-neutron system can provide more information about field distribution of gamma ray and neutron to achieve higher evaluation sensitivity, giving it broad prospects.
