@article{zhang_zhang_yuan_wang_zhang_2019, title={A comparative study on the neutron-gamma density and gamma-gamma density logging}, volume={176}, ISSN={["1873-4715"]}, DOI={10.1016/j.petrol.2019.02.007}, abstractNote={Abstract With the increasing demand for the radioisotope-free operations, using pulsed neutron gamma density (NGD) instead of gamma-gamma density (GGD) has become increasingly important for the development of logging while drilling (LWD) technology. However, due to the different logging mechanisms, there are huge differences between GGD and NGD in the instrument designs, measurement methods, and instrument specifications. To further promote the NGD development, a comparative study on GGD and NGD was carried out from theoretical and simulation aspects. Based on the theoretical methods, the differences between GGD and NGD in the gamma sources, field distribution, methods for density measurement and instrument specifications were systematically elaborated. Then, using the Monte Carlo simulation, the GGD and NGD models were built to quantitatively verify the theoretical results. Last, the detailed performances of NGD and GGD methods in different logging environments were compared. Results showed that the theoretical results are highly consistent with the simulation results. The density sensitivity of NGD is less than half that of GGD, but the depth of investigation (DOI) approaches twice that of GGD. The density precision and vertical resolution of NGD are less than those of GGD. Additionally, compared to GGD, NGD also has an excellent performance in different pore fluid and lithology formations, but it is easily affected by borehole factors. The research can provide quantitative performance evaluations for the NGD replacing GGD in LWD logging.}, journal={JOURNAL OF PETROLEUM SCIENCE AND ENGINEERING}, author={Zhang, Quanying and Zhang, Feng and Yuan, Chao and Wang, Xinguang and Zhang, Xiaoyang}, year={2019}, month={May}, pages={792–799} }
 @article{zhang_zhang_gardner_yan_wu_tian_chen_ti_2018, title={A method for determining density based on gamma ray and fast neutron detection using a Cs2LiYCl6 detector in neutron-gamma density logging}, volume={142}, ISSN={["0969-8043"]}, DOI={10.1016/j.apradiso.2018.09.011}, abstractNote={With the increasing demand for radioisotope-free operations, pulsed neutron-gamma density (NGD) has become increasingly important for logging-while-drilling (LWD) development. However, current NGD tools, adopting the multiple-detector array design, are not conducive to the simplification of instrument design and measurement system. To break obstacles, based on the fast neutron-gamma coupled theory, a new density measurement method was proposed. Further, combined with the neutron-gamma simultaneous detection characteristics of the Cs2LiYCl6 (CLYC) detector, an NGD measurement system consisting of a D-T source and one CLYC detector was used. Results show that the new method is capable of determining formation density using a single CLYC detector, which can not only avoid complex instrument systems but also improve density sensitivity. Moreover, the applicability of the new density method was well verified by Monte Carlo simulation. Additionally, the method was successfully applied in a simulated well, and density results are in good agreement with the benchmarked formations. The research provides theoretical guidance for NGD instrument design.}, journal={APPLIED RADIATION AND ISOTOPES}, author={Zhang, Quanying and Zhang, Feng and Gardner, Robin P. and Yan, Huizhong and Wu, Guoli and Tian, Lili and Chen, Qian and Ti, Yongzhou}, year={2018}, month={Dec}, pages={77–84} }
 @article{zhang_zhang_gardner_liu_zhang_qiu_chen_tian_wang_2018, title={Quantitative monitoring of CO2 sequestration using thermal neutron detection technique in heavy oil reservoirs}, volume={79}, ISSN={["1878-0148"]}, DOI={10.1016/j.ijggc.2018.10.003}, abstractNote={CO2 enhanced oil recovery (CO2-EOR) project is of significance for CO2 sequestration and heavy oil recovery. Quantitative monitoring of CO2 saturation (SCO2) is essential to recognizing and understanding the migration and distribution of CO2 injected into the geological formations. In this paper, based on the difference in the neutron moderation ability of CO2, water and heavy oil, thermal neutron detection technique is applied in heavy oil reservoirs to monitor CO2 sequestration. By Monte Carlo simulation, the responses of thermal neutron count ratio versus different porosities and CO2 saturation were studied. Then, a mathematical model of CO2 saturation versus thermal neutron count ratio and formation porosity was established to quantitatively calculate CO2 saturation. Besides, the effects of formation pressure and temperature, heavy oil density, lithology, and other factors on the method were studied. Results show that variations of formation pressure, formation temperature, and density of heavy oil have little impact on the CO2 saturation measurement. However, the change of formation lithology results in larger CO2 saturation errors and needs corrections. In addition, the method has a low discrimination between CO2 and CH4 gas, and the results are easily affected by the CH4 content. Finally, a simulated case demonstrates the application of the method. For the heavy-oil sandstone with different porosities, the method shows a perfect performance: the SCO2 errors are less than 1% for the high and low gas saturated formation. This research provides an effective strategy to monitor CO2 storage and residual oil saturation in CO2-EOR reservoirs.}, journal={INTERNATIONAL JOURNAL OF GREENHOUSE GAS CONTROL}, author={Zhang, Feng and Zhang, Quanying and Gardner, Robin P. and Liu, Juntao and Zhang, Xiaoyang and Qiu, Fei and Chen, Qian and Tian, Lili and Wang, Yang}, year={2018}, month={Dec}, pages={154–164} }