Improving the accuracy of CO2 sequestration monitoring in depleted gas reservoirs using the pulsed neutron-gamma logging technique.

• A new method for quantitatively monitoring CO 2 saturation in depleted gas reservoirs is proposed. • The ratio of the near detector's neutron capture gamma-ray count rate to the far detector's thermal neutron count rate measured by a four-detector pulsed neutron logging tool can effectively differentiate between CO 2 and CH 4. • The new method has higher CO 2 sensitivity compared to conventional count ratio methods. With favorable trap conditions and comprehensive infrastructure, depleted gas reservoirs create ideal spaces for the economic and efficient storage of carbon dioxide (CO 2). Accurate monitoring of CO 2 saturation is important for understanding CO 2 storage efficiency and distribution status. This paper proposes a new method for the quantitative monitoring of CO 2 saturation in CO 2 sequestration projects within depleted gas reservoirs. The new method uses the ratio of the near detector's neutron capture gamma-ray count rate to the far detector's thermal neutron count rate (RCN) measured by a four-detector pulsed neutron logging tool. In this paper, the RCN responses for different CO 2 saturation levels are studied using the Monte Carlo method, and the methods for roughly and accurately calculating CO 2 saturation are given. Moreover, the effects of formation factors such as lithology, clay mineral content, bound water salinity, and borehole environment on the RCN response are studied. Finally, the applicability of the RCN method is verified through a simulation example. The results show that the RCN method not only has high CO 2 sensitivity, but it also can accurately determine CO 2 saturation in depleted gas reservoirs and control the error of CO 2 saturation calculation within 2%. This study offers robust technical support for the evaluation of CO 2 saturation in depleted gas reservoirs. In addition, the proposed method has been proved applicable in the evaluation of CO 2 saturation in low-porosity reservoirs. [ABSTRACT FROM AUTHOR]