A Monitoring CO2 Method by the Dual Cross Section Pulsed Neutron Logging Technology in Heavy Oil Reservoirs

Capturing industrial carbon dioxide (CO2), one of the most common greenhouse gases, and injecting it into deep formations for long-term storage is a promising method. CO2 enhanced oil recovery (CO2-EOR) technology meets this requirement, as well as realizes the secondary exploitation of the oil field, which can increase oil and gas production to increase economic benefits. In CO2-EOR technology, gas saturation is a key parameter to characterize the distribution and migration of CO2. Many methods of monitoring CO2 storage status have been successfully applied all over the world, such as seismic exploration, CT and well temperature logging, etc. As an important method of gas saturation evaluation in nuclear geophysical exploration, pulsed neutron logging technology also plays an indispensable role in the evaluation of CO2 distribution. Based on the three-detector pulsed neutron logging tool, this paper proposes a method for monitoring the gas saturation of CO2 injection heavy oil reservoirs that combines the fast neutron scattering cross-section (FNXS), capture cross-section (Σ) and formation element logging to eliminate the influence of formation matrix composition and formation water salinity and improve the accuracy on the monitoring of gas saturation. The intensity of secondary gamma depends on the influence factors such as the inelastic gamma field distribution, the gamma absorption coefficient, and additional characteristics of nuclear propagation, as well as formation and logging tool properties that can all be simulated. We use the combination of inelastic gamma count of the far detector, inelastic gamma count ratio and capture gamma count ratio to characterize the FNXS quantitatively, eliminating the influence of formation lithology and density. The gas sensitivity of FNXS and Σ for CO2 monitoring in the reservoirs are analyzed, a CO2 monitoring method combining FNXS and Σ is established. The CO2 saturation can be calculated without formation porosity information, and the gas detection accuracy is improved. Using the element content information obtained by the three-detector pulsed neutron measurement system, a calculation method for determining the FNXS and the Σ of the formation matrix directly from the element content is established, which avoids the limitation in the conversion process from element content to mineral content and eliminates the influence of complex lithology on CO2 gas saturation calculation. In addition, the element logging data is used to calculate the formation water salinity, and the Σ of the formation water is obtained. Finally, a complex lithology CO2 injection heavy oil reservoir numerical calculation model is established, and the CO2 gas saturation evaluation is carried out using dual cross-section and element logging to verify the effectiveness of the method. The results show that this method can control the absolute error of CO2 gas saturation calculation within 10%, which verifies the feasibility of using this method to evaluate the CO2saturation, provides technical support for monitoring the gas saturation of CO2 injection heavy oil reservoirs.