Quantitative monitoring of CO2 sequestration using thermal neutron detection technique in heavy oil reservoirs

Abstract CO 2 enhanced oil recovery (CO 2 -EOR) project is of significance for CO 2 sequestration and heavy oil recovery. Quantitative monitoring of CO 2 saturation (S CO2 ) is essential to recognizing and understanding the migration and distribution of CO 2 injected into the geological formations. In this paper, based on the difference in the neutron moderation ability of CO 2 , water and heavy oil, thermal neutron detection technique is applied in heavy oil reservoirs to monitor CO 2 sequestration. By Monte Carlo simulation, the responses of thermal neutron count ratio versus different porosities and CO 2 saturation were studied. Then, a mathematical model of CO 2 saturation versus thermal neutron count ratio and formation porosity was established to quantitatively calculate CO 2 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 CO 2 saturation measurement. However, the change of formation lithology results in larger CO 2 saturation errors and needs corrections. In addition, the method has a low discrimination between CO 2 and CH 4 gas, and the results are easily affected by the CH 4 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 S CO2 errors are less than 1% for the high and low gas saturated formation. This research provides an effective strategy to monitor CO 2 storage and residual oil saturation in CO 2 -EOR reservoirs.