A novel borehole/annulus holdup calculation method based on pulsed neutron logging

Pulsed neutron logging tool (PNL), particularly small-diameter multifunction PNL, is an essential instrument used in casing well for evaluating residual oil saturation. In a borehole with a multiple casing string structure, PNL tools log in the oil tube and even the space between the casing and oil tube. Borehole/annulus holdup have a significant influence on the evaluation of oil saturation. Hence, the requirement of the estimation of borehole/annulus liquid holdup is highlighted. This study proposes a new method to determine the annulus oil holdup in a casing well with a complex string structure. For this method, the annulus holdup are derived from Delta S, which is defined as the difference between the relative variations of carbon/oxygen and carbon/hydrogen ratios. To demonstrate the feasibility of the proposed method, we first simulate the responses of Delta S under various borehole and formation conditions using the Monte Carlo method. The outcome shows that Delta S covaries with annulus oil holdup, confirming the feasibility of the proposed method in theory. The proposed method is applied to real data measured in an offshore production well. The result confirms that the formation water saturation calculated by the proposed method agrees well with the actual well production status, further confirming that the proposed method has a promising application in residual oil dynamic monitoring.