Optimizing Reservoir Monitoring: Improving PNL Logs in Changing Borehole Environments

Abstract

The growing demand for oil has emboldened producing companies to reenter old wells to further improve productivity and recovery. This requires monitoring the water saturation. Successes in reservoir saturation monitoring petrophysical analysis have increased the confidence to drill sidetracks in watered wells that have bypassed oil potential. Several techniques can be used to perform the analysis.The Pulsed Neutron Capture (PNC) log is one of the most popular slim cased hole formation evaluation logging tools, which allow running the surveys without having to pull out the production string. Under the right conditions, the PNC logs can be run periodically in the time-lapse mode to monitor changes in water saturation and movements in the oil-water contact and gas-oil contact.The wellbore environment may change between runs and this can complicate the analysis. For example, the borehole fluids may be different: gas, oil or brines of varying salinities. Also, changes in the downhole completion hardware can require running the logs through different tubular configurations. One has to be cognizant of all these environmental effects and appropriately correct for them to obtain the true formation properties, and to make comparisons between runs in the timelapse analysis.Different vendors use different correction schemes. In this paper we will discuss case studies with a methodology from one Service Company that uses a weighted database approach, which relies on characterizing the tool response in known environments. The paper shows some of the advantages and disadvantages of this technique, and in particular, when the downhole conditions deviate from the characterized environment. Alternative methodologies will be proposed to get the best possible results, based on this study. Finally, examples from Saudi Arabian wells will be shown where the computed capture cross section and neutron porosity were successfully corrected in challenging borehole conditions.