Water alternating gas (WAG) is employed to improve flood mobility, and sweep efficiency during gas injection by squeezing more oil out of the reservoir. Injected water sweeps the lower part of the reservoir, while gas tends to sweep the upper part due to gravitational forces. Accurate mapping of fluids and their distribution across the reservoir is an important aspect of characterizing reservoir properties. Understanding the performance of the injected fluids, and their flow pattern is critical to understand reservoir behavior, the interaction between the fluids at reservoir level, and quality check the reservoir dynamic model.
Techniques such as tracer injection and time lapse logging are utilized as a method to monitor, and confirm flood front arrival. Pulse neutron decay time log (PNL) is a common method used to monitor reservoir fluid movement behind casing. Multi detector pulse neutron decay improves the tool capabilities and enhances its potential in detecting gas flood front and its miscibility.
Interpreting reservoir monitoring logs requires combining the innovative knowledge on the tool physics and understanding reservoir properties. Integrating all available data is an important step towards the correct interpretation.
Multi detector neutron pulse decay was run in time lapse logging using same tool provider to allow consistency. The tool has successfully mapped the approach of injected gas, it clearly showed the change in fluid properties over time, and most importantly has shown the type of fluid approaching. Few months later gas tracer was found in the close-by producer.
This paper shows a case study of successful integration of the results of six years’ time lapse logging with the dynamic simulation model, in a field located onshore Abu Dhabi UAE, and subject to rich gas EOR recovery through WAG cycling system. Gas tracer and PNL were used to monitor the flood front and break through. The flood movement mapped from neutron pulse decay with time lapse, was successfully history matched in the dynamic simulation model. The calibrated simulation model is used to monitor the further advancement of flood front.