Predicting Inter-Well Porosity by Comparing the Breakthroughs of Polymeric and Molecular Tracers
Authors: Chen Hsieh , Yoon Bora , Thomas Gawain , Poitzsch Martin E 
Published: October 2023

Abstract
Understanding the porosity distributions across whole reservoirs is crucial in all stages of the exploration and production, such as estimating the original oil/gas in place and recoverable resources, selecting primary/secondary recovery mechanisms, optimizing enhanced recovery methods, etc. Nevertheless, there are no direct methods to probe inter-well porosity beyond near wellbore core analysis or loggings. Here, we propose a new method to directly measure the inter-well porosity using polymeric and molecular inter-well tracers.
Specifically, we utilize the transport property of polymers in porous media that the polymers can bypass small pores, i.e., the inaccessible pore volume (IPV), resulting in accelerated breakthrough. In contrast, small molecular tracers will flow through all pores without accelerated breakthrough. By comparing the breakthrough curves of the polymeric and molecular tracers, the inter-well porosity can be measured. We performed reservoir simulations to demonstrate the workflow. In the meantime, we synthesized model low-retention polymer tracer candidates and characterized their IPV in carbonate cores using coreflood experiments.
In reservoir simulations, we constructed waterflooding scenarios with both polymeric and molecular water tracers co-injected into injectors and observed their breakthrough curves from producers. Depending on the different porosity distributions between injector-producer pairs, the polymeric tracers can either breakthrough much faster than the molecular tracers, or both polymeric and molecular tracers may breakthrough at a similar time. Ensemble smoother with multiple data assimilation with tracer data (ES-MDA-Tracer) algorithms were then used for history matching and predicting the inter-well porosity. Encouragingly, including both polymeric and molecular tracers resulted in much improved inter-well porosity predictions.
In our experimental effort, we synthesized different sizes of the low retention sulfozwitterionic poly(1-vinylimidazole) (PZVIm) polymers that are good candidates for inter-well porosity-sensing tracers. Coreflood experiments co-injecting sulfozwitterionic PZVIm polymer tracers with reference NaBr water tracers in representative carbonate cores showed an IPV of ~10% for the polymers with molecular weight of 46,000 g/mol. Larger polymers may be synthesized to increase the IPV to have more dramatic breakthrough contrasts in the proposed filed applications.
In this paper, we presented a novel approach for the direct measurement of inter-well porosity by means of the different transport properties of the polymeric and molecular inter-well tracers, which the polymers are pore-sensitive (with IPV) while the molecular tracers are pore-insensitive. Detailed workflows were demonstrated using reservoir simulations and history matching algorithms. Finally, novel candidate polymers (sulfozwitterionic PZVIm) for this application were experimentally synthesized and verified, which greatly strengthened the validity of our approach.