Interpretation of an Interwell Partitioning Tracer Test in a Multi-Layer Carbonate Reservoir

Interwell Partitioning Tracer Test (IPTT) estimates remaining oil for economic evaluation of reservoir operations. It involves waterflooding and co-injection of two chemicals: one water soluble (conservative) and another that also partitions into the oil phase. The downstream concentration of injected chemicals yields concentration history (CH). Cooke's interpretation of CH estimates remaining oil from tracer arrival times. Complex reservoirs need extra interpretation effort. This study reanalyzes IPTT data from a giant Saudi Arabian reservoir, as published by Sanni et al. (2016) [1]. We introduce the following reservoir features that were overlooked in the original study: multi-layer structure, mass transfer resistances, stagnant zones, and physical properties continuity. We combine these features in a physically reasonable way to capture the first-order effects. Namely, we add extra terms to the well-known advection-dispersion equation (ADE) for each feature and use separate ADE for each reservoir layer preserving physical properties continuity in each layer. Consequently, partitioning tracer transport is described by an ADE with mass-transfer resistance to the stagnant oil phase in each layer. The oil phase is continuous and allows for partitioning tracer communication between the layers. The conservative tracer follows classic ADE in both layers. The system of differential equations is solved numerically with COMSOL. The key results from our analysis include improved mass conservation and overlooked remaining oil in the second layer. Specifically, our model accounts for all recovered mass for both conservative and partitioning tracers. More importantly, our model allows for estimation of remaining oil in the second layer. Remaining oil saturation in the first layer is 17% — close but slightly less than in the original study. The second layer hosts 25% of remaining oil. To the best of our knowledge, this is the first published result that interprets IPTT data for a multi-layer reservoir with stagnant zones, inter-layer oil communication, and slow partitioning tracer diffusion in oil phase. Our analysis revealed hydrocarbons in a second layer, missed in the original report. This study also demonstrates how proper physics-based test interpretation can provide guidance on essential data collection to improve IPTT analyses.