Tertiary Low Salinity Waterflooding LSWF in Sandstone Reservoirs: Mechanisms, Synergies and Potentials in EOR Applications
Authors: Rock Alexander , Hincapie Rafael E. , Hoffmann Eugen , Ganzer Leonhard 
Published: April 2018

Abstract
This work provides an extensive review on Low Salinity Water Flooding (LSWF) recovery mechanisms, as well as an evaluation of its synergies with Polymer Flooding (PF). Thereby, a critical state-of-the-art evaluation on LSWF and PF mechanisms is combined with selective laboratory experiments, performed to illustrate the observations and findings. This evaluation can be used as a guidance to understand the expected behavior of both processes when applied in combination.
The work presented here comprises two main steps: 1) Comprehensive review of the mechanisms responsible of oil recovery in each process and 2) Predefined secondary and tertiary mode flooding experiments. First, oil recovery mechanisms associated to LSWF and PF have been analyzed in detail. Second, different field cases were compared in order to draw the main conclusions with regards to performance and recovery factors. This also helped to define the synergies of LSWF and PF in terms of technical and economic efficiency. Finally, secondary and tertiary mode experiments were performed to evaluate the feasibility of applying both processes.
Despite of the over 15 mechanisms reported in the literature for LSWF, six main mechanisms were identified that contributes to oil recovery. Mechanisms are described as: 1) Wettability alteration 2) Multi-ion exchange, 3) Fine migration, 4) Salting-in, 5) Double-Layer-Expansion and, 6) Other mechanisms, such as osmotic pressure and IFT reduction. Thereby, wettability alteration and fine migration have the highest significance. On the other hand, PF mechanisms were found to be: 1) Viscous fingering reduction, 2) Enhanced flow between layers, 3) Pull-out effects, 4) Shear thickening/elastic turbulence and, 5) Relative permeability reduction. LSWF field cases revealed incremental recoveries of up to 13% OOIP whereas synergies between LSWF and PF yielded to an additional recovery of 15% OOIP, underlining the potential of the combination of both EOR technologies. Selective LSWF-PF experiments performed in sandstones core-plugs in this work, allowed the verification of the additional recoveries reported in the literature. Tertiary flooding with solely LSWF, showed a lower recovery than tertiary LSWF-PF flooding. Moreover, this observation confirms the potentiality of polymer-combined LSWF in sandstones. Additionally, with the combined processes, a lower polymer concentration was required than applying a typically designed polymer flooding. This can be translated to an economic benefit for field applications.
Tertiary mode flooding experiments in sandstones and the analysis of field cases provided clear evidence of the advantages of LSWF-PF. This could yield that the processes -when applied in tandem- become a leading EOR strategy, ensuring the extension of the reservoir lifetime. Moreover, fellow researchers can benefit because the work provides a comprehensive review of Low Salinity Water Flooding and Polymer Flooding mechanisms. To the authors understanding, literature is currently lacking of such a review.