Low Polymer Retention Opens for Field Implementation of Polymer Flooding in High Salinity Carbonate Reservoirs

Abstract

AbstractPolymer flooding has been a successful EOR method in sandstone reservoirs for decades. Extending polymer flooding to carbonate reservoirs has been challenging due to adsorption loss and polymer availability for high temperature, high salinity (HTHS) reservoirs. In this study, polymer flooding for carbonate reservoirs is moved forward as we show that HTHS polymers can exhibit low adsorption and retention in carbonate reservoir rock at ultra-high salinity conditions.Carbonate reservoir core plugs with permeabilities ranging from 10 mD to Darcy range were used for the adsorption/dynamic retention studies. The dynamic retention experiments made use of water-soluble tracers as comparison for the polymer transport in porous medium. The synthetic formation water had ultra-high salinity, i.e. 180 000 mg/L TDS with a hardness of 19 %. In addition to experiments performed on single phase water saturated cores, experiments were also performed on crude oil aged reservoir core samples. The aged core samples were flooded to Sorw by water flood or by centrifugation.The implementation of polymer flooding in carbonate reservoirs has long been restricted due to adsorption and retention. Most polymers for Enhanced Oil Recovery (EOR) are negatively charged. It has long been assumed that the rock surface is primarily positively charged in carbonate rock. The argument of electrostatic attractive forces indicates high loss of polymer due to adsorption. However, the rock surface charge in carbonates may vary with presence of surface biofilms, oil layers and highly varying pore geometries. Measurements of surface charge have shown that this assumption has been too simplistic and that some surfaces are negatively charged or near-neutral due to earlier mentioned effects or ion composition, hardness and total ionic strength. Furthermore, wettability is a key factor. The results of the retention experiments on aged carbonate reservoir rock, show that retention is reduced by a factor of 4-6 when remaining oil is present. This is highly surprising when the ultra-high salinity (180 000 ppm TDS) is considered. The paper shows detailed analysis of retention experiments and discusses the influence of oil, salinity, and polymer chemistry on retention in carbonate rock.Retention is a key factor for implementation of polymer flooding in carbonate reservoirs. This study shows that new, commercially available HTHS polymers can be applied for polymer flooding in carbonate reservoirs without a high loss of chemicals. The results obtained show a positive impact on economic feasibility of polymer EOR in carbonates. The solutions found here can be applied to similar reservoir conditions and facilitate polymer flooding in HTHS carbonate reservoirs. Pilot tests based on these results are ongoing at the time of writing.