Polymer flood in low permeability carbonate reservoirs

Polymer flood is one of the most economical and efficient chemical enhanced oil recovery (EOR) techniques. The injected polymer mostly flows within formations for multiple years before being produced. In the past ten years, polymer flooding has been considered in carbonate reservoirs which hold the majority of the existing oil reserves and usually under high-temperature and high-salinity conditions. These challenging environments can accelerate the degradation of polymers leading to the failure of polymer-based floods, especially considering the lengthy propagation of the injected chemicals. Some of the reservoirs have permeabilities of less than 50 mD. Injection of polymers in these tight porous media has a high chance of the injectivity issue. The main objectives of this research are to investigate the long-term thermal stability and transport of polymers in these challenging environments. Thermal stability study of HPAM, ATBS-incorporated PAM, scleroglucan, and PEO was investigated at temperatures up to 116°C. Polymer solutions were prepared inside an anaerobic chamber with dissolved oxygen levels less than 15 ppb and then stored in stainless-steel cylinders for aging. The solution viscosity, dissolved oxygen, and pH were monitored during the aging. At 116°C, the HPAM solution prepared in the softened seawater with the addition of 2 wt% of Na₂CO₃ and 0.75 wt% of NH₄OH retained more than 75% of the viscosity at 10 s⁻¹ after three years. Meanwhile, ATBS-polymer ZLPAM@50525 prepared with 2 wt% of Na₂CO₃ was more viscous than the initial solution. Adding ammonia to the ZLPAM@50525 solution impaired the polymer stability; only 76.4% of the original viscosity was preserved. The high ATBS-content polymer SAV 10 and SAV 10xv were prepared in a synthetic seawater. No sign of degradation was observed for the SAV 10 from Day 1 to the end of the test. Hydrolysis of SAV 10xv was noticed after 1097 days, that is, the solution viscosity at 10 s⁻¹ increased from 19.4 cp to 30.2 cp. Scleroglucan solution aged at 100°C lost more than 70% viscosity in two years. Adding glutaraldehyde into the solution accelerated the biopolymer degradation with the viscosity being completely lost after two months. PEO solutions had shown a clear viscosity drop on the first day and then remained stable in the subsequent 80 days. Polymer transport study was conducted in carbonate cores with and without the residual oil saturation. The retention was measured by the double bank method and the material balance method with the polymer concentration being characterized by both total organic carbon analyzer and capillary tube rheology. In tight carbonate rocks, HPAM exhibited high retention (> 100 µg/g), inaccessible pore volume (IPV) greater than 5%, and a high residual resistance factor (> 9). The sulfonated polyacrylamide, AN132, showed low retentions (< 15 µg/g) and zero IPV. The retention of HPAM was reduced by 50 µg/g in oil-wet rocks due to thin oil films in pores that prevented the direct adsorption of the carboxyl group of polymers on the mineral surface. The residual resistance factor of AN132 in the water-saturated rock was less than 2, indicating minimal blocking of pore throats in these tight rocks. The residual resistance factor of the AN132 polymer increased slightly in the presence of residual oil saturation due to partial blocking of the smaller pore throats available for polymer propagation in an oil-aged core. Low salinity polymer flood was carried out using HPAM prepared in diluted seawaters with modified sulfate concentrations. After extensive water floods, HPAM prepared in the 10 times-diluted seawater produced the same incremental oil recovery of 4% original oil in place (OOIP) as the ATBS-polymer AN132 prepared in the seawater. Increasing the sulfate concentration by four- and eight-folds doubled the incremental oil from low salinity polymer floods. With proper preparation, both polymers with the molecular weight of 6 MDa could be successfully injected into the oil-aged carbonate rocks with the absolute permeability of 10 mD and the end-point brine permeability of 1 mD.Petroleum and Geosystems Engineerin