Screening, Design, and Application of Chemical EOR to Control High Water-Cut and Reduce Residual Oil in a Complex Sandstone Oilfield in Kazakhstan

Abstract

This paper presents the results acquired during first laboratory-scale chemical flooding evaluation for a giant waterflooded oilfield in Kazakhstan. A carefully-designed chemical flood recipe involves the injection of a mixture of surfactant and alkali/nanoparticles followed by polymer to reduce oil-water interfacial tension (IFT) by surfactant, minimize surfactant and polymer adsorption by alkali or nanoparticles (NPs), and provide mobility control by polymer. Collectively, such an efficient design yields considerable improvement in residual oil mobilization and recovery. Field A in Kazakhstan, one of the oldest fields in the country, has been waterflooded for decades. Currently, the water cut of the field is more than 90%, with a high residual oil saturation. For the targeted reservoir conditions, four hydrolyzed polyacrylamide (HPAM) based polymers and around 10 different surfactant formulations were tested. Alkali and nanoparticles were then assessed for chemical adsorption control for the most optimum polymer and surfactant. The evaluation was done at reservoir temperature of 63 °C and Caspian seawater of 13000 ppm salinity was used as the makeup brine for all the formulations. The performance of the screened chemicals in the porous media was analyzed by a series of coreflood experiments on the reservoir cores. The critical parameters such as chemical adsorption, IFT, mobility ratio, resistance factor, and oil recovery were obtained and compared to select the best chemical enhanced oil recovery (CEOR) scheme. During screening phase of the study, one of the polymers, ASP3, displayed pronounced resistance against bacterial attack under reservoir conditions. Adsorption for the same polymer was also 13-14% less compared to its counterparts. Optimum surfactant was selected based on the generation of Winsor Type III microemulsion and a minimum IFT of 0.2 mN/m. The adsorption study indicated a 9-21% reduction in surfactant adsorption by alkali. In the case of polymer, NPs demonstrated better performance and caused an 18% decrease in polymer adsorption whereas alkali showed negligible effect. Corefloods were performed for various combinations of screened chemicals. In comparison with NPs-surfactant-polymer (NSP) design, surfactant-polymer (SP) and alkali-surfactant-polymer (ASP) schemes recovered more residual oil by effectively generating and producing microemulsion. However, ASP design outperformed the rest by recovering 96% of the remaining oil, which translated to 11% higher recovery compared to polymer flooding and 13% more oil compared to NSP flooding. This screening and design study demonstrates that the selection of chemicals for EOR strictly depends on the oil, formation and injection water, and reservoir rock interactions. Our study proved that appropriate design of chemical EOR constituents can yield favorable results in high salinity challenging formations that contain waxy oils with high paraffin content.