Selection of EOR Polymers for Carbonates from Laboratory Scale to Yard Scale: Observations and Insights

Abstract

Abstract Qualification of polymers for Chemical EOR applications typically involves following standard screening laboratory procedures to evaluate mixing efficiency, filterability, long term injectivity and oil displacement characteristics. However, to evaluate mixing performance of a selected polymer at the field scale, yard-scale tests are an important component in the overall screening process. Yard-scale tests identify polymer mixing operating envelopes in terms of mother solution concentrations and more importantly sensitivity of the mixed polymer with respect to variable water quality. The objective of this work is to understand mixing efficiency of different powder polymers with current commercial solid-liquid wetting setups and how it affects polymer filterability and subsequent injectivity in carbonate rocks. Several powder polymers, with different molecular weights were selected to test the polymer propagation in lower permeability carbonate rocks. To reduce inaccessible pore volume effects, lower molecular weight polymers were selected for screening and yard tests. Synthetic brine was prepared using de-ionized water to maintain optimal water quality at the yard scale. The wetting of powder and brine at high shear and mixing velocities was obtained through vendor proprietary solid/liquid contact device. Polymers were either mixed to higher concentration mother solutions and diluted to the target concentration using a laboratory overhead mixer or were mixed to target concentrations directly. The matured polymer solutions were injected into carbonate rocks and long term injectivity was monitored and was used as the final criteria to qualify a polymer for a given molecular weight. Injectivity of commercial scale lower molecular weight polymers performed better in terms of resistance factors and polymer retention in the lab scale. The selected lower molecular weight polymers were commercially manufactured by the different vendors for understanding mixing at the yard scale. Yard scale mixed polymer solutions did not have any gels or fisheyes, indicating optimal dissolution. Injection in carbonate rocks showed stable injectivity for more than 25 Pore Volumes of polymer solution injected. In cases where the synthetic brine quality was not optimal, the filterability and injectivity of yard mixed solutions were poor. Marginally higher molecular weight polymers showed plugging behavior despite having good mixing and filterability indicating very high polymer retention in tighter pore throats. This paper will provide guidelines on laboratory tests and scale-up yard tests for selection criteria of EOR polymers, especially in carbonate reservoirs. Yard tests designed to validate commercial polymer mixing can prove extremely useful to identify failure conditions caused by process upsets that can lead to suboptimal polymer mixing. Such tests are a low-cost exercise which can lead to better design of robust facilities that leads to higher uptime during field injection and project success.