Improving Transport of Nanoparticles in Carbonate Media by pH Alteration

Abstract

Improving long-term stability and reducing retention are active areas of research for nanoparticle-based technologies for the oil and gas industry. A common strategy to improve nanoparticle stability and reduce retention is the use of polymer and/or surfactant coatings. This manuscript describes a method to improve the transport properties of FITC-dextran through carbonate media. The proposed method is based on the observation that during Alkaline-Polymer-Surfactant flooding, polymer retention reduces as pH increases, likely as a consequence of transitioning through the point of zero (PZO) charge for the porous media. Multiple alkali agents have been identified in the past, but most of them are incompatible with brines containing a high concentration of divalent cations, such as Ca2+ and Mg2+, and are therefore incompatible with carbonate reservoirs. Sodium metaborate, however, has been reported as being compatible with hard brines and carbonate reservoirs. This study evaluates the effectiveness of sodium metaborate as an alkali agent to reduce FITC-dextran retention through carbonate matrices as a proxy for dextran-coated nanoparticles. A series of transport experiments were conducted using chromatography columns packed with fine marble powder to evaluate the impact of pH on FITC-dextran retention. The columns were initially saturated with treated saline water and let rest for three weeks followed by the injection of three pore volumes (PVs) of treated saline water for washing purposes. Next, five PVs of a solution of FITC-dextran dissolved in treated saline water (with or without sodium metaborate tetrahydrate) was injected through the columns. Finally, 5 PVs or treated saline water (with or without sodium metaborate tetrahydrate) were used to displace the injectant. Effluent samples were collected during each phase of the experiment and analyzed using a fluorescent spectrometer. Fluorescence intensity data was converted to concentration and plotted as a function of injected volume to create a breakthrough curve and to estimate FITC-dextran recovery. The results show a slight decrease in retention when using sodium metaborate to increase the solution pH. FITC-dextran recovery was estimated to be 47% for the injectant without sodium metaborate tetrahydrate, and 49% for the injectant with it. This trend is in agreement with previous studies looking at polymer retention and FITC-dextran retention. The experiments suggest that pH plays a significant role during the flushing phase. The ability to transport nanoparticles through oil reservoirs can lead to a whole new range of applications including smart tracers, contrast agents and improved EOR agents and other technologies for reservoir characterization.