Study of geochemical interactions during chemical EOR processes

Sharma, Himanshu
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Geochemical interactions of injected fluids with reservoir fluids and minerals determine the fate of the injected species in the porous media. In addition, these interactions could result in affecting the properties of the porous media such as permeability, porosity, and wettability. With a growing energy demand and continuous depletion of easy oil, chemical enhanced oil recovery (CEOR) techniques are being investigated to satisfy future energy needs. An understanding of geochemical interactions that occur during CEOR techniques is essential to make these processes robust and economical, and make reliable field predictions. In this study, geochemical interactions during alkali surfactant polymer (ASP) floods and low salinity wettability alteration in carbonates were investigated. Experiments were performed to understand interactions of various alkalis in sandstone and carbonate cores containing gypsum. The experiments included single-phase static and transport experiments, surfactant phase behavior experiments and oil recovery corefloods. The ionic compositions of aqueous solutions were carefully monitored to understand the geochemical interactions of these alkalis. The effect of injection rate was investigated to understand if reactions reached equilibrium at the injection rates typically used in lab corefloods. The study showed sodium metaborate to be most suitable, in comparison to sodium carbonate, sodium hydroxide and sodium silicate, for sandstone and carbonate cores containing gypsum. The reaction of sodium metaborate with gypsum was found to be rate dependent and did not reach equilibrium in the lab corefloods. Ammonia was investigated as an alternative alkali for ASP floods. Single-phase static and transport experiments were performed to study its interactions with gypsum. Single-phase static and transport experiments were performed to investigate the effect of adding ammonia on surfactant adsorption. Zeta potential measurements were performed using ammonia and sodium carbonate. Ultralow IFT surfactant formulations were developed, using ammonia as the alkali, for cores containing gypsum or otherwise. Polymer stability experiments were performed to identify polymers suitable for ASP corefloods in cores containing gypsum using ammonia as the alkali. The results showed ammonia to maintained a high pH in presence of gypsum without causing any calcium precipitation. The dissolved calcium ions, however, affected surfactant phase behavior and polymer stability. Single-phase static and transport surfactant adsorption experiments showed reduction in surfactant adsorption on sandstones by adding ammonia. The surfactant adsorption results were, however, not obvious for carbonates when ammonia was used as the alkali. Good oil recoveries and low surfactant retentions were observed during ASP corefloods in sandstone and carbonate cores using ammonia. Interaction of various alkalis with acidic crude oils was also investigated to develop low-cost alkali cosolvent polymer (ACP) floods for such oils. Alkali scans were performed with various alkalis (with and without adding a cosolvent) and low IFT regions were identified. The effect of cosolvent type and divalent cations on the phase behavior results was investigated. ACP corefloods were performed in sandstone cores. The phase behavior experiments showed low IFT regions to vary for different alkalis. ACP corefloods showed good oil recoveries in sandstone cores. Single-phase static and transport experiments were performed to understand geochemical interactions during low salinity waterfloods in carbonate cores at high temperatures. Various low-salinity brines were injected in a limestone core and the ionic composition of the effluent samples were monitored. The effect of injection rate on the composition of the effluent ions was investigated. The experiments showed calcite dissolution, dolomitization and sulfate adsorption to occur on injecting low-salinity brines in limestone cores at high temperatures. The reactions reached equilibrium within 2 PV at 1 ft/d when seawater was injected in a limestone core, initially saturated with the formation brine. However, the reactions did not reach equilibrium when the SW was subsequently displaced with SW/50, even after injecting more than 3 PV. Modeling and simulation work was performed with PHREEQC, UTCHEM and UTCHEM-IPHREEQC to model lab experiments. Single-phase alkali floods and oil recovery corefloods performed using sodium metaborate and ammonia were simulated. A good agreement was obtained between experimental and simulation results. The effect of ion exchange reactions on surfactant floods was investigated. In addition, a mechanistic model was developed for low-salinity wettability alteration in carbonates by incorporating key geochemical interactions observed during the experiments. The model results showed good agreement with effluent ions of static and dynamic experiments, assuming local equilibrium. A good agreement of modeling results was observed with the low salinity oil recovery corefloods reported in the literature.