Smart Water as a Sustainable Enhanced Oil Recovery Fluid: Covariant Saline Optimization

Abstract

Although the migration to renewable energies is accelerating, for several decades the fossil fuels will still be essential for mankind. More environment friendly EOR methods are a current challenge. Recently, injection of aqueous fluids with low salinity (Smart Waters) had good performance and reduced environmental impact. However, there still is no consensus on the best ion concentrations, which may be due to experimental designs being linear whilst EOR processes are covariantly synergistic. This work optimized the pre-salt oil recovery factor in limestone by injecting smart water with 30 formulations defined by covariant statistical experimental design, with Doehlert Matrix, for NaCl, CaCl2 and NaHCO3. Injected and produced fluids were characterized by rheology, pH, conductivity, molecular fluorescence spectroscopy and X-ray fluorescence. No direct univariate relationships between recovery factor and pH, TDS or ionic concentration were found. The recovery factor was maximized from 35.9% up to 52.5%. Two optimal regions were discovered, one with slightly lower salinity and the other with slightly higher salinity, which may explain the lack of convergence in the literature on linear concentration variation. NaHCO3 concentration presented the highest statistical significance, which is attributed to the maximization of the reduction in the oil/oil and oil/rock interface tension. Its presence synergistically favored lower concentrations of NaCl. Separately, NaCl CaCl2 showed opposite statistical significances. However, their synergy was evident in the high positive significance of their association, due to the association of monovalent and bivalent cations. In addition, molecular dynamics calculation revealed that monovalent ions improve the wettability of water on the carbonate rock. There was no preferential production of any type of asphaltenes. Total dissolved solids influenced the rheological profiles when at high concentrations. The dynamic evolution of the roles of Cl− and Ca+2 ions was identified as more porous volumes of smart water are injected, through variations in their concentration in the aqueous fraction produced, which was attributed to the fact that, as oil is produced and the intelligent injection of water continues, the interaction of the chlorine anions with the rock intensifies, leading to the gradual release of calcium cations until the surface saturation process occurs.