Review of low salinity waterflooding in carbonate rocks: mechanisms, investigation techniques, and future directions.

This review analyses the fundamental thermodynamic theory of the crude oil-brine-rock (COBR) interface and the underlying rock-brine and oil-brine interactions. The available data are then reviewed to outline potential mechanisms responsible for increased oil recovery from low salinity waterflooding (LSWF). We propose an approach to studying LSWF and identify the key missing links that are needed to explain observations at multiple length scales. The synergistic effect of LSWF on other chemical enhanced oil recovery methods such as surfactant, alkaline, nanoparticle and polymer flooding are also outlined. We specifically highlight key uncertainties that must be overcome to fully implement the technique in the field.

Electrostatic Origins of CO2-Increased Hydrophilicity in Carbonate Reservoirs.

Injecting CO2 into oil reservoirs appears to be cost-effective and environmentally friendly due to decreasing the use of chemicals and cutting back on the greenhouse gas emission released. However, there is a pressing need for new algorithms to characterize oil/brine/rock system wettability, thus better predict and manage CO2 geological storage and enhanced oil recovery in oil reservoirs. We coupled surface complexation/CO2 and calcite dissolution model, and accurately predicted measured oil-on-calcite contact angles in NaCl and CaCl2 solutions with and without CO2. Contact angles decreased in carbonated water indicating increased hydrophilicity under carbonation. Lowered salinity increased hydrophilicity as did Ca2+. Hydrophilicity correlates with independently calculated oil-calcite electrostatic bridging. The link between the two may be used to better implement CO2 EOR in fields.

Alkaline flocculation of Phaeodactylum tricornutum induced by brucite and calcite.

Alkaline flocculation holds great potential as a low-cost harvesting method for marine microalgae biomass production. Alkaline flocculation is induced by an increase in pH and is related to precipitation of calcium and magnesium salts. In this study, we used the diatom Phaeodactylum tricornutum as model organism to study alkaline flocculation of marine microalgae cultured in seawater medium. Flocculation started when pH was increased to 10 and flocculation efficiency reached 90% when pH was 10.5, which was consistent with precipitation modeling for brucite or Mg(OH)2. Compared to freshwater species, more magnesium is needed to achieve flocculation (>7.5mM). Zeta potential measurements suggest that brucite precipitation caused flocculation by charge neutralization. When calcium concentration was 12.5mM, flocculation was also observed at a pH of 10. Zeta potential remained negative up to pH 11.5, suggesting that precipitated calcite caused flocculation by a sweeping coagulation mechanism.

Critical conditions for ferric chloride-induced flocculation of freshwater algae.

The effects of algae concentration, ferric chloride dose, and pH on the flocculation efficiency of the freshwater algae Chlorella zofingiensis can be understood by considering the nature of the electrostatic charges on the algae and precipitate surfaces. Two critical conditions are identified which, when met, result in flocculation efficiencies in excess of 90% for freshwater algae. First, a minimum concentration of ferric chloride is required to overcome the electrostatic stabilization of the algae and promote bridging of algae cells by hydroxide precipitates. At low algae concentrations, the minimum amount of ferric chloride required increases linearly with algae concentration, characteristic of flocculation primarily through electrostatic bridging by hydroxide precipitates. At higher algae concentrations, the minimum required concentration of ferric chloride for flocculation is independent of algae concentration, suggesting a change in the primary flocculation mechanism from bridging to sweep flocculation. Second, the algae must have a negative surface charge. Experiments and surface complexation modeling show that the surface charge of C. zofingiensis is negative above a pH of 4.0 ± 0.3 which agrees well with the minimum pH required for effective flocculation. These critical flocculation criteria can be extended to other freshwater algae to design effective flocculation systems.

Surface chemistry of K-montmorillonite: ionic strength, temperature dependence and dissolution kinetics.

The surface chemistry of K-montmorillonite was investigated by potentiometric titrations conducted at 25, 50 and 70 degrees C and at ionic strengths of 0.001, 0.01 and 0.1 M KNO(3). Proton adsorption decreases with electrolyte concentration at all pHs. The pH of zero net proton charge (PZNPC) decreases from 8.1 to 7.6 when the ionic strength increases from 0.001 to 0.1 M. Temperature has a very small effect on surface charge. A constant capacitance model that accounts for protonation/deprotonation of aluminol and silanol edge sites and basal plane H(+)/K(+) exchange is used to fit the experimental data. H(+) and OH(-) adsorption to specific surface sites appear to account for the pH-dependence of the K-montmorillonite dissolution.

Effects of clay minerals on Cr(VI) reduction by organic compounds.

The objective of this study is to examine the effect of clay minerals (illite, montmorillonite, and kaolinite) on chromate (Cr(VI)) reduction by several low molecular weight organic compounds. Batch experiments at pH ranging from 3.0 to 6.0 and 25 degrees C showed that 2:1 layered clays illite and smectite catalyzed Cr(VI) reduction by oxalate. The catalytic effect increased as pH was decreased. The 1:1 clay kaolinite had no catalytic effect under comparable conditions. Direct Cr(VI) reduction by reactive moieties associated with illite and montmorillonite was observed, but at a much slower rate than the catalytic pathway. Cr(VI) reduction by glyoxylic acid, glycolic acid, lactic acid, and mandelic acid was accelerated by illite, although aqueous phase reduction might occur in parallel. These results suggest that Cr(VI) reduction rates in subsurface environments rich in organic compounds may be elevated through catalysis of surface-bound metals and/or soluble species from the clay minerals, and as a result, higher than those expected from aqueous phase reaction alone. Such rate enhancement for Cr(VI) reduction needs to be accounted for when developing new remedial techniques for chromium site remediation or assessing its natural attenuation.