Relationship between molecular charge distribution and wettability reversal efficiency of cationic surfactants on calcite surfaces

Effect of ethylene oxide groups on calcite wettability reversal by nonionic surfactants: An experimental and molecular dynamics simulation investigation

HYPOTHESIS

Ethoxylated nonionic surfactants are promising candidates for enhanced oil recovery (EOR) from oil-wet carbonate reservoirs due to their ability to reverse the mineral wettability. The wettability-reversal efficiency increases with the number of the ethoxy (EO) groups in the surfactant molecule.

METHODOLOGY

Contact angle measurements, scanning electron microscopy (SEM) and molecular dynamics (MD) simulations were combined to investigate the wettability reversal of an oil-wet calcite by three ethoxylated nonionic surfactants with 1, 4 and 8 EO groups, respectively, to directly probe the role of the EO groups and to uncover the molecular mechanism responsible for the wettability reversal.

FINDINGS

Both experiments and simulations consistently show a clear correlation between the number of EO groups and the wettability reversal efficiency of the surfactants, whereby the higher number of EO groups results in greater degree of wettability reversal. This is due to 1) the more hydrophilic surfactant headgroup weakening the carboxylate interactions with the surface by expanding the surface-adjacent water layer, and 2) the physically larger surfactant molecule attracting the carboxylates more strongly, thus aiding in their removal from the surface.

Surfactant-induced wettability reversal on oil-wet calcite surfaces: Experimentation and molecular dynamics simulations with scaled-charges

HYPOTHESIS

Surfactant flooding is the leading approach for reversing the wettability of oil-wet carbonate reservoirs, which is critical for the recovery of the remaining oil. Combination of molecular dynamics (MD) simulations with experiments on simplified model systems can uncover the molecular mechanisms of wettability reversal and identify key molecular properties for systematic design of new, effective chemical formulations for the enhanced oil recovery.

EXPERIMENTS/SIMULATIONS

Wettability reversal by a series of surfactant solutions was studied experimentally using contact angle measurements on aged calcite chips, and a novel MD simulation methodology with scaled-charges that provides superior description of the ionic interactions in aqueous solutions.

FINDINGS

The MD simulation results were in excellent agreement with the experiments. Cationic surfactants were the most effective in reversing the calcite wettability, resulting in complete detachment of the oil from the surface. Some nonionic surfactants also altered the wettability, but to a lesser degree, while the amphoteric and anionic surfactants had no effect. From the tested cationic surfactants, the double-tailed one was the least effective, but the experiments were inconclusive due to its poor solubility. Contributions of specific interactions to the wettability reversal process and implications for the design and optimization of surfactants for the enhanced oil recovery are discussed.

Experimental Investigation on a Novel Polyelectrolyte Molecular Deposition Film for Improved Injectivity in Low-Permeability Reservoirs

Acrylamide and dimethyl diallyl ammonium chloride were used as monomers to synthesize a polyelectrolyte molecular deposition film (PMDF) injection agent for solving the problem of high injection pressure of water wells in low-permeability reservoirs. The structure of the PMDF injection agent was determined through IR and ¹H nuclear magnetic resonance (NMR). The performance evaluation results show a change of wettability from hydrophilic to neutral wetting with the contact angle changing from 22.32 to 73.31° because of agent injection. It can also change the negative ζ-potential on the surface of the sand to a positive value. For comparison, core displacement experiments involving the commercial Gemini surfactant DF-G reveal that the effects of the depressurization of PMDF are more obvious than those of DF-G. The adsorption stability of the former is better than that of the latter. Especially, under a high-speed water flow of 30 pore volume (PV) injection, the depressurization rate of PMDF is still as high as 43.59%. Finally, the oil-water relative permeability curves and core nuclear magnetic resonance (NMR) experiments demonstrate that the PMDF treatment can reduce the irreducible water saturation, which indicated that the porosity of the flowable part of the core increased and the swept volume was increased. The suitable range of PMDF according to NMR pore-radius distribution within a low-permeability reservoir: the flowable partial pore required the throat radius greater than 0.01 μm.