# Chemical EOR with Methyl Ester Sulfonate: Achieving Residual Oil Saturation via 2–4-Order Capillary Number Increase

## TL;DR

This study shows that methyl ester sulfonate (MES) can effectively recover oil in harsh conditions by reducing oil-water tension and altering rock wettability.

## Contribution

The study demonstrates that MES enables capillary number increases of 2–4 orders of magnitude, leading to significant oil recovery in high-salinity and high-temperature environments.

## Key findings

- MES reduces oil–water interfacial tension to ∼0.02 mN/m at 80 °C.
- MES increases capillary numbers by 102–104, enabling residual oil saturation.
- Oil recovery exceeded 28% under optimized salinity in spontaneous imbibition tests.

## Abstract

This study extends the application of methyl ester sulfonate
(MES),
a biodegradable, anionic surfactant derived from renewable resources,
for chemically enhanced oil recovery (EOR) in high-salinity (up to
700 mM NaCl) and high-temperature (80 °C) conditions. MES is
demonstrated to reduce oil–water interfacial tension to low
values (∼0.02 mN/m at 80 °C), alter sandstone wettability
to preferentially water-wet, and form stable Winsor III microemulsions.
We highlight a key advancement by systematically compiling and comparing
published capillary desaturation data, showing that increasing the
capillary number by a factor of 102–104 enables the system to approach irreducible water or residual oil
saturation. Our experimental results align with this trend, where
MES significantly increases capillary numbers through synergistic
effects of interfacial tension reduction and wettability alteration.
Zeta potential measurements confirm colloidal stability across a broad
concentration and salinity range, while thermal analysis supports
MES stability up to 90 °C. Spontaneous imbibition tests show
oil recovery exceeding 28% under optimized salinity, further validating
the MES efficacy. This work not only expands the operational envelope
of MES for chemical EOR but also reinforces the mechanistic link between
the capillary number scaling and residual oil displacement. By integrating
lab-scale results with a broad data set from the literature, this
study delivers a compelling foundation for deploying MES in field-scale
EOR operations, particularly in mature reservoirs where sustainable
and cost-effective solutions are critical.