English abstract
More than 50% of the world's oil reserves are held in carbonate reservoirs. In recent years,
EOR methods developed for sandstone reservoirs can be enhanced for application in
carbonates. One promising method is the use of engineered water polymer flooding
(EWPF) with the synergistic effect of combining engineered water (EW) and polymer
flooding (PF). The combination of EW with PF could result in a modification of the rock's
wettability, reduction of the mobility ratio, promotion of polymer stability, and reduction
of the required polymer concentration. However, limited studies addressed the
simultaneous effect of engineered water compositions (i.e. potential determining ions
(PDIs)) on the viscosity of polymer and wettability alteration of carbonate surfaces at harsh
reservoir conditions of salinity and temperature. Accordingly, this study aimed to
investigate the effect of salinity, PDIs, and polymer functional groups (partially hydrolyzed
polyacrylamide (HPAM), and, acrylamide tertiary butyl sulfonic acid (ATBs)) on the
wetting properties of calcite, chalk, and dolomite surfaces as well as the viscosity of
solutions. The experimental protocol included viscosity, contact angle (CA), pH, and static
adsorption tests. All tests were carried out at 75 °C, a representative temperature of an
Omani carbonate reservoir. Three combinations of EW were considered in the study:
[Mg2+/SO4
2-
], [Ca2+/SO4
2-
], and [Mg2+/Ca2+]. The results revealed that as the salinity
decreases and polymer concentration increases, the HPAM polymer showed higher
viscosity than the ATBs-based polymers. However, there were optimum ratios of EW at
which the viscosity of ATBs-based polymers was enhanced. The wettability alteration
results showed that the affinity of crude oil towards dolomite surfaces was the highest. The
tendency of shifting the wettability of carbonate surfaces towards a more water-wet state
using EW decreased in the following order: Chalk˃ Calcite˃ Dolomite. The synergetic
effect of combined PDIs and ATBs-based polymer resulted in a further reduction in the
CA. However, the performance of the polymer varied with different EWs for dolomite.
Moreover, the HPAM polymer was more effective in altering the wettability of dolomite
towards a water-wet state compared to the ATBs-based polymer. The overall results
indicate that HPAM polymer suffered from high adsorption on the rock surface and loss of
viscosity for the considered EW systems in this study. Therefore, ATBs-based polymer is
a potential candidate for optimizing the economics of EWPF projects in carbonates.
