الملخص الإنجليزي
Improving oil recovery in carbonate reservoirs using low salinity/smart water has
been proven to be an efficient technique in the petroleum industry. It has been agreed
in literature that a brine of salinity lower than that of formation water enhances the
sweep efficiency by wettability alteration. However, the efficiency of the smart water
depends on the chemical activity of the surface modifying agents known as the potential
determining ions (PDIs); Ca2+, Mg2+ and SO4
2-
, which is temperature dependent.
Therefore, a comprehensive understanding of the effect of temperature on the PDIs
during smart water injection is a key factor for the success of the process.
This study aimed to investigate the effect of temperature on the activity of the PDIs
as well as their interactions in modifying the wettability of oil-aged carbonate surfaces
(calcite and dolomite). Temperatures of 25, 50, 75, 100 and 125 °C, representing global
carbonate reservoir temperatures, were selected for the study. Wettability alteration was
evaluated through contact angle (CA) measurement, while the effect of temperature on
the surface charges at the fluid/rock interface was analyzed using zeta potential
measurement. The concentrations of the PDIs before and after treatment were analyzed
to gain an insight of the driving mechanism that led to wettability alteration towards a
water-wet state.
Results showed a decreasing trend of CA and a shift in wettability towards more
water wetness for both mineralogies with temperature elevation, more significant at
temperatures greater than 75 °C for all the three individual PDIs. However, this shift in
wettability necessitates both high temperature and the presence of PDIs in the treating
solution as no significant change in wettability was recorded after treatment of the
carbonate surfaces with deionized water (DIW). Additionally, it was observed that
Mg2+/SO4
2-
combination had a better performance than Ca2+/SO4
2-
combination at all
temperatures for both carbonates. The strongest water-wet calcite surface resulted after
treatment with smart water containing all the three PDIs at 125 °C, while the same was
observed for dolomite after treatment with Mg2+/SO4
2-
combination at the same
conditions.
Zeta potential results showed a charge reversal from negative to positive after
treatment of the carbonate particles with all tested smart waters except the smart water
containing only SO4
2-
. A significant shift in surface charge was observed as temperature
increases to 125 °C, which is in-line with CA results. Ions analysis of the smart water
solutions after treatment with both carbonates revealed that the concentration of SO4
2-
ions decreases with temperature, which is due to adsorption of the negatively charged
ions on the rock surface. The final concentrations of Mg2+ and Ca2+ ions in the treating
solution is however greater than its initial concentrations, which is a result of both
dissolution and removal of the Ca/Mg- Carboxylate complexes from the surface,
producing water-wet surfaces.
Overall, the results confirm that an increase in temperature enhances the multi-ion
exchange (MIE) between the ions in the solution and the carbonate surface. However,
the interplay between the ions and the reaction kinetics not only depend on the
temperature, but also on the surface mineralogy as well as the ion composition of the
smart water.
