English abstract
Altering the wettability of rock surface to a favorable wetting condition is one of the
main mechanisms of enhanced oil recovery methods applied in carbonates. In order to
obtain desired wetting state, it is essential to have a clear understanding of the original
wetting properties of the rock surface. This study used different measurements to
investigate the individual effects of an organic acid and a base at different
concentrations as well as their combinations on wettability alteration of calcite surface.
Wettability of the calcite was examined before and after treating the surfaces with
different model oil solutions using contact angle and zeta potential tests. Besides,
Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis
(TGA) were used to recognize the adsorbed functional groups and the amount of
adsorbed organic compounds on the calcite surface, respectively. Moreover, interfacial
tension (IFT) measurements were used to evaluate the interfacial activity of the model
oil systems. Stearic acid represented the crude oil acidic component while N,Ndimethyldodecylamine (N,N-DMDA) represented the basic component of crude oil.
Different model oil systems were used for acid-base combinations in which the acid
concentration was kept constant for each system while the base concentration was
adjusted to prepare base to acid ratios of 0.25, 0.5, 1, 2 and 4. The results revealed that
by increasing the acid number, wettability of calcite surface shifted toward more oilwetness and adsorption of stearic acid on calcite surface increased. Stearic acid with
low concentration (AN=0.25) was enough to alter the wettability of calcite surface
from water-wet to oil-wet. TGA results showed vertical adsorption of stearic acid
molecules on the calcite surface. Treating calcite surfaces with the base revealed that
low concentration of N,N-DMDA (BN=0.25) was able to shift the calcite wettability
from water-wet to slightly oil-wet. It was observed that as BN increases, wettability
alteration toward oil-wetness is slightly improved. TGA results showed low adsorption
of the base with high surface area occupied by the base molecule which indicate that
the base is oriented horizontally on the calcite surface. Possible mechanisms of
wettability alteration are proposed based on possible interactions between the base
molecules/cations and calcite surface. For acid-base mixtures, at constant AN, the
wettability alteration toward oil-wetness increases in the presence of the base until
BN/AN ratio is equal to 1, then oil-wetness decreases. IFT measurements indicated
lower IFT of acid-base mixtures compared to those of individual acidic and basic
model oil solutions which could be due to synergistic effect between acid and base by
forming acid-base complexes. The presence of the base hindered the acid adsorption
the surface. However, the basic component could contribute to the wettability
alteration by forming acid-base complexes which can adsorb on the calcite surface and
impact its wetting preferences. In addition, the base molecules may adsorb on the
calcite surface at high BN/AN. Hence, ratio of BN to AN would determine the
contribution of acid molecules, base molecules and acid-base complexes in wettability
alteration of calcite surfaces. Mechanisms of wettability alteration by acid-base
combinations at different BN/AN ratios are presented.
