A pH-Resolved View of the Low Salinity Effect in Sandstone Reservoirs

Abstract

Core plug and field tests have shown that significantly more oil can be produced from sandstone reservoirs when the water that is injected to maintain pressure in the reservoir has lower salinity than regular seawater. We investigated the adhesion between functional groups known to dominate in crude oil and grains from reservoir sandstone as a function of pH and salinity. We used atomic force microscopy (AFM) in chemical force mapping mode, and we functionalized the AFM tips both with −CH 3 , to model adhesion of hydrophobic molecules, and with −COO(H), to model polar compounds. As the pH increased, the adhesion force decreased. The behavior was similar for the two types of tips, but adhesion was higher for the polar tip. The average adhesion at pH 4.5 for the −COO(H) tip in artificial seawater (ASW) was close to 300 pN, and 200 pN in ASW diluted by a factor of 20. For the −CH 3 tip, adhesion was ∼200 pN in high salinity and 140 pN in low salinity. Regardless of the salinity, adhesion decreased as pH increased. To gain understanding about the controlling processes, we examined our data in light of the Dejarguin−Landau−Verwey−Overbeek (DLVO) theory and the Henderson−Hasselbalch expression. The commonly used form of the DLVO relationship could not account for the pH dependence. It predicts adhesion to be independent of pH in high-salinity solution and strongly dependent on pH in low salinity, opposite to the experimental evidence. Although DLVO theory cannot be expected to quantitatively represent real systems, it is unlikely that DLVO would predict reversed behavior. Our results indicate that the extended double layer, which is one of the prevailing explanations for the low salinity effect, is not the full explanation. Decrease in surface charge density at low salinity plays an important role. The change in adhesion as pH increases above 5.5, which is the typical pH of sandstone reservoirs, to the neutral range could enhance oil recovery. This is probably a factor in the effectiveness of alkaline flooding and might be an inherent factor in the production that results from flooding with seawater, which is typically pH 8.3. However, the buffering effect in sandstone reservoirs, which maintains pH at ∼5.5, suggests that benefits from controlling pH for EOR could be marginal.

Chemical Force Microscopy Study on the Interactions of COOH Functional Groups with Kaolinite Surfaces: Implications for Enhanced Oil Recovery

“…The observed increase in measured adhesion with increasing pH, however, is not in agreement with either CFM measurements performed on sandstone samples [33], nor with the studies on the adsorption of benzoic acid [37] and crude oil [38] by kaolinite. In the former experiments, the presence of organic groups in the surface was invoked to explain the results, so a direct comparison to the results shown here is not appropriate.…”

“…Austad et al have shown experimental data where an increase in the pH of the effluent solution is observed after injection of low salinity water [5]. In addition, Shi et al [33] observed a small decrease in the measured adhesion for CFM experiments performed on sandstone grains using -CH3 and -COOH functionalised AFM probes. For these reasons, it was decided to perform a series of experiments where the Ca 2+ solution composition was kept constant (at 0.001 M), but the pH was varied.…”

“…This was done to try to disentangle a pure MIE mechanism from one combined with a variation in pH, as it has been reported that pH can play a large role in determining the oil adsorption capacity (or adhesion) [5,7,33]. Figure 3 shows the results from experiments kao-Si-1 and kao-Si-2.…”

“…In this model, a reduction in Ca 2+ concentration would result in a smaller amount of bridging and therefore to less adhesion. Authors have, however, usually linked the MIE mechanism to a variation in pH as well [5,33] as, in their experiments, this parameter was allowed to vary, or it is varied in a controlled manner. In the case of the experiments shown here, the pH was kept constant throughout its duration, and only the concentration was changed.…”

“…This is in contrast to some previous studies that have attempted to explain results via a single theory such as double layer expansion or multicomponent ion exchange mechanisms. Recent studies on the sorption of -CH 3 and -COOH groups on sandstone surfaces acknowledge the deficiencies of accounting for changes in adhesion by solely relying on the double layer expansion model, and, in particular on the use of DLVO theory (Derjaguin, Landau, Verwey, and Overbee), which allows for the calculation of electric double layer force, in order to address the complex adhesion behaviour shown by sandstone surfaces [33]. One of the main problems highlighted is the assumption that the surface charge density does not depend on the ionic strength, or salinity, of the solution.…”