Glycine for Enhanced Water Imbibition in Carbonate Reservoirs – What is the Role of Amino Group?

Previous studies indicated the efficacy of the simplest amino acid, glycine, as an aqueous additive for enhanced water imbibition in carbonate reservoirs. The objective of this research was to investigate the importance of the amino group of glycine in its enhanced water imbibition. To this end, glycine was compared with two carboxylates (acetate and formate) with/without adding hydrogen chloride (HCl) for adjusting the solution pH. Note that the amino group is the only difference between glycine and acetate. Contact-angle experiments on calcite were carried out at 347 K and atmospheric pressure with 68000-ppm reservoir brine (RB), and 4 different concentrations of glycine, acetate, and formate solutions in RB. To test the hypothesis that calcite dissolution is one of the main mechanisms in wettability alteration by glycine, we performed another set of contact angle experiments by adding HCl to brine, acetate, and formate solutions. HCl was added to match the pH of the glycine solution at the same concentration. We also performed imbibition tests with Texas Cream Limestone cores at 347 K with brine, glycine, acetate, and formate solutions (with and without HCl) in RB at 5.0 wt%. Contact-angle results indicated that glycine changed calcite's wettability from oil-wet to water-wet (45°). However, acetate solution was not able to change the wettability to water-wet; and formate moderately decreased the contact angle to 80°. The pH level increased from 6.1 to 7.6 after the contact angle experiment in glycine solution, indicating the consumption of hydrogen ions due to calcite dissolution. The levels of pH in formate and acetate solutions, however, decreased from 8.4 to 7.8. The acidity of glycine above its isoelectric point arises from the deprotonation of the carboxyl group. Imbibition tests with carbonate cores supported the observations from the contact-angle experiments. The oil recovery was 31% for glycine solution, 20% for RB, 21% for formate solution, and 19% for acetate solution. This re-confirmed the effectiveness of glycine as an additive to improve the oil recovery from carbonates. An additional set of imbibition tests revealed that acetate at the pH reduced to the same level as glycine was still not able to recover as much oil as glycine. This showed that glycine recovered oil not only because of the calcite dissolution and the carboxyl group, but also because of the amino group. It is hypothesized that the amino group with its electron donor ability creates a chelation effect that makes glycine entropically more favorable to get attached to the calcite surface than acetate. Another important result is that the formate solution at an adjusted pH resulted in a greater oil recovery than RB or RB at the same pH. This indicates that there is an optimal pH for the carboxyl group to be effective in wettability alteration as also indicated by the pH change during the contact-angle experiment.