Wettability alteration by glycine and seawater injection in carbonate reservoirs

Abstract

Carbonate reservoirs contain more than half of the world’s conventional oil reserves. However, since most carbonates are naturally fractured and oil- to mixed-wet there is often significant oil saturation remaining after waterflooding. This is because the injected water mostly flows through the fractures without imbibing into the oil-wet matrix. There is an increasing interest in finding low-cost and enviromentally-friendly wettability modifiers that promote water imbibition by shiftting the wetting state of the rock matrix. These injected chemicals, however, must be able to withstand the high temperatures and high salinity brines typically found in carbonate reservoirs. This study presents experimental investigation and modeling work of the application of glycine as a wettability modifier for carbonate reservoirs to improve oil recovery. We first investigated the potential of glycine in altering the wettability of carbonate surfaces. The experiment consisted of monitoring the contact angle of oil droplets placed on top of natural calcite pieces at 95°C for 5 days. The calcite surface remained oil-wet when submerged in formation brine with an average contact angle of 130°. Similar results were obtained with seawater (SW) with a contact angle of 128°. Low salinity water (LSW) was also tested by diluting SW ten times. It resulted in an average contact angle of 108°. In contrast, a strongly water-wet condition was obtained using FB with a glycine concentration of 5 wt% with an average contact angle of 50°. The oil droplets started to detach from the surface on the fourth day. This was direct evidence of the effect of glycine on altering the wetting-state of carbonate surfaces. We then investigated the enhance oil recovery in carbonate rocks by glycine. Spontaneous imbibition experiments were performed at 95 °C with Indiana Limestone cores. Glycine solutions were prepared with FB, SW, and LSW, with a concentration of 5 wt% and compared to LSW. On average, the glycine solutions recovered about 25% more oil than LSW. The recovery factor as a function of the squared root of time showed a linear trend typical of capillary-dominated flow. Glycine significantly enhanced oil recovery in high temperature and high salinity conditions by promoting spontaneous imbibition of water. An explanation to the previous experimental results is that glycine anion interacts with the positively charged surface of carbonate rocks. Wettability alteration then occurs by glycine adsorption and the corresponding removal of organic material from the rock surface. Based on this hypothesis, this research proposes a surface complexation reaction between glycine and carboxylic acids to model wettability alteration. The equilibrium constant was obtained by matching the zeta potential measurements of synthetic calcite in glycine solutions. The tuned surface complexation model (SCM) was used to investigate the desorption of carboxylic acids as a function of glycine concentration and temperature. The results correlated with the contact angle measurements and the recovery factor from the spontaneous imbibition experiments. High temperature was found to be critical for wettability alteration because it increases the concentration of glycine anion in the aqueous phase. Finally, we coupled the SCM in PHREEQC with a numerical model of two-phase flow displacement to investigate the major geochemical reactions driving wettability alteration in carbonates. We found that eight surface complexation reactions in the SCM can be simplified into a couple of anion exchange reactions between the injected wettability modifiers, glycine anion, sulfate ion, and the adsorbed carboxylic acids. Analytical solutions are then presented for the coupled two-phase and multicomponent reactive-transport model with anion exchange reactions to model the injection of wettability modifiers in carbonates.