Experimental study of calcium-enhancing terpolymer hydrogel for improved oil recovery in ultrodeep carbonate reservoir

Abstract

Along with the continuous water injection, water production problem is getting worse in ultrodeep carbonate reservoir (depth≥6000 m), but it is hard for common hydrogels to plug the water in this hostile reservoir condition (temperature≥130 °C, salinity≥220,000 mg/L). In this paper, acrylamide/acryl-acid/2-acrylamido-2-methyl-propanesulfonate (AM/AA/AMPS) hydrogel has been found to be a very effective plugging agent for this unconventional reservoir, and its thermal-resistance and salt-tolerance mechanism was studied detailedly. The results show that stable time of AM/AA/AMPS hydrogel can reach up to 60 days, and the syneresis rate is below 5% on the 120th day in the hostile condition (temperature = 130 °C, salinity = 223,000 mg/L, Ca2+ concentration = 11,000 mg/L). The viscosity and weight-average molecular weight of four polymers (PAM, AM/AA /AMPS, AM/DAC and AM/DMDAAC) decrease with the increasing aging time, but the descent speed of AM/AA/AMPS is the slowest. Besides, AM/AA/AMPS has strong hydrophilicity in the saline water since its Rh is the highest when the weight-average molecular weight of four polymers is nearly the same. FTIR, NMR and LS measurements show that the COO− of AM/AA/AMPS reacts with Ca2+, changing the hydrogel from the single-group crosslinking to double-groups crosslinking, which results in the microgrid structure of hydrogel becoming dense and strong. In contrast with the common hydrogel, the AM/AA/AMPS hydrogel shows better water plugging effect in the hostile condition. The enhancing hydrogel strength and outstanding absorption of polymer onto rock are considered as the important reasons to the better performance of AM/AA/AMPS hydrogel.