A Novel Thermal-Resistance and Salt-Tolerance Gel with Low-Concentration Crosslinkers for Water Shutoff in Tahe Oilfield

Abstract

Abstract Tahe Oilfield is an important oilfield of Sinopec in Northwest China. The reservoir temperature is up to 130°C, and the salinity is as high as 223,000 mg/L. Water production problem is getting worse along with the continuous water injection, but no gels can be successfully applied for water shutoff due to the hostile reservoir condition. Hence, obtaining a thermal-resistance and salt-tolerance gel is more and more important for Tahe Oilfield to decrease the unwanted water production. In this paper, a novel gel system with low-concentration organic crosslinker was developed for water shutoff in Tahe reservoir condition. The gel is based on the polyacrylamide/2-Acrylamido-2-methylpropane sulfonic acid (AM/AMPS) copolymer, and it employs hexamethylenetetramine, resorcinol and phenol as the crosslinkers. Addition of n-hydroxyethylethylenediamine as an anti-syneresis agent increases the gel stability, and the ethylenediamine can be as a crosslink delay agent to increase the gelation time. The system was mainly studied for effects of concentration of polymer and crosslinker on gel stability, and the thermal-resistance and salt-tolerance mechanism of the gel was investigated by the thermostability and scanning electron microscope (SEM) measurements. The strong gel can be obtained when the total crosslinker concentration is 800 mg/L, which is an ultralow low concentration for the crosslinker used in the gel for water shutoff. The gelling solution with a few tens of centipoise viscosity can be pumped with low pressures to effectively seal problem water zones thereby reducing operational costs and increasing recovery. The gel syneresis (defined as the decrease in the gel weight at a given time relative to the initial gel weight) is only 12.5% after the heat-treatment under the difficult environment condition (temperature=130°C, salinity=223,000 mg/L) for 100 days. Core flooding results have illustrated that the plugging efficiency of the gel, which has been heat-treated for 100 days under the above difficult condition, is more than 90%. The thermal treatment experiment shows that AM/AMPS is difficult to be degraded, and the SEM measurement indicates the heat-treated gel still has uniform grid structure, which may be the main reasons for the remarkable stability of the gel under high-temperature and high-salinity condition. The successful development of the novel gel with low-concentration organic crosslinkers not only reduces the production costs and the pollution to the environment, but also provides reference and guidance to obtain the methods to improve the gel stability under harsh conditions. By impeding water production, the gel developed here can be used to delay excess water influx and thus premature abandonment (or installation of expensive lift equipment), thereby extending the life and reserves of oil wells in Tahe Oilfield.