A novel profile modification HPF-Co gel satisfied with fractured low permeability reservoirs in high temperature and high salinity

Conformance control and water plugging are a widely used EOR method in mature oilfields. However, majority of conformance control and water plugging agents are unavoidable dehydrated situation in high-temperature and high-salinity low permeability reservoirs. Consequently, a novel conformance control system HPF-Co gel, based on high-temperature stabilizer (CoCl₂·H₂O, CCH) is developed. The HPF-Co bulk gel has better performances with high temperature (120 °C) and high salinity (1×10⁵ mg/L). According to Sydansk coding system, the gel strength of HPF-Co with CCH is increased to code G. The dehydration rate of HPF-Co gel is 32.0% after aging for 150 d at 120 °C, showing excellent thermal stability. The rheological properties of HPF gel and HPF-Co gel are also studied. The results show that the storage modulus (G′) of HPF-Co gel is always greater than that of HPF gel. The effect of CCH on the microstructure of the gel is studied. The results show that the HPF-Co gel with CCH has a denser gel network, and the diameter of the three-dimensional network skeleton is 1.5–3.5 μm. After 90 d of aging, HPF-Co gel still has a good three-dimensional structure. Infrared spectroscopy results show that CCH forms coordination bonds with N and O atoms in the gel amide group, which can suppress the vibration of cross-linked sites and improve the stability at high temperature. Fractured core plugging test determines the optimized polymer gel injection strategy and injection velocity with HPF-Co bulk gel system, plugging rate exceeding 98%. Moreover, the results of subsequent waterflooding recovery can be improved by 17%.