Mature oilfields usually encounter the problem of high watercut. It is practical to use chemical methods for water-shutoff in production wells, however conventional water-shutoff agents have problems of long gelation time, low gel strength, and poor stability under low temperature and high salinity conditions. In this work a novel polymer gel for low temperature and high salinity reservoirs was developed. This water-shutoff agent had controllable gelation time, adjustable gel strength and good stability performance. The crosslinking process of this polymer gel was studied by rheological experiments. The process could be divided into an induction period, a fast crosslinking period, and a stable period. Its gelation behaviors were investigated in detail. According to the Gel Strength Code (GSC) and vacuum breakthrough method, the gel strength was displayed in contour maps. The composition of the polymer gel was optimized to 0.25~0.3% YG100 + 0.6~0.9% resorcinol + 0.2~0.4% hexamethylenetetramine (HMTA) + 0.08~0.27% conditioner (oxalic acid). With the concentration increase of the polymer gel and temperature, the decrease of pH, the induction period became shorter and the crosslinking was more efficient, resulting in better stability performance. Various factors of the gelation behavior which have an impact on the crosslinking reaction process were examined. The relationships between each impact factor and the initial crosslinking time were described with mathematical equations.

Gel conformance control technology is widely used in moderate and high temperature reservoirs. However, there are few studies on shallow low-temperature and high-salinity reservoirs. The difficulties are that it is difficult to crosslink at low temperatures and with poor stability at high salt concentrations. Therefore, the PHRO gel was developed, which was composed of gelatinizing agent (polyacrylamide), crosslinking agents (hexamethylenetetramine and resorcinol) and crosslinking promoting agent (oxalic acid). The PHRO could form high-strength gels in both deionized water and high-concentration salinity solutions (NaCl, KCl, CaCl₂ and MgCl₂). The observation of the microstructure of PHRO gel shows that a strong “stem—leaf”-shaped three-dimensional network structure is formed in deionized water, and the network structure is still intact in high-concentration salt solution. The results show that PHRO has good salt resistance properties and is suitable for conformance control of low-temperature and high-salinity reservoirs.

Water plugging agent has been widely applied on oilfield EOR processes for decades. Conventional plugging agents used on reservoirs with low temperature (50°C) and high salinity (28.6 × 10⁴ mg/L) encounters with some limitations, such as slow gelling, weak gel strength, and poor instability. To overcome these problems, a novel acrylamide monomer polymer gel with strong gel strength, controllable gelling time and good stability was developed. The gelation processes of the plugging agent system at low temperature and high salinity is studied. The cross-linking reaction process of plugging agent is found to be in three stages: induction stage, rapid cross-linking, and stable stage. The composition of this plugging agent system is optimized: 5~7.5 wt% acrylamide + 0.5~0.75 wt% cross-linking agent + 0.005 ~0.12 wt% retarder + 0.5 wt% initiator. The plugging rate is over 96.5% in high permeability sandpack. It also displays good shear resistance. With the continuous flooding by water after the gel is formed, the polymer gel shows good flooding resistance. It can block the water effectively until 10 PV of injected water.