An in situ terpolymer gel system formed by resorcinol−hexamethylenetetramine (HMTA) was developed and systematically evaluated for water management in extremely high-temperature reservoirs. Suitable gelation time and favorable gelation performance were obtained by adjusting terpolymer and/or cross-linker system concentrations. With the increase of the terpolymer, the resorcinol−HMTA concentration increased both the gelation time and gelation performance. The gel system was prepared by deionized water and maintained good thermostability in a high-salinity environment. Very low concentrations of NaCl, KCl, and CaCl 2 can delay gelation time. After the critical concentrations are reached, these inorganic ions will boost the cross-linking reaction; however, the presence of MgCl 2 shortens gelation time. The gel system in ampules was kept stable at 150 °C for 5 months, and the differential scanning calorimetry measurement indicated that the gel system could be used for water management at temperatures up to 240 °C. Uniformly distributed three-dimensional network microstructures and dendritic structures among the gel grid pores could further increase the network structure strength and firmly lock water within the gel even under extremely high temperatures. The use of the gelation mechanism of an in situ terpolymer gel system formed by resorcinol−HMTA can help petroleum engineers control gelation time and performance.

Summary A terpolymer-gel system using low toxic polyethylenimine (PEI) as the crosslinker was developed for conformance improvement in high-temperature reservoirs. Suitable gelation time (GT), gel strength, and thermal stability could be obtained by selecting PEI molecular weight and adjusting terpolymer concentrations. With the increase of terpolymer concentration, GT decreases and the gel strength increases. However, in this research, the effect of PEI concentration on the gelation performance was much less obvious than that of the polymer concentration. Very low concentrations of sodium chloride (NaCl) can slightly shorten the GT. After critical concentrations were reached, the authors determined that the ions will delay the crosslinking reaction. Moreover, the addition of sodium carbonate (Na2CO3) can also lengthen GT. The gel systems were able to maintain thermal stability at 150°C. Uniformly distributed 3D network microstructures and the small size of the gel-grid pores made the network structure maintain thermal stability. The use of the terpolymer-gel-system gelation mechanism crosslinked by PEI can help petroleum engineers better understand and apply this terpolymer-gel system.

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The effect of four different phenolic compounds (i.e., phenol, catechol, resorcinol, and hydroquinone) on the performance of organically cross-linked terpolymer gel systems at the temperature of 150 °C was investigated. The phenol-based gelant systems were not able to form visible bulk gels at this extremely high temperature because the cross-linked clusters between phenol and hexamethylenetetramine (HMTA) only contained a small number of hydroxyl groups for cross-linking reactions. The catechol- and hydroquinone-based gelants were able to form relatively strong bulk gels because the amount of the cross-linked clusters between these two phenolic compounds and HMTA increased significantly. This increment also contributed to the decrease of the grid sizes of the gel network structures and the emergence of dendritic structures on them, thereby significantly increasing the viscosity, storage modulus, and thermal stability of the obtained gels. However, these two gel systems could not be maintained for long; syneresis began after only 3–12 days of the systems being held at 150 °C. When phenol was replaced by resorcinol, bulk gels with excellent strength and long-term thermal stability were able to form at 150 °C. The use of the gelation mechanism of the cross-linking reactions between the terpolymer and different cross-linker systems can help researchers and petroleum engineers better understand the differences between the different cross-linker systems and thus develop more suitable polymer gel systems for water management in extremely high temperature reservoirs.

Summary The PG Reservoir in Jidong Oil Field is at a depth of approximately 4500 m with an extremely high temperature of approximately 150°C. The average water cut has reached nearly 80%, but the oil recovery is less than 10% after only 2 years of waterflooding process. It is of great importance to develop a high-temperature-resistant plugging system to improve the reservoir conformance and control water production. An in-situ polymer-gel system formed by the terpolymer and a new crosslinker system was developed, and its properties were systematically studied under the condition of extremely high temperature (150°C). Suitable gelation time and favorable gel strength were obtained by adjusting the concentration of the terpolymer (0.4 to 1.0%) and the crosslinker system (0.4 to 0.7%). An increase of polymer and crosslinker concentration would decrease the gelation time and increase the gel strength. The gelant could form continuous 3D network structures and thus have an excellent long-term thermal stability. The syneresis of this gel system was minor, even after being heated for 5 months at the temperature of 150°C. The gel system could maintain most of the initial viscosity and viscoelasticity, even after experiencing the mechanical shear or the porous-media shear. Core-flow experiments showed that the gel system could have great potential to improve the conformance in Jidong Oil Field.