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.

Carbon capture and storage (CCS) technology has been widely investigated to decrease the greenhouse effect. Geological CO2 storage sites are targeted mainly on depleted petroleum reservoirs or deep saline aquifers. However, CO2 leakage might take place through wellbores, cap rocks, reservoir fractures, or faults during or after the process of CO2 storage leading to environmental problems. To minimize these hazards, different kinds of sealants have been developed and applied. This review aims to summarize those materials applicable to CO2 leakage remediation. On the basis of the sealing mechanisms and compositions of different sealant materials, they were divided into seven major types: Portland cement, geopolymer cement, resins, biofilms barriers, gel systems, foams, and nanoparticles. For different types of sealants, their application background, chemical and physical properties, CO2 leakage remediation mechanism, impact factors of sealing performance, advantages, and limitations were summarized. Future development directions for these sealant materials are also recommended. To solve the problem caused by the weak acid-resistant performance of Portland cement, anti-CO2 materials should be developed and added to the formulation. Environmentally friendly materials need to be designed to replace some current user-hostile compositions in the geopolymer cement. Moreover, chemicals that can control the geopolymerization process are also required because of the high curing temperature requirement for the recent geopolymer productions. The injectivity of Portland cement and resin limits its application for in-depth CO2 leakage control; however, gels with relatively low viscosity during the injection can be a good alternative, although their thermal stability and strength need to be further enhanced. Biotechnology and nanotechnology are perspectives to be applied in the CO2 leakage control process. Foams with good stability might be used for CO2 leakage remediation in the porous medium without fractures, but their life cycle should be prolonged.

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Polymer microspheres have been applied for petroleum reservoir enhanced oil recovery (EOR) in the past decade because they can overcome some drawbacks inherent in in situ polymer gel systems. A novel in situ surface decorated polymer microsphere technology was developed for chemical EOR in high-temperature reservoirs. The swelling performance of the conventional polymer microspheres at room temperature was systematically analyzed and verified by an environmental scanning electron microscope. Their swelling and degradation mechanisms at a high temperature (150 °C) were also examined. To improve the long-term thermal stability of the polymer microsphere, different concentrations of polyethylenimine (PEI) were used as an in situ surface decorating agent. The decorated microspheres remained stable at 150 °C for more than three months, and thermogravimetric analysis indicated that the in situ surface decorated polymer microspheres could remain stable at temperatures up to 310 °C. PEI-decorated 3D network structures on the surface of the microspheres prevented water molecules from entering inner structures, thereby inhibiting the polymer microspheres from swelling in aqueous solution under high temperatures. A sandpack flooding experiment showed how the in situ surface decorated polymer microspheres could effectively plug the porous media for water control and oil recovery improvement even after aging at 150 °C for two months. In situ surface polymer microspheres decorated with PEI can help petroleum engineers better apply this gel technology.

Summary The existence of high conductivity features such as fractures, karst zones, and void space conduits can severely restrict the sweep efficiency of waterflooding or polymer flooding. Preformed particle gel (PPG), as a cost-effective technology, has been applied to control excessive water production. However, conventional PPG has limited plugging efficiency in high-temperature reservoirs with large fractures or void space conduits. After water breakthrough, gel particles can easily be washed out from the fractures because of the lack of particle-particle association and particle-rock adhesion. This paper presents a comprehensive laboratory evaluation of a novel water-swellable high-temperature resistant hyperbranched recrosslinkable PPG (HT-BRPPG) designed for North Sea high-temperature sandstone reservoirs (130°C), which can recrosslink to form a rubber-like bulk gel to plug such high conductivity features. This paper systematically evaluated the swelling kinetics, long-term thermal stability, and plugging performance of the HT-BRPPG. Bottle tests were used to test the swelling kinetic and recrosslinking behavior. High-pressure-resistant glass tubes were used to test the long-term thermal stability of the HT-BRPPG at different temperatures, and the testing lasted for more than 1 year. The plugging efficiency was evaluated by using a fractured model. Results showed that this novel HT-BRPPG could recrosslink and form a rubber-like bulky gel with temperature ranges from 80 to 130°C. The elastic modulus of the recrosslinked gel can reach up to 830 Pa with a swelling ratio (SR) of 10. In addition, the HT-BRPPG with an SR of 10 has been stable for over 15 months at 130°C. The core flooding test proved that the HT-BRPPG could efficiently plug the open fractures, and the breakthrough pressure is 388 psi/ft. Therefore, this novel HT-BRPPG could provide a solution to improve the conformance of high-temperature reservoirs with large fractures or void space conduits.