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Microbial metabolic products, such as biosurfactants, bioemulsifiers, acids, solvents, and biogases, are useful for reducing the viscosity of heavy oils and enhancing oil recovery. Two heavy oil viscosity-reducing microorganisms, namely, SH-2 and SH-3, were selected from produced water which were collected from high-temperature reservoirs by enrichment culture technique. The screened bacteria produce biosurfactants and biogases that can biodegrade heavy crude oil components. The screened bacteria combined with indigenous bacteria were applied in a pilot test of microbial huff and puff. Temperature, porosity, and permeability of the reservoir were 50 °C, 14.32%, and 22 mD, respectively. After microbial treatment, the 50 °C degassing for crude oil viscosity of the produced oil was decreased from 750 to 634 mPa·s. Moreover, wax and resin–asphaltene contents of produced oil were reduced by 12.3% and 16.9%, respectively. The average oil production was improved from 2.2 to 3.5 t/day after microbial treatment. The production remained stable without the chemical viscosity reducer for 54 days. The analysis of bacterial community structure indicated that the number of bacteria species increased and that the microbial diversity was highly abundant. However, harmful microorganisms for microbe-enhanced oil recovery, such as sulfate-reducing bacteria, are inhibited during the progress of microbial huff and puff.

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.

Polymer gel systems as water management materials have been widely used in recent years for enhanced oil recovery applications. However, most polymer gel systems are limited in their ability to withstand the harsh environments of high temperature and high salinity. Those polymer gel systems that can handle high-temperature excessive water treatments are reviewed in this paper and categorized into three major types: in situ cross-linked polymer gels, preformed gels, and foamed gels. Future directions for the development of polymer gel systems for high-temperature conditions are recommended. For excessive water management with temperatures from 80 to 120 °C, current polymer systems are substantially adequate. Polymer gel systems composed of partially hydrolyzed polyacrylamide (HPAM)/chromium can be combined with nanoparticle technology to elongate their gelation time and reduce the adsorption of chromium ions in the formation. Phenolic resin cross-linker systems have reasonable gelation times and gel strengths; however, more environmentally friendly cross-linkers should be developed to meet the increasingly stringent environmental requirements. For particle gels, the addition of functional monomer(s) can improve the antitemperature performance. When the applied temperatures reach 120 °C, inorganic cross-linker systems are no longer applicable, and the gelation time of organic cross-linking polymer gel systems and gel thermal stability will decrease significantly due to fast cross-linking reactions. During this period, retarders can be used to elongate the gelation time, and gel strength enhancers (e.g., cement, silica) can also be applied to improve the gel strength at such extremely high temperatures. Most importantly, novel polymers (e.g., ter- or tetrapolymers), functional monomers, and environmentally friendly cross-linkers need to be discovered and developed for polymer gel applications. Second cross-linking systems can be applied to further enhance the strength of the particle gels in harsh conditions. On the basis of these developments, foamed gels can be well-implemented in fractures and wormholes to save the amount of injected gels.

The formulation and mechanism of a hybrid ultralow interfacial tension spontaneous emulsification (SE) system consisting of alkyl polyglucoside surfactant and low-concentration alkali were investigated to enhance oil recovery for low-permeability reservoirs. Multiple-light scattering was used to characterize the stability and demulsification process of oil-in-water (O/W) emulsions. The emulsions were evaluated in terms of droplet size distribution, ζ potential, and three-phase rheological property. The average droplet size of the optimum SE O/W emulsion is about 0.2 μm, which is smaller than the average diameter of the low-permeability throats (0.2–4.0 μm). The coalescence and floatation of emulsion droplets are well-deferred. The oscillation–shear–oscillation rheological property could be further improved with the presence of the optimum system. This can be attributed to the synergistic effect of surfactants and alkali, which enables the formation of a more tight arrangement film of surfactants at the oil–water interface. Subsequently, diffusion of the surfactant from the interface to the bulk phase is inhibited by enhancing the interfacial viscosity. Moreover, the flooding results from the visual micromodel experiments demonstrate that the small residual oil droplets could be displaced easily in the porous media. The newly formulated SE system can be applied to enhance oil recovery for low-permeability reservoirs.