A gel system composed of acrylamide (AM), N , N ′-methylenebisAM (BIS), and ammonium persulfate ((NH 4 ) 2 S 2 O 8 ) was developed and applied extensively in reservoirs to reduce water cut and increase oil production in mature fields. However, this gel system suffers from thermal stability loss and syneresis at high temperatures that reduces its ability to control water flow. It has been widely accepted that the loss of gel thermal stability can be explained via three aspects: the rupture of polymer chains, the breakage of cross-linker chains, and hydrolysis of polymer. The mechanism of hydrogel syneresis through polymer hydrolysis has been investigated extensively in other publications. However, research on the other two mechanisms is quite limited. In this article, we conduct a series of experiments to demonstrate how the rupture of polymer and cross-linker chains leads to the hydrogel instability at high temperatures. Viscosity and energy-dispersive system measurements suggested that polyAM chains were disrupted by the oxidation reactions involving free radicals. The method to measure the cross-linking degree was established and in combination with X-ray photoelectron spectroscopy measurements, the results showed that cross-linker chains were broken as a result of weaker C–N bond resulting from positively charged mesomethylene carbon and hydrolysis of amide groups on the cross-linker. Because of the application of deionized water in the experiments, nuclear magnetic resonance and FTIR measurements showed that the hydrolysis degree of polymer was weak. Hence, our results verified that breakage of polymer and cross-linker chains led to the rupture of the gel network at high temperature. Besides, cross-linker chains may play a more important role in the thermal stability of the gel, which explains some work into high-temperature-resistant gels.

Monoisopropanolamine (MIA) and monoethanolamine (MEA), which are two kinds of organic alkalis, present favorable potential for enhanced oil recovery. When MIA and MEA are respectively applied, the minimum oil/water interfacial tension (IFT) can be reduced to about 1 mN/m, the crude oil can be emulsified, and the originally oil–wet sand surface can be altered to weak water-wet status. To display the synergistic effect of surfactant and organic alkalis, a surfactant/compound organic alkalis (MIE/MEA) flooding system (SMM), which consists of 0.10 wt % SDBS, 0.15 wt % MIA, and 0.10 wt % MEA, is screened. This system can reduce the minimum oil/water IFT to an ultralow value which contributes to the emulsion stability. Results of sandpack flooding tests indicate that although the incremental oil recovery can be continuously enhanced with the increase of the SMM slug size, there is an optimal injection size (0.8 PV in this study) when the expense is taken into consideration. As for the effect of the injection type, the highest incremental oil recovery (21.86% OOIP) is achieved when organic alkalis (MIA/MEA) are simultaneously injected with the SDBS. During the SMM flooding, emulsification is a crucial flooding mechanism, and most of the incremental oil is extracted in the form of emulsified oil droplets. Moreover, the injection rate is optimized (0.5 mL/min) for the SMM flooding system to achieve a satisfactory incremental oil recovery.

In order to improve the CO2 foam stability at high temperature and salinity, hydrophilic silica nanoparticles (NPs) were added into a dilute zwitterionic surfactant solution to stabilize supercritical CO2 (SC-CO2) foam. In the present paper, the foaming capacity and stability of SC-CO2 foam were investigated as a function of NP concentration at elevated temperatures and pressures. It was observed that the drainage rate of SC-CO2 foam was initially fast and then became slower with NPs adsorption at the gas-liquid interface. The improved foam stability at high temperature was attributed to the enhanced disjoining pressure with addition of NPs. Furthermore, an obvious increase in the foam stability was noticed with the increasing salinity due to the screening of NP charges at the interface. The rheological characteristics including apparent viscosity and surface elasticity, resistance factor, and microstructures of SC-CO2 foam were also analyzed at high temperature and pressure. With addition of 0.7% NPs, SC-CO2 foam was stabilized with apparent viscosity increased up to 80 mPa·s and resistance factor up to 200. Based on the stochastic bubble population (SBP) model, the resistance factor of SC-CO2 foam was simulated by considering the foam generation rate and maximum bubble density. The microstructural characteristics of SC-CO2 foam were detected by optical microscopy. It was found that the effluent bubble size ranged between 20 and 30 μm and the coalescence rate of SC-CO2 foam became slow with the increasing NP concentration. Oscillation measurements revealed that the NPs enhanced surface elasticity between CO2 and foam agents for resisting external disturbances, thus resulting in enhanced film stability and excellent rheological properties.

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