This study presents the development of modified nanographite (MG) as a crosslinking agent to enhance the temperature and salt resistance of gel systems and enable their use in deep oil and gas reservoirs. The novel nanographite hybrid crosslinked gel was formed by combining polyacrylamide (PAM) as the polymer matrix with hexamethylenetetramine (HMTA), hydroquinone (HQ), and MG as the crosslinking agents. The effects of the MG concentration on the gelation time, gel strength, and thermal stability were investigated, and the plugging ability of the nanographite hybrid crosslinked gel system was evaluated. The results demonstrated that the gelation time of the hybrid gel system comprising 0.8 wt% PAM, 0.6 wt% HMTA, 0.6 wt% HQ, and 0.2 wt% MG was 4 h, and the maximum tolerance temperature was 154.9 degrees C. Furthermore, the addition of MG more than doubled the storage modulus (G') and loss modulus (G '') of the gel system. Compared to the conventional gel system without MG, the hybrid gel exhibited remarkable bound water contents as high as 21.14%, which represented a more than 5-fold improvement. Microstructural analyses revealed that the hybrid gel network contained numerous MG sheets, leading to a denser network structure compared to that of the conventional gel. The presence of the MG greatly increased the network strength and crosslinking density of the hybrid gel, thereby improving its thermal stability, temperature resistance, and salt resistance. Additionally, the hybrid gel exhibited excellent plugging performance, achieving a core plugging rate of 96.2%.

Bubble breaks up and coalesces continuously during migration in reservoirs, which determines the size distribution of bubbles. As a key parameter of foam technology, the bubble size greatly affects foam stability and plugging performance in the reservoir. Aiming to solve the problem of poor stability of ordinary foam under reservoir conditions, in this study, a high-stability foam for profile control was prepared using dispersed particle gel particles as a foam stabilizer. The characteristics of foam were mainly investigated by the modified Ross-Miles method and microfluidic experiments. The obtained results showed that the foam reinforced by dispersed particle gel particles had a longer half-life under reservoir conditions. It is easier to break up from parent bubbles to daughter bubbles during the migration, which can form foam with higher stability in the porous media. The average size of the daughter bubbles decreased with increasing liquid-phase flow rate, pore-throat ratio and capillary number. The average size varied with the capillary number in a power-law form. Because of the viscoelastic interfacial film and liquid-phase viscous network, the shear effect on the parent bubble in reinforced foam was increased when passing through the pore-throat structure with variable diameter. The large parent bubbles were more likely to break up into more small and stable daughter bubbles. And the interfacial film with higher mechanical strength makes it difficult for the daughter bubbles in reinforced foam to coalesce again. The results indicated that the reinforced foam had stronger dynamic stability and better plugging performance in the reservoir. This work revealed the breakup and coalescence mechanism of bubbles reinforced by dispersed particle gel particles in porous media and provided theoretical guidance for the application of foam technology in high water-cut reservoirs.

Chemical viscosity reduction and cold production technology has the characteristics of low cost, low energy consumption, and low emission, which makes it suitable for gradually replacing thermal production as the main development method of heavy oil reservoirs. Most of the existing evaluation processes of viscosity reducers used in chemical viscosity reduction and cold production adopt the dynamic method of preparing O/W emulsions by high-speed shearing. However, high-speed shearing cannot be achieved in the contact process between heavy oil and the viscosity reducer in porous media. There is only a weak seepage shear force, which makes heavy oil self-diffuse in the viscosity reducer solution. Therefore, the existing methods cannot accurately evaluate the self-diffusion performance of viscosity reducers under quasistatic conditions. In this work, a method for evaluating the self-diffusion performance of a heavy oil viscosity reducer based on UV-Vis absorption spectroscopy is proposed, and the influence of various factors on the viscosity reduction effect and self-diffusion performance of a viscosity reducer is investigated. The results show that there is a good correspondence between the viscosity reduction effect and the self-diffusion performance. The viscosity reducer concentration, oil content, temperature, and emulsification time have obvious effects on the self-diffusion performance of the viscosity reducer. Based on the results of the orthogonal experiment, a self-diffusion performance prediction model of the viscosity reducer is established. The verification experiment results show that the model has good applicability. This research provides a new idea for accurately evaluating the self-diffusion performance of viscosity reducers under quasi-static conditions.

Foam technology is currently recognized as one of the most promising technical means to further enhance oil recovery. However, the poor stability of foam in harsh reservoirs with high temperatures limits the further application of foam fluids in oil fields. Based on the Pickering emulsion template method, this paper proposed a synthesis strategy for Janus nanographene oxide with aerophilic/hydrophilic characteristics as a novel foam stabilizer. A high-stability foam reinforced by Janus nanographene oxide was constructed. In addition, this paper clarified the relationship between foam stability and the interfacial characteristics of foam and revealed the probable mechanism by which Janus nanographene oxide enhances foam stability. The results showed that the synthesized Janus nanographene oxide has a typical two-dimensional lamellar structure with an obvious asymmetric characteristic. The negatively charged Janus nanographene oxide showed a small particle size, high zeta potential, high thermal stability, satisfactory amphiphilicity, and high interfacial adsorption, which was beneficial for enhancing foam stability at high temperatures. The foam stability was positively correlated with the interfacial viscoelastic modulus of the foam. Janus nanographene oxide was irreversibly adsorbed onto the gas-liquid interface, which increased the interfacial viscoelastic modulus, especially the interfacial elastic modulus increased from 2.94 mN center dot m(-1) to 16.21 mN center dot m(-1). As a result, the ability of the foam to resist deformation due to external forces was enhanced, which can be reflected in the enhancement of the foam stability. The half-life of foam at high temperature increased from 26 min to 49 min. An appropriate amount of Janus nanographene oxide can form a solid-like film with high mechanical strength on the gas-liquid interface and gradually fill the Plateau boundary of the foam. It can not only effectively reduce the drainage rate of the liquid film but also hinder gas diffusion between adjacent bubbles, which delays the further thinning of the film and coarsening behavior of the foam. Therefore, the constructed reinforced foam can be considered as a promising candidate for further enhancing oil recovery in high-temperature reservoirs, especially for steam channeling control. In addition, this work contributes to a better understanding of the mechanism by which Janus nanomaterials can enhance foam stability and provides theoretical guidance for the further application of foam technology. (c) 2022 Published by Elsevier B.V.

The breakthrough flow channel generated by formation heterogeneity has severely restricted oil recovery improvement, and its identification and control methods have received extensive attention. In this study, we developed a novel fluorescent dispersed particle gel (FDPG) for identifying and regulating breakthrough flow channels based on the dispersed particle gel (DPG). A copolymer of acryloyl fluorescein and acrylamide (PAMAF) was synthesized by free-radical copolymerization for the preparation of the FDPG. The fluorescence properties and identification method of FDPG were systematically investigated. The results showed that the FDPG had stable fluorescence properties, and its concentration showed a linear relationship with fluorescence intensity. The excitation and emission wavelengths of FDPG fluorescence fall within the interval that can avoid the influence of crude oil on its detection. FDPG can accurately reflect the migration of particles in the formation and avoid the chromatographic separation effect. FDPG has excellent injection and migration properties and can effectively control the high permeability layer after ageing. By analysing the output of the FDPG, breakthrough flow channels that exist between formation layers and within well groups can be effectively identified. FDPG provides a promising approach for identifying breakthrough flow channels and monitoring particle profile control agents.

Conventional flooding systems have very low recovery rates in ultralow permeability reservoirs in virtue of the characteristics of high injection pressure, low permeability and small pores in such reservoirs. In this study, a novel surface-modified carbon black (MCB) nanofluid was prepared as a flooding system to resolve this problem. The preparation process of MCB nanoparticles was divided into three stages: (1) oxidation of carbon black (CB) nanoparticles, (2) acyl chlorination of oxidized carbon black (OCB) nanoparticles and (3) grafting of active groups onto the surface of acyl chloride carbon black (ACB) nanoparticles, and the main synthesis conditions, such as modifier concentration and grafting temperature, were optimized. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analysis indicated that the factional groups were successfully introduced onto the surface of CB nanoparticles, and the crystallinity of CB nanoparticles decreased gradually during the reaction process. Additionally, the dispersed stability, temperature resistance, salinity tolerance, wettability alteration and oil stripping ability were evaluated so as to obtain the performance of the MCB nanofluid. The results show that the average size of the MCB nanoparticles is approximately 72.3 nm, and the dispersed stability of the MCB nanofluid is much better than that of the CB nanofluid. Compared with the carbon black (CB) nanofluid, the MCB nanofluid shows stronger wettability alteration and oil stripping ability, and can also withstand the high temperature and high salt environment of oil reservoirs. 1 PV of MCB nanofluid can enhance oil recovery by 28.9%. Furthermore, the enhanced oil recovery (EOR) mechanisms for the MCB nanofluid are revealed through analysis of low oil-water IFT (IFT), wettability alteration and Wasan's theory of structural disjoining pressure. This new type of environment-friendly flooding system injects vitality into the development of ultralow permeability reservoirs.

A high-temperature- and high-salinity-resistant dispersed particle gel (DPG) was successfully prepared by shearing a modified nanographite-strengthened bulk gel. Through the preparation requirements, a modified nanographite-strengthened bulk gel was prepared using a modified nanographite hybrid polymer and monomer cross-linkers. The effect of the nanographite hybrid polymer, cross-linkers, temperature, and salinity on the gel performance of the strengthened bulk gel was systematically studied. The higher proportion of bound water and the dense three-dimensional network structure of the strengthened bulk gel are the keys to temperature and salinity resistance. Rheological test results show that the bulk gel had excellent viscoelasticity. The effects of temperature, salinity, and aging time on DPG coalescence were studied from the perspective of micromorphology and particle size. The accelerated thermal motion at a high temperature increases the probability of collision and coalescence between particles. The high-salinity compression diffusion double layer reduces the electrostatic repulsion between particles. Finally, the strengthening mechanism of DPG by the modified nanographite was discussed. The high-temperature- and high-salinity-resistant DPG may be an alternative for profile control and water plugging in deep oil reservoirs.

The reservoir heterogeneity and unfavorable oil/gas mobilityratiolead to gas channeling and low CO2 sweep efficiency duringCO(2) flooding in low-permeability reservoirs. A dispersedparticle gel (DPG) could migrate deep into the reservoir and coalesce,which has the potential for CO2 gas channeling control.In this work, a CO2-resistant bulk gel was prepared bya cross-linking reaction of a copolymer with acid-resistant groupsand catechol-hexamethylenetetramine, which exhibited excellentCO(2) resistance. Subsequently, a CO2-resistantdispersed particle gel (SCDPG) used in supercritical CO2 was successfully prepared from the CO2-resistant bulkgel by a high-speed mechanical shearing method. Meanwhile, the coalescencebehavior of the SCDPG particles in supercritical CO2 wassystematically investigated from the microstructure, particle size,& zeta; potential, and mechanical strength. The results showed thatthe SCDPG particles were dispersed in the liquid phase as a singleparticle. SCDPG had good dispersion stability during storage and injection.In supercritical CO2, the dispersion stability of SCDPGdecreased, and the particles coalesced with each other to form aggregateswith a stereostructure instead of degradation. The SCDPG particlesmaintained high mechanical strength, showing the long-term effectivenessfor gas channeling control during CO2 flooding. In addition,the interparticle force of SCDPG particles was measured by an AFMcolloid probe based on the reservoir characteristics. The interparticleforce of SCDPG particles changed from repulsion to adhesion with increasingsalinity. At a salinity of 0.5 mol/L, the adhesion force increasedwith the increase of temperature and the decrease of the pH value.According to the type of intermolecular force, the adhesion forceoriginated from the hydrogen bond, & pi;-& pi; stacking,and cation-& pi; interaction. This work provides theoreticalsupport for the field application of the dispersed particle gel andpromotes the development of utilization of carbon dioxide in oilfields.

Insufficient mobility control capability and limited swept volume are the primary challenges faced by polymer flooding technology. In this work, a graphite oxide nanoparticle-reinforced polymer (GORP) flooding system was constructed. Compared with the conventional polymer (CP) flooding system, the GORP flooding system with 0.15% graphite oxide nanoparticles has a better viscosity increasing effect, shearing resistance and aging stability. In terms of mobility control capability, due to the reinforcing effect of graphite oxide nanoparticles on the three-dimensional polymer network structure, the GORP flooding system has a higher resistance factor and residual resistance factor than the CP flooding system in different permeability cores. In addition, the NMR online displacement experiment results further prove that the enhanced oil recovery performance of the GORP flooding system is more prominent, with a total recovery of 72.87%, which is 5.04% higher than that of the CP flooding system. The GORP flooding system may have the potential to become a long-term effective technology to control the water cut of oil wells.

In recent years, a newly developed dispersed particle gel (DPG) has attracted significant attention because of its excellent properties and a good application prospect in an enhanced oil recovery process. The preparation method is convenient and easy to scale up for the field application. The dispersed particle gel with sizes ranging from submicron to micron can block the high permeability layers by accumulating in large pore spaces or directly plugging small pore throats. Furthermore, the dispersed particle gels can achieve in-depth profile control due to the elastic deformation and migration into the reservoir's porous media. These characteristics have demonstrated great potential for the dispersed particle gels to strengthen the alkali/surfactant/polymer combination flooding system. The polymers used are the principal sources of viscosity in the system; the surfactant and alkaline produce the synergistic effect by generating the ultralow oil-water interfacial tension. The added dispersed particle gel has a synergistic viscosity increase effect, temperature tolerance, and thermal stability. However, the combined flooding system suffers from the surfactant loss and chromatographic separation that affect the chemical components' synergistic effect for enhanced oil recovery. In this paper, the chromatography separation effects linked to a new method of oil displacement mechanism based on the dispersed particle gel strengthened Alkali/Surfactant/Polymer as a novel combination flooding system was investigated. The addition of dispersed particle gel in the Alkali/Surfactant/Polymer combination flooding system could interleave in the system's *Corresponding network structure and increase the viscosity stability by strengthening the flooding system to improve the oil recovery capacity. The novel dispersed particle gel strengthened alkali/surfactant/polymer flooding systems have a high displacement efficiency and a better-swept volume capacity considering the oilfield requirement for the enhanced oil recovery process. The effects of polymer, surfactant, alkaline, and dispersed particle gel concentration on the combination flooding system were evaluated, furthermore the impact of external factors on the system such as salinity; aging time was described. The results showed that the effects of external factors aging time and salinity slightly affected the system's interfacial reduction capacity. The increase in interface elasticity produced significantly favorable effects on system stability. For the displacement mechanism, when the dispersed particle gel strengthened alkali/surfactant/polymer combination flooding system moved in a porous medium, the dispersed particle gel passed through the pore throat directly or by deformation depending on the system's pressure variation. Furthermore, the phase separation was formed due to the combined component's effects, resulting in the differential migration between the systems, which led to different degrees of chromatographic separation phenomenon to affect the displacement mechanism. As we can see in the micro visualization simulation experiment, the residual oil interacted with Alkali molecules and form the in-situ surfactant, which makes the remaining oil emulsified, as well the surfactant adsorbed by DPG particles, make the crude oil emulsified and get enhanced. The chromatography and micro visualization experiments results of the dispersed particle gel strengthened alkali/surfactant/polymer combination flooding system indicated a favorable application of the system in an enhanced oil recovery process.

To solve the problem of poor stability of ordinary foam in high-temperature conditions, nano-graphite (NG) was used in this study as a foam stabilizer to enhance stability. First, the foam was statically evaluated by the stirring method, and the influence of surfactants, NG, and temperature on foamability and stability were evaluated. Then, the surface tension, apparent viscosity, micromorphology, and contact angle of the foam were measured using a rheometer, microscope, and contact angle measuring instrument. Finally, the plugging ability of the foam was evaluated through a water flooding experiment. The results showed that the synergistic effects of nano-graphite and surfactant significantly improved foam stability, and the best surfactant and nano-graphite concentrations were determined. Compared with ordinary foam, NG-stabilized foam had better stability with increasing temperature. Using the apparent viscosity, surface tension, micromorphology, and contact angle, the mechanism of nanographite stabilization of foam was explained: the mixture of NG and surfactant formed a small number of flocs, which increased the apparent viscosity of the foam. Nanoparticles were adsorbed on the gas-liquid interface, which increased the thickness of the liquid film, improved the mechanical strength of the liquid film, and delayed the drainage of the liquid film and the coalescence of bubbles, thereby improving the stability of the foam. After injecting foam, the pressure difference generated by NG-stabilized foam was approximately twice that of ordinary foam, showing stronger plugging ability. (C) 2021 The Author(s). Published by Elsevier B.V.

A novel dispersed particle gel-strengthened polymer–surfactant (DPS) combination flooding system was proposed and demonstrated for enhanced oil recovery in high water cut mature oilfields. As compared to a conventional polymer–surfactant (PS) combination flooding system, DPS systems have a higher viscosity and a more stable network structure. The polymer is mainly a source of the viscosity, while the surfactant plays a key role in reducing the interfacial tension (IFT). The added dispersed particle gel (DPG) has a synergistic viscosity increase effect, whereas for the DPG particles, the salinity and aging time have a slight effect on the IFT reduction capacity of the DPS system. On the basis of sand-pack flowing experiments, the DPS system has a better mobility control capacity than the PS system in the combination flooding stage and the following water flooding stage. Parallel sand-pack flowing experiments indicate that injection of a DPS system can effectively improve the profile control. The added DPG ...