High-temperature performance and cross-linking mechanism of different types of gel systems in saline environment

Profile modification of injection wells or water plugging of production wells are the most common ways to improve oil recovery with the continuous development of oil reservoirs. For high temperature and high-salt oil and gas reservoirs, the plugging agent is required to have certain stability under high temperature and high-salt conditions. Polymer gel is one of the commonly used plugging agents in oil fields. This article conducted detailed experimental research and mechanism analysis on six gel systems composed by two types of polymer (hydrolyzed polyacrylamide [HPAM] and terpolymer L-1) and three types of cross-linker systems (phenol/hexamethylenetetramine [HMTA], resorcinol/HMTA and bisphenol-A/HMTA). The mechanisms of cross-linker systems and polymer were studied, and the experimental researches were done on their gelation process, long-term thermal stability, salt resistance, microstructure observation, rheological properties, and so on. The number of high-temperature resistant cross-linking points determines the performance of the gel system formed by cross-linker systems with the polymer. The cross-linker systems of bisphenol-A/HMTA have four high-temperature resistant cross-linking points, and its performance was the best. Due to the introduction of ATBS and NVP groups on the polymer chain, the performance of gel system formed by the terpolymer L-1 was significantly more stable than that of formed by HPAM. This study shows that the bisphenol-A/HMTA-L-1 gel has excellent long-term thermal stability and salt tolerance, and can be used in ultrahigh temperature (150°C) and high-salt oil and gas reservoirs to improve oil recovery.     

Effect of a polymer on chromium(III) diffusion during gelant injection in fractured media

The effect of a polymer on chromium diffusion during gelant injection into fractured media was explored with a polymer/chromium(III) (Cr³⁺) gelant. The capture and barrier effects are two main affecting mechanisms of polymer molecules on the diffusion of Cr³⁺, and the formation of the polymer leak‐off layer is a key influencing factor. The experimental results show that when the polymer molecular weight (MW) or concentration increased, both the diffusion rate of Cr³⁺ and the leak‐off degree of the gelant decreased sharply. This resulted in the delayed initial production and advanced final production of Cr³⁺. Because of the changing diffusion rate of Cr³⁺ and the dilution effect of brine during gelant injection, the change trends of the Cr³⁺ production from the fracture outlet and matrix ports reversed after the injection volume exceeded a value; this value was named the critical injected volume. During gelant injection into the fractures, under the effects of gravity action and the disproportionate diffusion of Cr³⁺, the chromium diffusion profiles measured at the fracture top and bottom were different. The chromium diffusion profile along the matrix varied with increasing gelant injected volume, but it finally reached the diffusion equilibrium state in the matrix. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43447.     

Flow characteristics of three enhanced oil recovery polymers in porous media

This article presents an experimental study aiming to explore the relationship among rheological properties, flow characteristics in porous media, and enhanced oil recovery (EOR) performance of three typical EOR polymers. The results suggest that xanthan gum exhibits a very pronounced shear‐thinning behavior, which is probably also the reason explaining its moderate adsorption extent within porous media (thickness of adsorbed layer, e = 3.1 μm). The advanced viscoelastic properties coupled with the less adsorption extent compared to the hydrophobically modified copolymer (HMSPAM) allow xanthan gum to establish a “piston‐like” displacement pattern and lead up to 49.4% original oil in place (OOIP) of the cumulative oil recovery during polymer flooding. Regarding HMSPAM, the significant permeability reduction of the porous media induced by multilayer adsorption (e = 5.6 μm) results in much higher drive forces (ΔP) in the extended waterflooding stage, which further raises the cumulative oil recovery by 18.5% OOIP. In general, xanthan gum and HMSPAM totally produced 84% OOIP which is 15% higher than the extensively used EOR polymer, hydrolyzed polyacrylamide (HPAM), under the same experimental conditions. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41598.     

Influence of water on phase transition and rheological behavior of cellulose/ionic liquid/water ternary systems

The influence of water content on liquid crystalline (LC) gel formation and the rheological behavior of a ternary microcrystalline cellulose (MCC)/1‐ehtyl‐3‐methylimidazolium acetate (EmimAc)/water system was investigated using polarized optical microscopy (POM) and rheometry. POM indicated that the distinct water content range for forming a fully anisotropic LC gel with 14 wt % MCC was 4–10 wt %. Adding water to the MCC/EmimAc system resulted in enhanced complex viscosity and storage and loss moduli, and ultimately LC gel formation. Comparison of creep compliance vs. time revealed that the system without water showed representative viscoelastic behavior, while the time dependence of creep compliance disappeared as the water content increased, suggesting elastic‐solid behavior. Additionally, hydrogen bonding between cellulose and EmimAc weakened as water content increased, whereas the intra‐ and intermolecular hydrogen bonds of cellulose became stronger because of strong self‐association. This strong bonding caused aggregation, chain entanglement, and self‐supporting LC gel network formation. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44658.     

Comprehensive insight of crosslinking agent concentration influence on starch-based aerogels porous structure

In this work, we evaluated starch-based aerogels prepared using freeze-drying, and an eco-friendly crosslinking agent (trisodium citrate), in three different concentrations: 12.8, 19.3, and 25.7 mg ml⁻¹. Aerogels were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, relative density, porosity percentage, scanning electron microscopy (SEM), and water uptake. The results indicated a higher reticulation and crystallinity and lower water uptake, increasing crosslinking agent concentration (Aero 1:2). SEM images revealed the prominent presence of macroporous structure (more than 50 nm) within the aerogel, containing small and large pores within the aerogel structure. The small pores were statistically similar for Aero 1:1 and Aero 1:1.5, as observed by analysis of variance analysis, but they were statistically different from Aero 1:2. The large pores diameter increased with higher crosslinking concentration and induced them as closed pores, resulting in a less porous structure. This result is directly associated with a higher reticulation degree. Overall, the influence of crosslinking agent concentration was studied and indicated that Aero 1:1.5 presented valuable properties.     

Tackling slip effects in the nonlinear flow properties of gellan fluid gels

Gellan gum is a biopolymer widely used in the food, pharmaceutical, chemical, and agrochemical fields. Its ability to form a strong gel makes it possible to produce fluid gels. These materials present an apparent yield stress, but its value could be influenced by the wall-slip effect when performing the rheological measurements by which it is determined. In this work, the influence of the measuring surface and gap on flow behavior was first determined. The tests revealed the need to use geometries with rough surfaces, although the sample thickness using a parallel plate has no influence. Subsequently, the value of yield stress was obtained by means of creep tests (found to be 4.3 Pa), and, finally, the effect of wall slip on the dynamic viscoelastic behavior was assessed. There was an influence on the extension of the linear viscoelastic region, but not on the viscoelastic functions of the mechanical spectra. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 46900.     

Degradation behavior of ionic‐covalent entanglement hydrogels

Ionic‐covalent entanglement (ICE) hydrogels are a recently introduced new type of robust materials for potential future application in the fields of tissue engineering and soft robotics. Here the degradation behavior of gellan gum/polyacrylamide ICE hydrogels immersed in PBS or enzyme solutions is presented. It is demonstrated that ICE gels immersed in enzymes became stiffer, whereas under cyclic testing their mechanical responses stabilize after 10 loading/unloading cycles whether immersed in PBS or enzyme solutions. The leachates of the ICE hydrogels were found to be non‐cytotoxic for the growth of L929 and PC12 cells. These findings will be of benefit to the future development of tissue engineering applications based on these gel materials. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41216.     

油气井化学反应气动力深穿透热解堵试验研究

高能气体压裂是解决长庆低渗透油气层解堵的一项简便易行的技术手段.由于火药燃烧随压力增高呈指数增加趋势,装药量受到极大限制.火药燃烧产生大量的CO气体也是一个潜在的危险因素.国外特别是乌克兰等国家都在研究一种新型化学反应气动力热解堵的新技术.根据国外研究的思路,笔者用铝粉、NaOH、NH4NO3甲醛等数种化学物质组配进行了实验室试验研究.以期得到一种新的化学反应气动力热解堵的技术途径.     

Study of the rheological properties and solution microstructure of progressively cross-linked aqueous gels

A stepwise cross-linking polyacrylamide hydrogel was prepared using the laboratory homemade stepwise crosslinker PVC-PP-Cr and partially hydrolyzed polyacrylamide. The microstructure was investigated by using the scanning electron microscope and dynamic light scattering. The results show that the system has a controllable cross-linking time of 4 ~ 6 h, good heat resistance, and maximum gel strength of >300,000 mPa.S. In terms of viscoelastic behavior, a stepwise cross-linking process between crosslinker and hydrolyzed polyacrylamide (HPAM) is presented, and the viscoelasticity is increased step by step. During the stepwise cross-linking process, the dendritic structure, network structure and lamellar structure to a stable spatial network structure is obtained. Here, the cross-linked structure between polymer molecular chains is formed after addition of the crosslinker, which increases the steric hindrance of macromolecular chains and further enhances the stability of the system. With the gradual release of chromium ions from the PVC-PP-Cr stepwise crosslinker, the gel system is formed continuously. When intermolecular friction stopped increasing, intermolecular entanglement and interaction are enhanced, and elasticity is also increased.     

The use of environmental scanning electron microscopy for imaging the microstructure of gels for profile control and water shutoff treatments

The use of gel systems as profile control and water shutoff agents has become a wide practice in recent years for oil recovery applications. But few systematic studies have been carried out on the microstructure of gels. In this study, environmental scanning electron microscopy (ESEM) was employed to investigate the microstructure of gels in their natural state. It was directly observed that a three‐dimensional network structure formed in chromium or zirconium crosslinked polymer gels is the same as organic/ inorganic crosslinked gels. However, there is a dense flat structure in phenol resin crosslinked polymer gels or phenolic resin gels. The differences between the microstructures can be attributed to the different crosslinking sites and crosslinking density, and this affect the gel's stability at reservoir temperatures. In addition, this paper also introduced the crosslinking mechanism and macro‐morphology of the bulk gels. It can contribute to a better understanding of the gel's microstructure. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39946.     

Evaluation of pH‐sensitive hydrogels to control the permeability anisotropy of oil reservoirs

In stratified or fractured oil reservoirs, the oil recovery efficiency tends to be low as the injected fluid flows mainly through the matrix's most permeable regions, leaving behind part of the displaceable oil in the matrix's unswept zones. Given this issue, this study aims to evaluate the potential of applying seven commercial samples, based on poly(acrylic acid), to control the anisotropic permeability profile of reservoirs. To perform this study, first, continuous and oscillatory shear tests were conducted to characterize the hydrogel's rheological and viscoelastic behavior in various subsurface conditions (salinity, temperature, and/or pH value). Second, polymer dispersion elution tests were performed in a porous medium to evaluate the matrix's permeability reduction after treatment with hydrogels. The seven commercial samples were classified as pseudoplastic fluids at pH values ranging from 1 to 10. Under typical reservoir conditions, PAAr 70 (which has intermediate molar mass and intermediate number of crosslinks) was the only sample to behave as a strong gel (G′/G″ > 10). Elution tests confirmed that the PAAr 70 hydrogel gelified inside the consolidated sandstone plugs and reduced the matrix's permeability four‐fold. Therefore, samples based on poly(acrylic acid) with high crosslink density proved to be the most promising for controlling the anisotropic permeability profile of heterogeneous oil reservoirs. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40665.     

Synthesis and application of nonionic polyacrylamide with controlled molecular weight for fracturing in low permeability oil reservoirs

Nonionic polyacrylamide (NPAM) with controlled molecular weight was successfully synthesized as a gel fracturing fluid by aqueous solution polymerization. The effects of the monomer concentration, initiator concentration, reaction time, feeding temperature and reaction temperature on the molecular weight were systematically investigated through single‐factor and orthogonal experiments. The NPAM molecular weight can be controlled by adjusting these factors. The decisive factor is the acrylamide concentration, whereas the initiator concentration and feeding temperature are secondary factors. These synthetic NPAMs can be crosslinked with zirconium acetate to produce a gel fracturing fluid for use in low permeability oil reservoirs. Gel fracturing fluids based on synthetic NPAMs have high shear resistance, low filtration performance, easy gel breaking performance, good proppant carrying capability and low core damage capability. By adjusting the NPAM or crosslinker concentrations, the gel fracturing fluid can be adapted for use in low permeability oil reservoirs for a wide temperature range (60–120°C). © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41637.     

Equilibrium swelling of thermo-responsive core-shell microgels

Design of new routes for preparation of hydrogels with fast response and enhanced mechanical and physical properties requires adequate modeling of their swelling. A model is developed for the equilibrium swelling of thermo-responsive gels. A characteristic feature of the model is that it accounts for a strong increase in the elastic moduli above the volume phase transition temperature T_c driven by aggregation of hydrophobic segments into clusters that serve as extra physical bonds between chains. The model is applied to the analysis of swelling diagrams on poly(N-isopropylacrylamide) macroscopic gels, microgel latices, and core-shell microgels with rigid cores. Good agreement is shown between the experimental data and results of simulation. It is demonstrated that the elastic moduli of microgels are higher, while their degrees of swelling in the stress-free state are lower compared with those of macroscopic gels.     

High‐temperature‐resistant polymer gel system with metal–organic mixed cross‐linking agents

A high‐temperature (200°C)‐resistant polymer gel system was developed from partially hydrolyzed polyacrylamide (HPAM), chromium lactate (CrL), and water‐soluble phenol/formaldehyde resin (WPF) mixed cross‐linkers. Rheological measurements indicated that the gelation process of the gel system could be divided into four successive steps: induction, first cross‐linking with metal cross‐linker, secondary cross‐linking with organic cross‐linker, and stabilization. Effects of various parameters that affect the gelation time and gel strength including polymer concentration, cross‐linker concentration, salinity, pH, and the gelation temperature were evaluated. Gelant formulated with 0.5 wt % HPAM, 0.1 wt % CrL, and 0.9 wt % WPF and treated at 80°C for 48 h showed sufficient gelation time, high rigidity, and good thermal stability. Morphology observation by scanning electron microscopy (SEM) and atomic force microscopy (AFM) revealed that the gel had compact network microstructure. A cross‐linking mechanism for the gel system was proposed based on the gelation process and experimental results. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42261.     

Evaluation of CO2-philicity and thickening capability of multichain poly(ether-carbonate) with assistance of molecular simulations

Phenyl-centered tri-chain poly(ether-carbonate) (TMA-PEC), phenyl-centered double-chain poly(ether-carbonate) (TPA-PEC), and phenyl-centered four-chain poly(ether-carbonate) (TFA-PEC) were synthesized to act as CO₂ thickener. Their solubility in CO₂ was measured by cloud point pressure. In order to explore the material characteristics that affect the solubility, dynamic simulations were used to analyze intermolecular polymer interactions, and the interaction between polymers and CO₂. It was found that TPA-PEC and TMA-PEC has better solubility than TFA-PEC in CO₂ among the three polymers while the thickening effect is poor, TFA-PEC possess the best viscosity thickening effect while the solubility in CO₂ is unfavorable. The silicone unit 1,1,1,3,5,5,5-heptamethyl-3-(3-[oxiran-2-ylmethoxy] propyl)trisiloxane modified TFA-PEC (TFA-PEC-SAGE) combine good solubility and good thickening ability together. The molecular simulations show that TPA-PEC and TMA-PEC have weaker intermolecular interactions and TPA-PEC and TMA-PEC have stronger interaction with CO₂ which are beneficial to the solubility.     

Boric acid incorporated on the surface of reactive nanosilica providing a nano‐crosslinker with potential in guar gum fracturing fluid

Small molecule borate crosslinker widely used in hydraulic fracturing treatment has a low crosslinking efficiency of less than 1%. Thus boric acid was introduced onto the surface of reactive nanosilica (denoted as nano‐SiO₂; size: about 20 nm) containing ? NH₂ group to obtain a nanosilica‐based crosslinker (denoted as nano‐crosslinker) with increased crosslinking efficiency, thereby broadening the application of nano‐SiO₂ in guar gum fracturing fluid. Moreover, the influence of the as‐prepared nano‐crosslinker on the rheological behavior of guar gum gel was investigated with borate crosslinker as a reference. Results show that boric acid chemically reacts with the amino group of the reactive nano‐SiO₂ to form N? B bond, which is beneficial to the formation of the network structure of guar gum gel. The guar gum gel crosslinked with the 57 ppm of borate based on the carrier of nano‐SiO₂ exhibits better temperature tolerance and shear resistance as well as breaking behavior than the counterpart crosslinked with 200 ppm of borate. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 45037.     

Research of phenolic crosslinker gel for profile control and oil displacement in high temperature and high salinity reservoirs

To further enhance oil recovery of high temperature and high salinity reservoir at Tahe Oilfield, field test results from profile control and oil displacement of gel were carried out in this article. Static and dynamic evaluations were performed through gel strength code method, environment scanning electron microscope, and physical simulation experiment devices. The field test results show that, under the conditions of high temperature (100.8 °C) and salinity (19.8 × 10⁴ mg/L), the stable gel system was formed with gelling time range from 26 to 45 h, gel strength ranging from E to H, and dehydrating amount lower than 3.0% after ageing 60 days. Meanwhile, the microstructure is very stable. When the permeability and gel strength ranges from 0.212 to 0.970 μm² and E to H, respectively, the plugging ratio is larger than 85%, and the plugging performance becomes better with the increase of permeability or gel strength. Due to the profile improvement rate of 99.8% and the oil recovery up to 28.5%, profile control and oil displacement technology of gel can effectively promote fluid diverting. The water cut reduced from 95.2% to 89.0% during field test carried out in Tahe Oilfield, which means that profile control and oil displacement technology of gel could stabilize oil production by water control effectively. Also, this technology has a wide application prospective that provides with strong technical support for further enhanced oil recovery in high temperature and high salinity reservoirs. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46075.     

Cationic starch derivatives as reactive shale inhibitors for water‐based drilling fluids

One of the major problems associated with the use of water‐based drilling fluids is the interaction of water with specific rock formations, such as shales, and the consequent swelling of reactive clays that may be present in that type of rock. Several types of clays reactivity inhibitors have been used by the oil industry, and the most effective ones are the cationic polyelectrolytes, such as the poly(diallyldimethylammonium chloride) (PDADMAC). However, this polyelectrolyte is very toxic. In this work, a series of cationic starch derivatives with different cationic degrees were synthesized with the objective of evaluating their potential as environmentally correct shale reactivity inhibitors. The results showed that the synthesized derivatives presented a good capacity of adsorption on bentonite and an efficient inhibition of the shale reactivity. The derivative with an intermediate cationic degree presented the best performance. In the tests with the formulated fluids this derivative provided an intact cuttings recovery of 84.8% and a total recovery of 92.3%. These values are very close to those found for the PDADMAC additive, therefore indicating that this cationic starch derivative presents a good potential as inhibitor of the shale reactivity. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46621.     

Gelation performance of poly(ethylene imine) crosslinking polymer–layered silicate nanocomposite gel system for potential water‐shutoff use in high‐temperature reservoirs

Polymer gels have been widely used for water shutoff in mature oil fields. In this paper, polyacrylamide (PAM)–montmorillonite (MMT) nanocomposites (NC) were prepared through in situ intercalative polymerization. Fourier transform infrared spectroscopy and X‐ray diffraction were conducted to characterize the prepared PAM/MMT nanocomposites. The gelation performance of poly(ethylene imine) (PEI) crosslinking PAM/MMT nanocomposite gel system (NC/PEI gel system) was systematically investigated by bottle testing and viscosity measurement methods. The results showed that the gelation performance of the NC/PEI gel system was greatly affected by the total dissolved solids, PAM/MMT nanocomposite concentration, and PEI concentration. The NC/PEI gel system exhibited much better thermal stability and gelation performance than the PAM/PEI gel system at the same conditions. The gelation performance after flowing through porous media of the NC/PEI gel system before injection and that of the subsequently injected gel system was different. The gel strength decreased and the gelation time was delayed after the gel system before injection was flowed through porous media. However, the gel strength of the subsequently injected gel system did not decrease, and only the gelation time was delayed after flowing through porous media. This study suggests that the NC/PEI gel system can be used as a potential water‐shutoff agent in high‐temperature reservoirs. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44243.