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Experimental investigation on nano-silica reinforcing PAM/PEI hydrogel for water shutoff treatment Lifeng Chen, Jinjie Wang, Long Yu, Quan Zhang, Meilong Fu, Zhongcong Zhao, and Jiaqi Zuo Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00840 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 1, 2018

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Energy & Fuels

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Experimental investigation on nano-silica reinforcing PAM/PEI hydrogel for

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water shutoff treatment

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Lifeng Chen, Jinjie Wang, Long Yu, *ξ Quan Zhang, Meilong Fu, Zhongcong Zhao and Jiaqi

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Zuoa

†

†

5 6 7

‡

‡

†

†

†

†

College of Petroleum Engineering, Yangtze University, Wuhan, 430100, China

Faculty of Earth Resources, China University of Geosciences, Wuhan, 430074, China ξ

Department of Chemical Engineering, University of Calgary, T2N1N4, Canada

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Abstract: Polyacrylamide hydrogel used in conformance control subjects to a poor stability at

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elevated temperature. In this paper, nano-silica has been found to be a very effective stabilizing

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agent for the polyacrylamide/polyethylenimine hydrogel, and the reinforcing performance of

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nano-silica on the hydrogel was investigated detailedly. The result showed that the strength of

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hydrogel is enhanced obviously by the nano-silica, and the hydrogel syneresis is also significantly

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inhibited by the nano-silica, which can increase the hydrogel stability time from 18 days to 180

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days at 130℃. Besides, nano-silica can increase the shear viscosity and hydrodynamic radius of

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polyacrylamide, and the bound water in the hydrogel is also grown in the presence of nano-silica.

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Compared with the micro-morphology of common hydrogel, the dense and strong mesh structure

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is formed due to the reinforcing behavior of nano-silica. According to the above results, it is

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inferred that the silanol group of nano-silica crosslinks with the amidogen of polyacrylamide via

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the hydrogen bond, which is considered as the primary cause to the reinforcing performance of

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nano-silica for the hydrogel. In addition, due to the improving strength, stability and the increasing

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absorption of polymer to rock surface, the hydrogel enhanced by nano-silica has higher water

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shutoff ratio, residual resistance factor and longer period of validity. *Corresponding author at: Department of Chemical Engineering, University of Calgary, T2N1N4, Canada . E-mail address: [email protected] or [email protected]. 1

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Keywords: hydrogel; nano-silica; reinforcing performance; water shutoff

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1. Introduction

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Profile modification is a means of enhancing oil recovery by diverting flood water into

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previously unswept zones. The hydrogel is emplaced into the highly-permeable, flooded-out layers

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of the formation proximate to the wellbore, and reduces the permeability of the target zone to the

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flooding fluid, thereby modifying the flow profile and diverting injected fluids into zones of

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greater residual oil content. Since the profile modification is characterized by quick response time,

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low treatment costs, low risk, and frequently favorable payback, it has been widely used to

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improve the oil recovery, especially in oilfields of China 1,2.

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Polyacrylamide (PAM) hydrogel has been used extensively as water shutoff agent to reduce

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water production in common oilfield on account of its low price and simple preparation. However,

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further application of this hydrogel in difficult reservoir condition is limited due to its

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unsatisfactory stability. Although a fairly wide number of new acrylamide polymers have been

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described in the literature

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copolymers of N-vinyl pyrrolidone (N-VP), 2-acrylamido-2-methyl-propanesulfonate (AMPS),

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and N-vinyl acetamide (N-VA), can be commercialized. These novel polymers were generally

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found to possess excellent thermal stability and salt tolerance, but their suitability for petroleum

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applications is limited by their unreasonable cost.

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, only a relatively small number of them, including acrylamide

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In addition, the hydrogel crosslinked by the metallic cross-linker, such as Cr3+ and Al3+, has

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poor temperature-resistant and salt-resistant property 6,7. As a result, the organic crosslinking agent

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such as formaldehyde and phenol, has been employed to prepare the hydrogel used in high

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temperature condition. Moradi-Araghi et al.

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reported a stable phenol-formaldehyde gel which 2


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survived over 9 months at 149 °C in seawater containing 3.4% TDS and 1700 mg/L hardness.

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However, due to the toxicity of the phenol and formaldehyde, the application of these crosslinkers

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has been restrained in most oilfields. Polyethylenimine (PEI), which is environmentally friendly,

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is one of the hot topics of crosslinkers nowadays. Among PEI-based hydrogels, the t-butyl acrylate

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(PAtBA)/PEI system is popular and has been reported in some literatures. Al-Muntasheri et al.

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and Jia et al. 10 obtained stable PAtBA/PEI hydrogels at 130 and 40 °C, respectively, proving that

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PEI could form stable hydrogels with PAtBA within a wide range of temperature. Al-Muntasheri

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reported the favorable efficiency of the PAtBA/PEI hydrogel system, which has been performed in

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more than 450 field jobs around the world to deal with water production problems, such as water

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coning/cresting, high permeability streaks, and gravel pack isolation. The main shortcoming of the

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PAtBA/PEI hydrogel for water shutoff is the high cost because the required polymer concentration

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is too high (as much as 7%). Hence, finding the method of reinforcing hydrogels and applying the

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common PAM and PEI in the hydrogel for water shutoff is a better way to improve the

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effectiveness of the profile modification treatment.

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Due to the high reaction activity, the nanoparticle has been proved to be a reinforcing agent

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for the structural strength, and widely employed in the drug carrier and detergent. There have been

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some papers to describe different effects of nanoparticles on polymer solution like rheology

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enhancement or thermal property improvement

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modification of the hydrogel performance has almost never been reported in the literature.

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Furthermore, there is nearly no information of how the dispersed nano-silica interacting with the

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PAM of high molecular weight. Hence, in this study, the hydrogel prepared by PAM and PEI had

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been developed, and the effect of nano-silica on the gel stability, strength and gelation time was

11-13

. However, the role of nanoparticles in

3


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investigated. To clarify the mechanism of nano-silica affecting on the hydrogel, the influence of

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nano-silica on the viscosity of PAM solution, hydrodynamic radius of PAM molecule, the bound

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water amount and micro-morphology of hydrogels was studied. In the end, the shut-off

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performance and the wash-out resistance of the hydrogel reinforced by nano-silica was evaluated

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with the core flowing tests, and the mechanism of nano-silica enhancing the water shutoff

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performance was put forward. This investigation provides a clear understanding of the effect of

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nano-silica on PAM/PEI hydrogel, and the findings can be utilized to improve the stability and

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strength of the hydrogel applied in the profile modification and other stimulation treatments.

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2. EXPERIMENTAL SECTION

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2.1. Materials

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PAM was purchased from Baome Chemical Group Corporation, and its viscosity-average

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molecular weight is 8000 kDa. PEI with an average molecular weight of 10 kDa was gotten from

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Qianglong Chemical Group Corporation, and its purity was over 99%. The average particle size of

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nano-silica, which is produced by Hungbai Chemistry Co., Ltd., is 28.6nm. In this paper, the

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solution salinity is 0.5% NaCl, and all concentrations are on a weight basis.

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2.2. Measurements of gelation time, strength and syneresis rate of hydrogels

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First, PAM stock solution was prepared at 25°C by dissolving solid PAM in water with 0.5%

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NaCl. A container with a known amount of water was vigorously stirred to create a deep vortex.

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PAM was slowly added to the shoulder of the vortex to effectively wet the polymer beads. The

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container was sealed to minimize evaporation and was stirred continuously for 12 h to ensure

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complete dissolution of PAM. PEI, whose amount was carefully tuned, was dissolved in the above

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brine to prepare a crosslinker solution. Finally, the gelling solution was prepared by mixing the 4


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PAM stock solution and PEI solution, and it (20g) was sealed in a bottle and put into an oven at

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130℃. All hydrogels in this paper were prepared as the formula of 0.4%PAM + 0.65%PEI +

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0.3%thiourea + x%nano-silica (x: 0-1). The gel strength code method 14, which is showed in Table

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1, was used to determine the hydrogel strength and gelation time, and the gelation time is

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considered as the period of time when gelling solutions in code A state turn to code F in this paper.

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Syneresis rate is defined as the decrease in the gel weight at a given time relative to the initial gel

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weight, and the onset of the syneresis is referred to the time from which the gel actually formed,

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not from the moment that the solutions were placed in the oven.

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2.3. Measurements of viscosity

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PAM solutions with 0, 0.3% and 1% nano-silica were prepared respectively, and the viscosity

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of these solutions was measured at the shear rate in the range of 0.34s-1-77.17s-1 by the Brookfield

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viscometer (DV-II+Pro) at 30℃.

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2.4. Measurements of dynamic light scattering (DLS)

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For the DLS measurements, the polymer solutions were carried out in the light scattering

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vials. All glassware was kept dustfree by rinsing in hot acetone prior using. The solutions were

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filtered through membrane filters (pore size=0.8 µm) directly into the vials, and this process was

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carried out in a dustfree glovebox. The determination concentration of PAM solution with

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nano-silica of different concentrations is 5mg/L. DLS measurements were carried out at 25℃

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using a commercial multi-angle light scattering BI-200SM (Brookhaven Instruments Corporation)

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equipped with a digital correlator BI-9000AT. The scattering angle and wavelength were fixed to

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be 90°and 532nm respectively. The result of hydrodynamic radius (Rh) was directly obtained by

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Dynamic Light Scattering Software Ver. 5.74. 5


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2.5. Measurements of differential scanning calorimetry (DSC)

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The hydrogel samples with nano-silica of different concentrations were taken to be measured

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by DSC. A Pyris Diamond DSC (Perkins Elmer) equipped with a cooling device was used to

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measure the phase transition of water sorbed in the gel samples. DSC curves were obtained by fast

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sample cooling from 40℃ to -40℃ at the scanning rate of 10℃/min. The crystallization

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temperatures of water sorbed in the sample were determined from the temperature at the

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maximum point of the corresponding enthalpy peaks. 15

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2.6. Measurements of scanning electron microscope (SEM)

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The micro-morphology of hydrogel was investigated by SEM with the refrigeration system of

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Emitech K1250X. The SEM samples were prepared by a cryogenic preparation method and

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gold/palladium was used for sputter coating. Determinations were conducted at accelerating

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voltage of 15 kV and working distance from 5 mm to 10 mm.

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2.7. Sandpack flow experiment

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The plugging ability of hydrogels was evaluated by the sandpack flow experiment. A series of

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sandpack columns of 2.55 cm in diameter and 30 cm in length were used to prepare cores. These

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sandpack columns were made of stainless steel and they were nontransparent. The sandpack

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experimental procedure was showed in the following steps: (1) Clean quartz sand with different

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mesh size (60−140 mesh) was used to fill the sandpack (the permeability of water in each

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sandpack core approximates 2800mD and the porosity is about 39%), and then the sandpack was

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saturated with the brine (5000mg/L NaCl). (2) The brine was injected into the sandpack at the flow

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rate of 1cm3·min-1 (9.3ft/day) to measure the pressure difference between the two ends of the

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sandpack, and then the permeability (kw0) was obtained by Darcy's equation. (3) After that, the 6


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gelling solution of 0.5 PV (pore volume) was injected into sandpack with the same flow rate, and

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then the sandpack was kept at 130℃ for 2 days and 180 days respectively. (4) When the

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heat-treatment of the gel is completed, the sandpack was flooded with the brine again to measure

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the pressure difference to get the permeability kw1. (5) Finally, the effectiveness of the gel in

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reservoir conformance control was evaluated by the water shutoff ratio (Fs) and residual resistance

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factor (Fr), which were obtained by the following equation Fs=(kw0-kw1)/kw0×100% and Fr=kw0/kw1

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respectively.

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3. RESULTS AND DISCUSSION

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3.1. Effect of nano-silica on the basic property of hydrogels

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3.1.1. Stability of hydrogels

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In general, hydrogels typically used in profile modification applications share a common

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phenomenon, termed syneresis, in which the solvent phase separates from the gel phase as a result

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of gel shrinkage. Syneresis shortens the validity periods of water shutoff and decreases the

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economic benefit. Therefore, inhibiting the syneresis is the key to extend the duration of water

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shutoff treatment. Through many lab screening experiments, the best PAM/PEI hydrogel

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(0.4%PAM + 0.65%PEI + 0.3%thiourea) was developed in the absence of nano-silica, and it could

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be stable for 18 days at 130℃ (Figure 1). However, the stabilization time of 18 days is too short

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for the hydrogel to apply in the profile modification. In order to reduce the syneresis rate, the

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nano-silica with the concentartion in the range of 0.1%-1% was employed to reinforce the

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hydrogel in the research. It can be observed from Figure 1 that the hydrogel syneresis is

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significantly decreased by the nano-silica. The syneresis rate of hydrogel with 0.2% nano-silica on

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the 180th day falls below 10%, and 0.5% nano-silica can make the syneresis problem nearly 7


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disappear. As can be seen, nano-silica can obviously improve the stability of the hydrogel.

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3.1.2. Gelation time and strength of hydrogels

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The gelation time is an important construction parameter of water shutoff application, which

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is closely related to the distance of profile control. In this research, the effect of nano-silica on the

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gelation time was investigated, and the result was showed in Table 2. The influence rule of

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nano-silica on the gelation time of hydrogels is very clear, that is, the gelation time decreases with

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the growing concentration of nano-silica, reducing from 27h to 12h. It is well known when the

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polymer and the experiment condition are invariable, the gelation time is determined by the

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crosslinker. In this research, the crosslinker (PEI) is unchanging, so it can be inferred that the

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nano-silica plays the crosslinker role and then reduces the gelation time. Besides, the effect of

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nano-silica on the strength of hydrogels was studied, and the result was also showed in Table 2.

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When the nano-silica was employed in the gelling solution, the hydrogel strength was enhanced,

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which was improved from F to I along with the increasing nano-silica. The improvement of

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hydrogel strength further indicates that the nano-silica has the crosslinking effect to reinforce the

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hydrogel structure.

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3. 2. Mechanism investigation on nano-silica reinforcing hydrogel

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3.2.1. Effect of nano-silica on the viscosity and Rh of PAM

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Figure 2 shows the effect of nano-silica on the viscosity of PAM solution. It is obvious from

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the figure that the viscosity enhancement by nano-silica in PAM solution is significant, especially

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when the concentration of nano-silica is 1%. Therefore, it can be concluded that nano-silica is a

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thickening additive for PAM solution. Zeyghami et al.

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nano-silica/AM-AMPS (acrylamide copolymer of 2-acrylamido-2-methyl-propanesulfonate)

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reported the same viscosity increase for

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solution. However, they pointed out that nano-silica reduced the viscosity of nano-silica/HPAM

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(partially hydrolyzed polyacrylamide) solutions. The different effect of nano-silica on the polymer

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viscosity was attributed to the size of negatively charged groups (carboxyl and propanesulfonate).

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Generally, nano-silica can be adsorbed at the surface of polymer molecules via hydrogen

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binding, hydrophobic interaction, or electrostatic binding, depending on the nature of the polymer

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chains. Both HPAM and AM-AMPS are negatively charged and so their interaction with a

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negatively charged nano-silica surface is electrostatically unfavorable. There have been, however,

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some reports on adsorption of negatively charged polymers on silica surface. Hommer

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investigated the interaction of silica fume with negatively charged polycarboxylate ethers. The

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adsorption mechanism for negatively charged polycarboxylate ether was considered to be

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hydrogen bonding of the side chains with the silica particles. These evidences reveal that

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adsorption interaction of negatively charged polymers with negative silica surface is possible,

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provided that a non-electrostatic attraction mechanism, such as hydrogen bonding, compensates

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for the repulsive electrostatic forces. Therefore, the reason why nano-silica can obviously enhance

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the viscosity of PAM is clarified, that is, since PAM is non-charged, there is no electrostatic

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repulsion and then many attachment points are easy to be formed via hydrogen binding between

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PAM and nano-silica, which is favourable to the viscosity increase of PAM.

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The effect of nano-silica on the hydrodynamic radius of PAM was investigated to get a better

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understanding on the tackifying effect of nano-silica. As showed in Figure 3, the average

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hydrodynamic radius (aRh) of PAM increases with the rising concentration of nano-silica. The aRh

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is only 106.93nm in the absence of nano-silica, and it reaches up to 135.78nm when the

22

concentration of nano-silica is 1%. In general, the increase of the polymer hydrodynamic radius 9


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(Rh) is caused by two factors: improving the hydrophilicity of the polymer molecule by

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introducing the hydrophilic group to the polymer molecule, and increasing the polymer molecular

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chain length. In this research, the molecular structure and molecular weight of PAM is invariable,

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and nano-silica is hydrophilic, so it can be concluded that: (1) nano-silica increases the PAM

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molecular chain length via the bridge connection between nano-silica and PAM, (2) the

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hydrophilic nano-silica improves the hydrophilicity of PAM. These factors result in the increase of

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Rh, and contribute to the tackifying effect of nano-silica for PAM solution.

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3.2.2. Effect of nano-silica on the bound water of hydrogels

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Generally, polymer molecules in water are combined with a certain amount of water

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molecules through the attraction of molecules, such as van der Waals and hydrogen bonds. This

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kind of water is called the bound water, which has close interaction with polymer matrix, so there

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is no solid-liquid phase transition for this water in the process of temperature changing. Besides

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the bound water, the other water in polymer solution is called free water, which has weak or no

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interaction with polymer molecule, and its crystallization temperature is usually below 0℃. The

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relative content of these two types of water are mainly affected by the concentration and

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hydrophilicity of polymer. The higher the concentration and hydrophilicity are, the more the

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bound water in the polymer solution will be 18.

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To investigate the effect of nano-silica on the bound water of hydrogels, the DSC

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measurements for different hydrogels were conducted. As showed in Figure 4, all hydrogels with

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different concentrations of nano-silica have an exothermic peak which appears in the range from

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-19.4℃ to -17.4℃ respectively. According to the definition of the free water and bound water, it

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can be concluded that these exothermic peaks result from the heat release of the free water. Closer 10


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inspection of the data in Figure 4 indicates that the phase transition temperature of the free water

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in the hydrogel with nano-silica is lower than that of the hydrogel without nano-silica, and more

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nano-silica leads to lower phase transition temperature. Besides, the melting enthalpy obtained in

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the DSC measurement is showed in Table 3, and the mass fraction of a type of water was obtained

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as: Wn= ∆Hn/∆H0*100%, where ∆Hn is the melting enthalpy of one type of water, and ∆H0 is

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assumed to be the same as that of pure water (333.5 J/g)

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increase of nano-silica, the free water in the hydrogel decreases from 84.0% to 75.8%, and the

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bound water increases from 16.0% to 24.2%. The increase of the bound water may be due to the

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following reasons: (1) nano-silica is hydrophilic and can form plenty of silanol groups with water,

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which results in the free water changing to bound water, (2) When the grid size of the hydrogel is

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less than 10-25nm, the water in this micropore is bound water. The presence of nano-silica can

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increase grid density of the hydrogel, which is favorable to form this micropore whose size is less

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than 10-25nm, and then the amount of the bound grows. These indicate that the nano-silica can

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increase the hydrophilcity of the hydrogel, which will inhibit the hydrogel syneresis.

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3.2.3. Effect of nano-silica on the micro-morphology of hydrogels

19

. As showed in Table 3, due to the

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The micro-morphology of hydrogels is the fundamental factor that determines the basic

17

property of hydrogels, such as stability, strength and so on. To better understand the mechanism of

18

nano-silica reinforcing hydrogel, the observation analysis by the SEM was conducted. As showed

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in Figure 5, the microstructures of the two hydrogels in the same amplification are obviously

20

different. The microstructure of the hydrogel (Figure 5a and 5b) without nano-silica is the network

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structure, and there is large amount of big pore space in the network structure to hold the water in

22

hydrogels. In this case, the water in the big hole which can’t be directly contacted with the PAM 11


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molecule has weak interaction with the hydrogel skeleton, and is easy to be separated from the gel

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phase. However, the microstructures of the hydrogel with 0.5% nano-silica (Figure 5c and 5d) is

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the tabular structure with small holes, and there is more water can absorb on the polymer chain,

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leaving less free water in the holes. As a result, the hydrogel with nano-silica is hard to generate

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the syneresis problem.

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3.2.4. Analysis of the mechanism of nano-silica reinforcing hydrogel

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Due to the large specific surface area, the nanoparticle has high reactive activity. On the

8

surface of nano-silica, there are many silanol groups, which are easy to form the hydrogen bond

9

with the hydrophilic group, such as hydroxyl, amidogen, etc 20. According to the above results of

10

this research, that is, nano-silica can (1) enhance the stability and strength of the hydrogel, (2)

11

reduce the gelation time of the hydrogel, (3) improve the viscosity and hydrodynamic radius of

12

PAM, (4) increase the bound water and the microscopic grid density of the hydrogel, it can be

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concluded that the silanol group of nano-silica crosslinks with the amidogen of PAM via the

14

hydrogen bond. As a result, the hydrogel with nano-silica is reinforced. The reaction mechanism

15

of the hydrogel with (or without) nano-silica is showed in Figure 6. The PAM molecule reacts

16

with PEI through transamidation reaction or nucleophilic attack (Figure 6-a), whereas the PAM

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solution with nano-silica has double crosslinking effect due to the addition of nano-silica (Figure

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6-b). To better clarify the mechanism of nano-silica reinforcing hydrogel, the schematic

19

illustration of the hydrogel in the absence and presence of nano-silica is showed in Figure 7.

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3.3. Results of sand pack flooding

21 22

Water residual resistance factor (Fr) is an important parameter to evaluate the shutoff performance of a hydrogel. The scientist

21

proved the necessity of improving the residual 12


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resistance factor for the water shutoff treatment, and found that a higher Fr resulted in a better

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water shut-off performance for the polymer hydrogel system.

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Data of core flood tests with the injection of the PAM/PEI/nano-silica hydrogel were

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collected and plotted in Figure 8-a. Fr was applied together with the shut-off ratio (Fs) to evaluate

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the shut-off performance of the PAM/PEI/nano-silica hydrogel. In the process of the subsequent

6

water injection after the hydrogel formation, the total injected water volume was 5.0 PV. Figure

7

8-a shows that the highest Fr is 306.3 with the injected water volume of 0.5 PV. Then, Fr sharply

8

decreases when the injected water volume is in the range of 0.5PV-2.0 PV. With a further

9

expansion of the injected water volume from 2.0PV to 5.0 PV, Fr changes slightly and finally

10

levels off at about 50. Meanwhile, Fs is still higher than 98% when the injected water volume

11

reaches 5.0 PV. Both Fr and Fs indicate that the shut-off performance and the wash-out resistance

12

of the PAM/PEI/nano-silica hydrogel are favorable.

13

However, as showed in Figure 8-b, the Fr and Fs of the common PAM/PEI hygrogel are

14

obviously less than the ones of the nano-silica reinforcing hydrogel, and they are reduced to 8.70

15

and 88.51% respectively after the injection of 5.0 PV water. The strength code of the common

16

hygrogel and nano-silica reinforced one is F and H respectively, which indicates that higher

17

strength will lead to better plugging effect. Besides, as it is generally accepted, nanoparticle can

18

change the rock wettability from oil-wet to water-wet, and improve the rock hydrophilcity. Several

19

scientists have reported that the high hydrophilcity is benefit to the generation of the hydrogen

20

bond and then more polymer molecule will adsorb onto the rock surface

21

enhancing hydrogel strength and rock hydrophilcity is the main reason for the excellent water

22

plugging effect of nano-silica reinforcing hydrogel. To better understand the plugging mechanism 13


22,23

. In conclusion, the

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

of nano-silica reinforcing hydrogel, the schematic illustration in pore scale is showed in Figure 9.

2

Due to the degradation of polymer and the damage of the hydrogel grid structure in the aging

3

process, the hydrogel strength inevitably is decreased which results in the reduction of Fr and Fs. In

4

order to investigate the long effectiveness of the hydrogel, the hydrogel in the sand pack was aged

5

at 130℃ for 180 days, and then the Fr and Fs of this hydrogel was studied. As showed in Figure

6

10-a, the Fr and Fs of nano-silica reinforcing hydrogel decreased obviously due to the aging. The

7

highest Fr is 48.90 with the injected water volume of 0.5 PV. With a further expansion of the

8

injected water volume from 0.5 to 5.0 PV, the Fr finally reduces to 5.30. However, the Fs is still

9

kept higher than 80% after the scouring action of 5.0 PV injected water, which indicates that this

10

hydrogel has excellent long effectiveness, and is benefit to extend the validity period of water

11

shutoff application. Compared with the performance of nano-silica reinforcing hydrogel, the Fr

12

and Fs of common PAM/PEI hydrogel are reduced to 1.16 and 13.79% respectively after the

13

injection of 5.0 PV water (Figure 10-b), and it shows that this hydrogel nearly has no plugging

14

effect at this condition.

15

4. CONCLUSIONS

16 17

A series of experimental investigations have been conducted to evaluate the reinforcing behavior of nano-silica for PAM/PEI hydrogel. The results indicate that:

18

(1) The strength of hydrogel is enhanced obviously by the nano-silica, and the hydrogel

19

syneresis is also significantly inhibited by the nano-silica, which (with the concentration of 1%)

20

can increase the hydrogel stability time from 18 days to 180 days at 130℃.

21

(2) Nano-silica can increase the shear viscosity and hydrodynamic radius of PAM, and the

22

bound water in the hydrogel is also grown in the presence of nano-silica. It is inferred that the 14


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1

silanol group of nano-silica crosslinks with the amidogen of PAM via the hydrogen bond.

2

Compared with the micro-morphology of common hydrogel, the dense and strong mesh structure

3

is formed due to the reinforcing behavior of nano-silica.

4

(3) In contrast with the PAM/PEI hydrogel, the hydrogel enhanced by nano-silica has higher

5

water shutoff ratio, residual resistance factor and longer period of validity. The improving

6

absorption of polymer caused by nano-silica is considered as an important reason to the better

7

performance of nano-silica reinforcing hydrogel.

8

AUTHOR INFORMATION

9

Corresponding Author

10

*E-mail: [email protected] or [email protected].

11

Notes

12

The authors declare no competing ﬁnancial interest.

13

ACKNOWLEDGMENTS

14

Financial support by National Natural Science Foundation of China (51704035, 51574266,

15

51604036), Yangtze Youth Fund (2016cqn42) and the tenth College Students' Innovative and

16

Entrepreneurial Training Project in Yangtze University (2017095) are gratefully acknowledged.

17

REFERENCES

18

(1) El-karsani, K. S. M.; Al-Muntasheri, G. A.; Hussein, I. A. Polymer systems for water shutoff

19

and profile modification: A review over the last decade. SPE J. 2014, 19(1): 135-149.

20

(2) Bai, Y.; Xiong, C.; Wei, F. Gelation Study on a Hydrophobically Associating

21

Polymer/Polyethylenimine Gel System for Water Shut-off Treatment. Energy Fuel. 2015, 29(2):

22

447-458. 15


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1

(3) Chen, L.; Zhang, G.; Ge, J. A novel thermal-resistance and salt-tolerance gel with

2

low-concentration crosslinkers for water shutoff in Tahe oilfield, SPE Asia Pacific Unconventional

3

Resources Conference and Exhibition, November 9-11, 2015, Brisbane, Australia.

4

(4) Vasquez, J.E., Eoff, L. S. Laboratory development and successful field application of a

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conformance polymer system for low-medium-and high-temperature applications. Presented at

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SPE Latin American and Caribbean Petroleum Engineering Conference, December 1-3, 2010,

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Lima, Peru.

8

(5) Chen, L.; Zhang, G.; Ge, J. An ultrastable hydrogel for enhanced oil recovery based on

9

double-groups crosslinking. Energy Fuel. 2015, 29(11):7196-7203.

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(6) Albonico, P.; Lockhart, T. P. Stabilization of polymer gels against divalent ion-induced

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syneresis. J. Petrol. Sci. Eng. 1997, 18(1): 61-71.

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(7) Bai, Y.; Li, J.; Xiong, C. Effect of a polymer on chromium(III) diffusion during gelant injection

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in fractured media. J. Appl. Polym. Sci. 2016, 133(20):566-572.

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(8) Moradi-Araghi, A.; Bjornson, G.; Doe, P. H. Thermally stable gels for near wellbore

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permeability contrast corrections. SPE Adv. Technol. Ser. 1993, 1, 140-145.

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(9) Al-Muntasheri, G. A.; Nasr-El-Din, H. A.; Zitha, P. L. J. Gelation kinetics and performance

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evaluation of an organically cross-linked gel at high temperature and pressure. SPE J. 2008, 13,

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337-345.

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(10) Jia, H.; Pu, W. F.; Zhao, J. Z.; Jin, F. Y. Research on the gelation performance of low toxic

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PEI cross-Linking PHPAM gel systems as water shutoff agents in low temperature reservoirs. Ind.

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Eng. Chem. Res. 2010, 49, 9618−9624.

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(11) Sharma, A.; Tripathi, B.; Vijay, Y. K. Dramatic Improvement in properties of magnetically 16


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aligned CNT/polymer nanocomposites. J. Membrane Sci. 2010, 361(1-2):89-95.

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(12) Kamphunthong, W.; Sirisinha, K. Thermal property improvement of ethylene-octene

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copolymer through the combined introduction of filler and silane crosslink. J. Appl. Polym. Sci.

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2010, 115(1):424-430.

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(13) Zhao, G.; Dai, C.; Wen, D. Stability mechanism of a novel three-Phase foam by adding

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dispersed particle gel. Colloids Surf. A, 2016, 497:214-224.

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(14) Vernáez, O.; García, A.; Castillo, F. Oil-based self-degradable gels as diverting agents for oil

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well operations. J. Petrol. Sci. Eng. 2016, 146:874-882.

9

(15) Shinyashiki, N.; Shimomura, M.; Ushiyama, T.; Miyagawa, T.; Yagihara, S. Dynamics of

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water in partially crystallized polymer/water mixtures studied by dielectric spectroscopy. J Phys

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Chem B, Condens Matter, 2007, 111(34): 10079-10087.

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(16) Zeyghami, M.; Kharrat, R.; Ghazanfari, M. H. Investigation of the Applicability of Nano

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Silica Particles as a Thickening Additive for Polymer Solutions Applied in EOR Processes. Energ.

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Source. Part A, 2014, 36(12):1315-1324.

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(17) Hommer, H. Interaction of polycarboxylate ether with silica fume. J. Eur. Ceram. Soc. 2009,

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29(10):1847-1853.

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(18) Cordova, M.; Cheng, M.; Trejo, J. Delayed HPAM gelation via transient sequestration of chromium in polyelectrolyte complex nanoparticles. Macromolecules, 2008, 41(12): 4398-4404. (19) Ping, Z. H.; Nguyen, Q. T.; Chen, S. M. States of water in different hydrophilic polymers-DSC and FTIR studies. Polymer, 2001, 42(20): 8461-8467. (20) Lai, N.; Guo, X.; Zhou, N. Shear Resistance Properties of Modified Nano-SiO2/AA/AM Copolymer Oil Displacement Agent. Energies, 2016, 9(12):1037-1044. 17


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(21) Zhao, J. Z.; Jia, H.; Pu, W. F.; Liao, R. Influences of fracture aperture on the water shutoff

2

performance of polyethyleneimine crosslinking partially hydrolyzed polyacrylamide gels in

3

hydraulic fractured reservoirs. Energy Fuel. 2011, 25, 2616−2624.

4

(22) Maghzi, A.; Mohammadi, S.; Ghazanfari, M. H. Monitoring wettability alteration by silica

5

nanoparticles during water flooding to heavy oils in five-spot systems: A pore-level investigation.

6

Exp. Therm. Fluid Sci. 2012, 40(7):168-176.

7

(23) Esmaeilzadeh, P.; Sadeghi, M. T.; Fakhroueian, Z. Wettability Alteration of Carbonate Rocks

8

From Liquid-Wetting to Ultra Gas-Wetting Using TiO2, SiO2, and CNT Nanofluids Containing

9

Fluorochemicals for Enhanced Gas Recovery. J. Nat. Gas Sci. Eng. 2015, 26:1294-1305.

10 60 0 0.1% 0.2% 0.3% 0.5% 1%

Syneresis rate (%)

50 40 30 20 10 0 0

30

60

11 12

90 120 Time (day)

150

180

Figure 1. Effect of nano-silica on the syneresis rate of hydrogel 160

Viscosity (mPa·s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 23

0 0.3% 1%

120

80

40

0 0

13

20

40 Shear rate (S-1)

60

18


80

Page 19 of 23

1

Figure 2. Effect of nano-silica on the viscosity of PAM solution 140

120

aRh (nm)

100

80 0

0.1 0.2 0.3 0.5 Concentration of SiO2 (%)

0

2 3 4

1

Figure 3. Effect of nano-silica on the hydrodynamic radius of PAM

0.5% 0.3%

Heat (Endo)/mW

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.2%

0.1%

0

-40

5 6 7

-20

0

20

40

Temperature/℃

Figure 4. DSC curve of hydrogels with different concentrations of nano-silica

a

b

30µm

10µm

8

19


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2 3 4

d

30µm

10µm

Figure 5. SEM images of different hydrogels. (a,b) hydrogel without nano-silica; (c,d) hydrogel with 0.5% nano-silica.

CH2 CH

5

c

Page 20 of 23

CH2 CH

a m

O C NH2

+

CH2 CH

m

m

O C RN-CH2 -CH2-[RN-CH2 -CH2]m NR C O

R'RN-CH2-CH2-[RN-CH2-CH2 ]m NR'R

CH2 CH

CH2 CH

m-1

2

CH2 CH

O C RN-CH2-CH2-[RN-CH2-CH2]m NR

O C

+ R'NH m

C O

H N H H CH2 CH

O m

O C NH2

+ R'RN-CH -CH -[RN-CH -CH ] 2

2

2

2 m

NR'R

+

HO

H O

SiO2

O H

OH

H O

b

H O

O H

CH2 CH

6

2

CH2 CH O C RN-CH2 -CH2-[RN-CH2 -CH2]m NR CH2 CH

C

m

O

m-1

Figure 6. Effect of nano-silica on the formation mechanism of the hydrogel (a: without nano-silica, b: with nano-silica).

b

a

10 11 12

+ R'NH

OH O H

H N H O C

7 8 9

O H

HO SiO2

Figure 7. Schematic illustration of nano-silica reinforcing the hydrogel (a: without nano-silica, b: with nano-silica).

20


300

Fs

250

95

200 150 100

FR

50

(a)

0

85 0

1 2 3

90

1

2 3 4 Injected water volume (PV)

350

100

300

FS

250

95

200 150 90

100 50

5

Shut-off ratio (%)

100

Water residual resistance factor

350

Shut-off ratio (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Page 21 of 23

FR

(b)

0

85 0

1

2

3

4

5

Injected water volume (PV)

Figure 8. Water residual resistance factor and shutoff ratio of hydrogel with aging time of 2 days (a: with nano-silica, b: without nano-silica).

a

b

sand grain

gel

water flow

4

c

d

water

flow

5

e

f

water

flow

6 7 8

Figure 9. Schematic of nano-silica influencing the water shutoff performance of hydrogels (a: water flooding in high-permeability zone; b: hydrogels filled in high-permeability zone; c, d: 21


Energy & Fuels

water shutoff performance of hydrogel with nano-silica; e, f: water shutoff performance of hydrogel without nano-silica).

95

30

90 20

FS 85

10

(a)

FR

0 0

1

3 4 5 6

80


60 50

2.5

40 2.0 30

FS 1.5

20

(b)

FR

1.0 0

5

1

10


5

Figure 10. Water residual resistance factor and shutoff ratio hydrogel with aging time of 180 days (a: with nano-silica, b: without nano-silica). Table 1 Gel strength code Gel strength code

Gel description No detectable gel formed: The gel appears to have the same viscosity as the original polymer solution. Highly flowing gel: The gel appears to be only slightly more viscous than the initial polymer solution. Flowing gel: Most of the gel flows to the bottle cap by gravity upon inversion. Moderately flowing gel: Only a small portion (5-10%) of the gel does not readily flow to the bottle cap by gravity upon inversion. Barely flowing gel: The gel can barely flow to the bottle cap and/or a significant portion (>15%) of the gel does not flow by gravity upon inversion. Highly deformable nonflowing gel: The gel does not flow to the bottle cap by gravity upon inversion. Moderately deformable non flowing gel: The gel deforms about half way down the bottle by gravity upon inversion. Slightly deformable nonflowing gel: only the gel surface slightly deforms by gravity upon inversion. Rigid gel: There is no gel surface deformation by gravity upon inversion.

A B C D E F G H I 7 8

9 10

3.0

Shut-off ratio (%)

40


100

50

Shut-off ratio (%)

1 2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 23

Table 2 Effect of nano-silica on the gelation time and gel strength Concentration of SiO2, %

0

0.1

0.2

0.3

0.5

1

Gelation time, %

27.0

22.5

19.0

15.5

13.5

12.0

Gel strength

F

G

G

H

H

I

Table 3 Melting enthalpy of free water and mass fraction of different states of water in hydrogels Concentration of SiO2, %

0

0.1

0.2

0.3

0.5

∆H，J/g

-280.1

-277.9

-273.1

-265.7

-252.8

Free water content, %

84.0

83.3

81.9

79.7

75.8

Bound water content, %

16.0

16.7

18.1

20.3

24.2

22


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PEI

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