Investigation of the Rheological Properties of Nanosilica-Reinforced

May 30, 2019 - Lost circulation is one of the serious problems in drilling operations .... PAM hydrolysis is known to increase with the temperature wh...
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Cite This: Energy Fuels 2019, 33, 6829−6836

Investigation of the Rheological Properties of Nanosilica-Reinforced Polyacrylamide/Polyethyleneimine Gels for Wellbore Strengthening at High Reservoir Temperatures Mohamed Shamlooh,† Ahmed Hamza,† Ibnelwaleed A. Hussein,*,† Mustafa S. Nasser,† Musaab Magzoub,‡ and Saeed Salehi*,‡ †

Gas Processing Center, College of Engineering, Qatar University, P.O. Box 2713, Doha, Qatar Department of Petroleum and Geological Engineering, University of Oklahoma, Norman 73019, Oklahoma, United States

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ABSTRACT: Wellbore strengthening has been introduced recently to resolve lost circulation problems by improving the fracture gradient and hence extending the mud window. Polymeric cross-linkable solutions showed outstanding strength with high thermal stability at elevated temperatures. In this study, silica with different sizes is used to reinforce the polymer solutions. The objective of this work is to investigate the effect of nanosilica sizes (8, 20, 50, and 85 nm) and concentrations (0.1−2 wt %) on the stability and viscoelastic behavior of polyacrylamide (PAM) cross-linked with polyethyleneimine (PEI) at 130 °C. Moreover, the effect of PAM Mw is also studied. The results have shown that nanosilica has reinforced the PAM/PEI solution; the gel of the base polymeric solution has upgraded from code “F” to codes “G” and “I” based on the Sydansk coding system. The strongest gel was formed with the addition of 2 wt % of 50 nm silica to the polymer base solution which enhanced the gel strength by more than 300%. Zeta potential confirmed that 50 nm silica was the most stable among the other sizes. Gel strength was observed to increase upon increasing the size of nanosilica initially, and then, it has decreased which gave an optimum nanosilica size of 50 nm. The stability of silica particles in the system is suggested as an explanation for this drop in strength. The interaction between silanol and carbonyl groups via hydrogen bonding is proposed as the controlling mechanism of gel formation. The results suggest the importance of selecting the proper size and content of nanosilica for reinforcing PAM/PEI gels.

1.1. Conventional LCMs. Conventional LCMs such as flaky, fibrous, or granular are used as additives to manage equivalent mud density.14 However, pumping of the formulated mixture, which contains high solids, into the wellbore is one of the major difficulties as well as the large required volume. High shear rate on the fluid inside the fissure openings and probability of pressure surges because the high density of solids limits the use of conventional LCMs. A more effective alternative is the injection of the cement slurry and allowing it to harden inside the lost circulation zone either alone or mixed with conventional LCMs. In spite of the high efficiency of cement pills, long waiting on cement time, testing the quality of the plug, and multistages job add more cost to treatment and nonproductive time. Therefore, polymer solutions could be a good alternative for such solid-containing materials.15 1.2. Polymers in Lost Circulation and Wellbore Strengthening. Polyacrylamide (PAM) and partially hydrolyzed PAMs (HPAMs), with different degree of hydrolysis, have been widely used in conformance control and wellbore strengthening. Organic cross-linkers showed high thermal stability at high temperatures compared to nonorganic crosslinkers. Polyethyleneimine (PEI) is commonly used to cross-link PAM where the cross-linking reaction becomes fast and

1. INTRODUCTION Lost circulation is one of the serious problems in drilling operations because of a partial or total loss of the drilling fluids through natural or induced fractures when a high-density mud is used to overcome high formation pressures.1,2 Frequent lost circulation extends the drilling nonproductive time and costs about 15%, on average, of the sum of high-pressure hightemperature (HPHT) wells.3 The abnormal conditions such as HPHT, the existence of fractures, and drilling in depleted reservoirs contract the mud weight window (MWW) between the pore pressure and fracturing gradient making the formation easily to be broken down by the high mud weight. 3 Consequences of kick and blow out events put human personnel and environment at risk. Moreover, technical problems during other operations such as mud logging and productive zone damage were reported.4 Recently, wellbore strengthening which implements plugging lost circulation materials (LCMs) to seal the fractures nearby the wellbore and consequently the formation of fracture gradient will be higher, leading to wider MWW.5−7 It is essential to prop the fracture rapidly to improve the treatment efficiency.8,9 If the fracture is successfully bridged at the wellbore wall or close to it, the hoop stress around the wellbore will increase.10,11 There are several challenges in implementing conventional LCMs for wellbore strengthening applications as one of the corrective techniques such as using large particles with high concentration.12,13 © 2019 American Chemical Society

Received: April 1, 2019 Revised: May 18, 2019 Published: May 30, 2019 6829

DOI: 10.1021/acs.energyfuels.9b00974 Energy Fuels 2019, 33, 6829−6836

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Energy & Fuels significant at high temperatures of more than 90 °C. PAM/PEI systems have been used successfully before for water shut-off applications, and the highest reported strength is around 2130 Pa.16−18 Silica nanoparticles are getting more attention because of their low cost and well-known physical properties such as high surface area, ordered structure, and simplicity of surface modification.19 Nanoparticles have been suggested as eco-friendly additives to improve the water-based mud properties as an alternative to the macro- to microsize organic additives (low efficiency at HPHT) and to avoid the toxicity of oil-based muds.20 Nanomaterials have been proved to be a good physical enforcer in conformance control because of their ability to withstand higher temperatures as well as water salinity.21−23 Adding 1 wt % nanosilica (NS, size of 28.6 nm) strengthened the gel produced using PAM/PEI in a slightly saline medium from code “F” to “I”24 on the Sydansk25 coding system. Similarly, 30 wt % smaller size (9−12 nm) silica reinforced 1 wt % HPAM (high molecular weight)/PEI at high temperatures.26 Introducing the nanoparticles prolonged the gelation time to 132 h and developed grade “G” gel with thermal stability for more than 27 days at 85 °C without observing syneresis. Liu et al.27 reported that 0.3 wt % SiO2 (13 nm) increased the storage modulus of the gel formed by PAM/hydroquinone to around 300% compared to silica-free gel. Various mechanisms have been proposed to explain the effect of silica in a polymeric system. Conradi28 suggested that physical enforcement happens in the presence of a polar group in the polymer which creates an electrostatic bond between silica and the polymer matrix which makes it more compacted and stronger. Chen et al.29 explained the reaction between PAM/ PEI and silica in a way that the multiple silanol groups on silicon create a hydrogen bond with the amide group. The binding energy on the atomic level for sizes between 2 and 3.68 nm indicated the presence of physical bonds in the form of van der Waals interactions between the hydroxyl groups on silica and the functional groups on PAM.30 The main limitation faced in a polymeric system with NS is the agglomeration and the difficulty in the dispersion.31 The percentage of agglomeration is a function of the concentration of the polymer and molecular weight (Mw) as the more entangled the polymer chains are, the more suspended silica is found. Previous studies have addressed the effect of adding NS to reinforce polymeric solutions using a fixed particle size. In this study, the effect of particle size on the rheology and cross-linking of the PAM/PEI system is investigated using different sizes of NS. Moreover, the combined effect of polymer size and silica size is studied by examining NS in low- and high-Mw PAM. Therefore, the detailed objectives of this work are to: a. study the effect of particle size of various NS particles on the viscoelastic behavior of PAM/PEI at high temperature; b. examine the efficiency of NS in reinforcing polymeric gels with different NS loadings; c. investigate the influence of NS on strengthening low and high Mw PAM; and d. explore the impact of NS size on the cross-linking of PAM/PEI and identify the controlling mechanisms.

polymer concentration enhances the produced gel strength and this formulation produces the highest gel strength.16−18,32 Moreover, the wellbore strengthening, which is the application of interest, requires stable gel with high strength; hence, NS is used as an additive to improve gel strength. 2.1. Materials. PAM with high Mw (>1 500 000 Da) and degree of activity of 12−15 wt % and low Mw (700 000 Da) with a concentration of 20 wt % was obtained from SNF Floerger, France. The viscosity of the low- and high-Mw PAM at 170.3 s−1 and 24 °C is 72 and 195 cP, respectively. PEI of Mw of 750 000 Da is used as a cross-linker with a concentration of 33.3 wt %. All of the materials were used as received. The solutions were prepared using deionized water. Nonfunctionalized NS in the liquid form with different sizes of 8, 20, 50, and 85 nm was used to reinforce the cross-linked polymer obtained from NYACOL Nano Technologies, Inc, Canada. The NS solutions were added in different concentrations 0.1, 0.25, 0.5, 1, and 2 wt %. 2.2. Preparation Procedure. PAM (9 wt %) was weighed, and then deionized water was added. Then, NS was added to the solution while stirring for 10 min. After that, the solution was exposed to sonic waves within a sonicating bath for 10 min. Finally, PEI was added to the solution in droplet wise during stirring, and the new formulation was left for 10 min in the stirrer to mix properly. It should be noted that the addition of silica to the PAM/PEI mixture resulted in poor gel strength because of poor dispersion of gels. Therefore, silica was added to PAM first, and then PEI was introduced at the end. pH values of all prepared systems have ranged between 10.17 and 10.32. This level of basicity suits the targeted application where the pH of drilling fluids usually lies between 9 and 11. 2.3. Mature Gel Preparation. Test tubes, which can withstand up to 180 °C, were filled by the polymeric solution before immersing into an oil bath. The temperature was gradually increased, over a period of 20 min, to 130 °C to avoid thermal shock and glass breaking. This temperature was selected to represent high-temperature oil and gas reservoirs. Silicon caps that can withstand high pressures were used to cover the test tubes. The polymeric formulation used has high water content; hence, the pressure increases inside the tubes. PAM hydrolysis is known to increase with the temperature which will enhance gelation. However, the pressure has no effect on the rheology of liquids or gels. For the initial screening, 24 h was set as a curing time. After that, the gel strength of mature gels was measured using a rheometer. 2.4. Dynamic Rheological Measurements. An Anton Paar MCR 302 rheometer was used to conduct all of the experiments. Frequency sweep tests were conducted at 25 °C and atmospheric pressure using the parallel plate geometry with a 25 mm diameter and 2 mm gap. The motor adjustment was done first before installing the parallel plate geometry. Zero gap and reset force were performed before each set of runs. Samples were loaded and trimmed carefully to fill the area of the parallel plate. All tests were implemented on mature gels where the strain was fixed at 10%, which is within the linear region and the frequency was varied in the range 0.25−30 Hz. Storage modulus at a frequency of 10 Hz was used to compare the gel strength of the different formulations. The storage modulus represents the elastic behavior of the produced gel as well as the amount of stored energy in the system. In engineering terms, the more the storage modulus is, the higher stresses the gel can withstand. Thus, it will result in higher well stability.

3. RESULTS AND DISCUSSION 3.1. Effect of NS Size on the Rheological Behavior PAM/PEI. 3.1.1. Gel Description according to Sydansk Codes. Sydansk25 suggested the code system between “A” and “I” to describe the gel strength. Table 1 illustrates the proposed codes in detail. Table 2 shows the gel codes for the different systems after an aging time of 24 h in an oil bath at a temperature of 130 °C. Compared to the baseline, which is a PAM/PEI system with a 9/1 concentration, all reinforced systems exhibited a noticeable increase in the gel strength upon the addition of NS. However, among the experimented systems, the maximum strength was achieved using NS of sizes 20 and 50 nm. The

2. EXPERIMENTAL WORK PAM (9 wt %)/PEI (1 wt %) was selected as the base fluid because previous studies with different PAM/PEI systems have shown that the 6830

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Energy & Fuels Table 1. Sydansk25 Gel Strength Codes gel strength code A

changing the frequency. Hence, the gel produced is stable in the tested range. The impact of silica addition on gel strength could be divided into three regimes, as shown in Figure 3. First, a slight increase in the gel strength is observed followed by a small drop and then a constant increase in the gel strength of the gel with the addition of more silica. Figure 3 illustrates this behavior where the effect has been clearer for the 8 nm silica. It is believed that in the first period, silica enhances the cross-linking reaction, which is initiated by the PEI, and creates hydrogen bonds between silanol groups and the polymers. This interaction gives extra strength to the polymer. A similar observation was noticed by Adibnia and Hill33 work in which they claimed that silica in the presence of a small amount of cross-linker is able to chemically cross-link the PAM. In the second interval, increasing the amount of NS in the system further decreases the possibility of interaction between the polymer and the cross-linker, which results in the observed drop in elasticity. Further increase in silica will no longer affect the cross-linkability, and additional silica in the third interval leads to an increase in the strength. This is mainly due to the increase in the total solid contents in the system as well as the creation of hydrogen bonding between silica and the crosslinked polymer. This observation was also reported by Chen et al.29 where it was suggested that the controlling mechanisms of adsorption of PAM on the surface of NS are:

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 flow to the bottle by gravity upon inversion (usually characterized as a tonguing gel) 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 nonflowing 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

B C D E F G H I

Table 2. Sydansk Codes for the Produced Gels NS concentration, wt % size, nm

0.1

0.25

0.5

1

2

8 20 50 85

G H H G

G H H G

G H H G

G I I G

G I I H

a. Binding through hydrogen bond between NS and PAM b. Hydrophobic interaction c. Based on the charge on the polymer chains, electrostatic binding.

increase of silica concentration to levels of up to 2 wt % has enhanced the gel strength of the base polymer solution from Sydansk code “F” to a rigid gel of code “I”. 3.1.2. Storage Modulus. Rheological tests for the produced gel were in agreement with the above Sydansk test results as the middle-sized silica achieved the best results (Figure 1). The

However, these forces are competing with each other because even with anionic PAMs such as HPAM interacted with NS with a negative charge on its surface. From the electrostatic point of view, this interaction is not desired; however, it has no effect on the adsorption of anionic polymers on silica. The adsorption mechanism for anionic polymers is accomplished through hydrogen bonding between the carbonyl on PAM chains with the silica which is stronger than the electrostatic repulsion.29 The weakening of the electrostatic repulsion is favorable because many attachment points are easy to form via hydrogen bonding between PAM and NS, which is preferred for better gel reinforcement. Increasing the size and concentration of silica will increase the surface area and consequently the total charge. Therefore, the optimum concentration significantly depends on the size and concentration of the silica because they control the adsorption of PAM on the silica surface and directly affect the controlling mechanisms, which in turn influence the gel strength. The concentration ranges of these three intervals were observed to shift to lower values as the size of NS is increased (Figure 4). The trend of the 20 nm reinforced gel (Figure 4a) shows shrinkage in the first interval as it ended at a concentration below 0.25 wt %, followed by the second and third intervals. Whereas in the 50 and 85 nm (Figure 4b,c), the first interval has disappeared which indicates its existence at lower concentrations than the studied range. Finally, the second interval has further contracted to a smaller range for the 85 nm silica (Figure 4c). This behavior could be directly linked to the surface area per unit mass where it is higher for smaller particles. The higher the surface area, the higher the cross-linking inhibition in the second interval, which shifts the whole range to higher concentrations.

Figure 1. Effect of silica size and concentration on the storage modulus.

dashed line in Figure 1 indicates the baseline followed which is a system of 9% of PAM with 1% of the cross-linker (PEI) in which a gel with a storage modulus of 1644 Pa was produced. The highest gel strength was achieved for the system with 50 nm silica at a concentration of 2 wt % reaching a storage modulus value of 5481 Pa which is more than 300% higher than the PAM/PEI base solution. Figure 2 shows the frequency sweep tests performed to evaluate the strength of the mature gel. There was no significant change of storage modulus noticed upon 6831

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Figure 2. Frequency sweep test of 9:1 wt % PAM/PEI combined with NS at a range of concentration from 0.1 to 2 wt % and different sizes (a) 8 nm, (b) 20 nm, (c) 50 nm, and (d) 85 nm.

ash. These observations verify the polymer−silanol interaction mechanisms discussed above. 3.1.3. Analysis of the Stability of NS Solutions Using Zeta Potential. For further understanding of the reinforced systems, zeta potential (ζ-potential) of the stock solutions of NS in water was tested. Figure 5 reveals that the 50 nm size is the most stable size in water as it has shown the highest magnitude of ζ-potential of −42.8 mV. These results were compatible with the rheological results as the 50 nm showed the highest gel strength. The 8 nm silica showed the least stable state between the tested nanoparticles which is again was reflected in rheology by producing the weakest gel. This is mainly interpreted by the fact that the more stable the system is, the easier is its dispersion through the viscous polymer solution resulting in improved bonding and higher strength. The high values of ζ-potential suggest that the effect of the hydrophobic interaction of all systems is negligible which means that no phase separation will occur. Accordingly, silica settling has not been observed for all samples even at high silica concentrations of the largest studied size, which was 85 nm. 3.1.4. Loss Factor. Loss factor, or mathematically tan(δ), is a good representation of the degree of viscoelasticity as it follows the following equation

Figure 3. Rheological behavior of gel with the variation in a concentration of 8 nm silica.

For the same reason, inhibition of cross-linking takes place at a lower concentration for the larger particles (50 and 85 nm). Coil contraction phenomena were reported after intramolecular hydrophobic interactions. However, it was confirmed that there is a “critical association concentration”, above which a stable network structure will be formed resulting in substantial viscosity enhancement and hence polymer solution reinforcement.34,35 Moreover, NS also has a critical concentration above which the nanoparticles will adsorb PAM chains because of the hydrogen bonding between the silanol group and carbonyl groups.32 A similar strengthening effect was noticed upon adding fly ash, which consists mainly of silica and alumina, to the PAM/PEI cross-linked system.36−38 Adewunmi et al.37 have verified through the X-ray diffraction test that ash particles disperse in the polymeric network which indicates the existence of physical or chemical bonds. Moreover, polymeric systems tend to shift to more crystalline rather than amorphous upon the addition of fly

tan(δ) =

G″ G′

(1)

Figure 6 shows that all of the tested samples had a tan(δ) of less than 1 which indicates the solid-like behavior of the gels which is expected as the tests were performed on a mature gel. However, Figure 7 shows that there is a clear trend followed by the magnitude of the loss factor as it decreases with the increase of nanoparticle size for a fixed amount of added silica. These results reflect that the produced gel shows a more solid-like behavior when larger particles of NS are added. From another 6832

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Figure 4. Rheological behavior of gel with the variation in concentrations of (a) 20 nm, (b) 50 nm, and (c) 85 nm silica.

Figure 5. Zeta potential of NS in water.

Figure 7. Loss modulus as a function of NS size.

cross-linking reaction, which is reflected in the shift of elasticity toward the viscous side. 3.2. Effect of PAM Molecular Weight. On one hand, highMw PAM cross-linked with PEI without the addition of any silica showed a significant increase in the strength of the final gel (Figure 8). The same ratio of PAM/PEI was used, and the Mw of PAM was the only variable. Thus, the final polymeric matrix produced by the high-Mw PAM was stronger with the gel

Figure 6. Loss factor as a function of NS concertation.

perspective, varying the concentration for a fixed NS size showed a polynomial-like trend where there is a minimum value mirroring an optimum concentration that shows the most solidlike behavior. This optimum concentration was found to be between 0.25 and 0.5 wt % of all sizes. The addition of more silica showed a higher strength in the rheological tests because of the increase of van der Waal forces between the silica and the polymeric matrix. However, the presence of silica hinders the

Figure 8. Effect of Mw on storage modulus. 6833

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Figure 9. Schematic representation of the dispersion of (a) small size NS in high Mw polymer, (b) large size NS in high Mw polymer, (c) small size NS in low Mw polymer, and (d) large size NS in low Mw polymer.

strength of the high-Mw PAM being more than 200% higher than the gel with low-Mw PAM. On the other hand, upon the addition of silica to PAM, the high-Mw PAM showed different behaviors than the low-Mw system. A previous research29 suggested that the addition of silica to polymers leads to increase in the hydrodynamic radius of the polymer because of the bridging connection between NS and PAM which increases the length of the molecular chain as well as the hydrophilic interaction of NS and PAM. On the basis of zeta potential results, the bridging connection is proposed to be the dominant mechanism because of the negligible effect of the hydrophilicity as suggested by the Chen et al.29 study. At high Mw, the coil contraction of PAM intermolecular chains would greatly influence the network structure because the chains would be extended. High strength was achieved using the high Mw PAM; however, using this system might result in problems with injectivity because of the high viscosity despite its favorable high strength. For the high-Mw PAM, Figure 8 shows a decrease in gel strength with the increase in particle size, whereas the reverse is observed for the low-Mw PAM. In the case of the high-Mw polymer, increasing the size of silica provides fewer crosslinkable sites because it hinders the accessibility of PEI to PAM chains and hence the coil contraction will be significant. On the other hand, using the 8 nm size silica provides more PEI accessibility for chemical cross-linking with PAM and higher surface area/mass for the physical carbonyl−silanol hydrogen bonding. Thus, the ease of penetration, which is reflected in the size of silica, is the dominant factor in the high-Mw system. In contrast, for low-Mw PAM, chains are shorter and the number of PAM chains is high so even large particles will have enough space to penetrate through the polymeric matrix. Therefore, the degree of suspension and stability of silica will be the determining step for the low-Mw polymeric matrix. Figure 9 shows a schematic description of the effect of the small and large sizes of NS on the adsorption of PAM and network structure of the reinforced composite. A similar mechanism was reported by Chen et al.29 using 28.6 nm silica, and the same previous approach has been extended in this study to explore the effect of size. 3.3. Effect of Aging Time on the Gel Strength. The thermal stability of the PAM/PEI system has been studied by exposing the gelled systems to high temperature for different aging times. Figure 10 shows that NS has enhanced the thermal stability of the 9 wt % PAM/1 wt % PEI at different aging times. The base solution PAM/PEI showed slight decrease in the storage modulus after a day of aging at 130 °C, and the same

Figure 10. Effect of aging time on the gel strength of 9 wt % PAM/1 wt % PEI with 2 wt % NS with different sizes at 130 °C.

trend can be clearly observed after the addition of 8 nm compared with the gel strength after 12 h. Similar behavior was reported by ElKarsani et al.17 for the base solution at different aging times at 150 °C. The ratio of gel strength reduction was 35.54% for the base fluids; however, adding 2 wt % of the smallest size NS showed about 19.9% drop which indicates the effect of silica in enhancing the thermal stability. For the bigger sizes (20, 50, and 50 nm), the gel strength increased linearly with time for the whole studied range. This behavior indicates that the presence of silica has delayed the thermal decomposition of the gel to more than 24 h. Additionally, the trend of gel strength for different aging times (Figure 10) was the same having the 50 nm silica as the optimum size which revealed a significant gel strength increase by 67.75% after raising the aging time from 1 to 24 h. Moreover, the produced gel with 50 nm silica after 1 h was stronger by 81.62% compared to the mature gel obtained from 9 wt % PAM/1 wt % PEI. Therefore, such gels can be used for the treatment of lost circulation because they produce stronger gel in a short time, hence reduce the drilling downtime. Gelation time of the studied systems can be fast for deep reservoir applications where the system will form a gel before reaching the designated area. Nevertheless, gelation time can be controlled through the addition of retarders. Many retarders have been proposed in the literature such as ammonium chloride and sodium carbonate.15

4. CONCLUSIONS In this work, NS with sizes in the range of 8−85 nm was used to reinforce a polymer solution consisting of 9 wt % PAM and 1 wt % PEI. The polymer and the cross-linker are examined for potential wellbore strengthening applications at an elevated temperature (130 °C). Such conditions are representative of deep reservoir conditions. Different concentrations between 0.1 6834

DOI: 10.1021/acs.energyfuels.9b00974 Energy Fuels 2019, 33, 6829−6836

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with gratitude to SNF Company for supplying the polymers used in this study. The acknowledgment is also extended to Oklahoma University for supporting this research.

and 2 wt % of various sizes were investigated. Although different metrics were used to evaluate the studied systems such as zeta potential, loss factor, and storage modulus, the latter has always given the conclusive decision as the gel strength is the targeted property in this study. On the basis of the findings of this study, it could be concluded that: NS with the studied nanosize range is an efficient reinforcing agent for the 9:1 wt % PAM/PEI solution for high-temperature wellbore strengthening applications and could be used to seal the fractures inside the reservoir because of the observed high gel strength. Sydansk codes “G” to “I” were observed for all mature gels after adding NS compared to code “F” for the base polymeric formulation. The addition of 2 wt % of 50 nm silica to the base polymer/ cross-linker solution has improved the storage modulus by more than threefold. Whereas the minimum gel strength enhancement was recorded after adding 1 wt % of 8 nm silica to the PAM/PEI solution which was stronger by 1.3 times compared to the polymeric solution at the same conditions. Thus, the optimum NS size is 50 nm for the high-Mw PAM/PEI system. Increasing the size of NS in the polymeric system tends to increase the elasticity. Adding 2 wt % of small size NS to high-Mw PAM increased the gel strength. However, for the high-Mw PAM, the increase in the size of NS leads to an initial increase in gel strength followed by a drop for the large size silica with an optimum size of 8 nm. On the other hand, the optimum for the low-Mw PAM was observed at 50 nm in agreement with zeta potential results. These observations are correlated with PAM chain size and ease of mobility of silica particles within the shorter chain of the low-Mw PAM. Hydrogen bonding between the silanol group and a carbonyl group on PAM was suggested to be the dominant mechanism that forms a stable network structure in which NS physically reinforces the new formulated systems with different nanosizes. Therefore, the gel strength developed due to the chemical crosslinking of PAM/PEI has further been enhanced by the addition of silica. The effect of silica was not limited to the physical enhancement, as it has also extended to enhancing the thermal stability of the gel. The storage modulus of the gel in the absence of silica was noticed to start decreasing after 24 h of aging under 130 °C. Yet, adding silica to the same system has extended this degrading effect to longer periods beyond the studied range (24 h) as the storage modulus was found to be proportional to the aging time within the first 24 h. The results suggest that the optimum size of NS that leads to the highest gel strength depends on PAM Mw.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (I.A.H.). *E-mail: [email protected] (S.S.). ORCID

Ibnelwaleed A. Hussein: 0000-0002-6672-8649 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the support of Qatar National Research Fund (a member of Qatar Foundation) through grant # NPRP10-0125-170240. The findings achieved herein are solely the responsibility of the authors. Special thanks 6835

DOI: 10.1021/acs.energyfuels.9b00974 Energy Fuels 2019, 33, 6829−6836

Article

Energy & Fuels

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DOI: 10.1021/acs.energyfuels.9b00974 Energy Fuels 2019, 33, 6829−6836