Effect of Ionic Strength on the Transport and Retention of

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Effect of Ionic Strength on the Transport and Retention of Polyacrylamide Microspheres in Reservoir Water Shutoff Treatment Chuanjin Yao, Dan Wang, Jing Wang, Jian Hou, Guanglun Lei, and Tammo S. Steenhuis Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01588 • Publication Date (Web): 26 Jun 2017 Downloaded from http://pubs.acs.org on July 1, 2017

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Industrial & Engineering Chemistry Research

Effect of Ionic Strength on the Transport and Retention of Polyacrylamide Microspheres in Reservoir Water Shutoff Treatment Chuanjin Yao,*,†,‡,§ Dan Wang,† Jing Wang,|| Jian Hou,†,‡ Guanglun Lei,† and Tammo S. Steenhuis*,§ †

School of Petroleum Engineering and ‡Shandong Provincial Key Laboratory of Oilfield Chemistry,

China University of Petroleum, Qingdao, Shandong 266580, China §

Department of Biological and Environmental Engineering, Cornell University, Ithaca, New York

14853, United States ||

School of Petroleum Engineering, China University of Petroleum, Beijing 102249, China

ABSTRACT: Knowledge of the effect of ionic strength (IS) on the transport and retention of polyacrylamide microspheres in porous media is essential for their application in reservoir water shutoff treatment especially at high IS conditions. In this work, retention and release experiments were conducted in a transparent sand-packed micromodel at various ISs from 0.001 to 0.20 M to investigate their pore-scale transport-retention-release processes, retention mechanisms and spatial distributions in porous media. DLVO interaction profiles, chamber dissections and mass balance calculations were used to quantitatively analyze the effect of IS on the transport and retention of polyacrylamide microspheres during the reservoir water shutoff treatment. Results indicated that retention of polyacrylamide microspheres increased with IS due to the diminution/elimination of

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energy barrier at high IS levels. Straining at pore-throats was the largest contributor to the retention of polyacrylamide microspheres for all IS conditions. IS reduction was beneficial to the release of polyacrylamide microspheres that were loosely retained on sand grain surfaces by secondary energy minima. Nevertheless, most of the released polyacrylamide microspheres were once again retained by straining which would further enhance the water shutoff performance. Irreversible retention by primary energy minima ranged between 0.37% and 4.65% with increasing IS and the elimination of energy barrier at high IS conditions could enhance this process. 1. INTRODUCTION With the current increasing global energy demands and depleting hydrocarbon reserves, enhancing the hydrocarbon recovery from existing oilfields has become more and more important.1,2 The enhanced recovery is usually associated with the pressurized gas (e.g., CO2)3-6 and water injection7-9 to displace the residual hydrocarbons in the reservoirs. One aspect, which is of great importance in enhanced hydrocarbon production, is the ability of the injected displacing fluids to flow through the section of the reservoirs that still contains a large amount of stagnant hydrocarbons.10 In most hydrocarbon reservoirs, there exist conductive fractures/channels, thief zones, or/and highpermeability layers.11 In this case, the injected displacing fluids prefer to flow along these preferential flow paths, and thus a large fraction (up to 70%) of the hydrocarbons is still retained in subsurface.12 Moreover, large quantities of produced water are pumped to the surface, which is very damaging to drinking water sources and the environment generally if not properly controlled. Making flow more uniform can result in enhanced hydrocarbon recovery, and prevent or reduce the amount of waste water production.13,14 Water shutoff treatment using polyacrylamide microspheres has long been seen as an effective method to modify the flow heterogeneity and improve the displacement efficiency of hydrocarbon 2 ACS Paragon Plus Environment

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reservoirs.15 It has been long recognized that polyacrylamide microspheres could penetrate into reservoir formation deeper than conventional rigid particles because they can pass through the pore-throats after a certain deformation under pressure depending on their elasticity.16 Briefly, the polyacrylamide microspheres (Figure 1a) are first injected into the reservoir formation. Then they continually transport in reservoir porous media and plug the preferential flow paths through the mechanisms of migration, plugging, remigration through deformation, and replugging, as shown in Figure 1b. Simultaneously, the subsequent displacing fluids are forced to flow through the section of the reservoirs that still contains the hydrocarbons. The initially stagnant hydrocarbons are constantly displaced from subsurface, and the amount of waste water production is also reduced accordingly.

Figure 1. Diagram of (a) microstructure and (b) dynamic profile modification and diplacement improvement process of the polyacrylamide microspheres. With respect to this water shutoff methodology, the polyacrylamide microspheres should be in appropriate particle size matching with the pore size of reservoirs, and also be stable in complex reservoir conditions. Much experimental and theoretical work has been carried out to understand the key factors affecting the profile modification and displacement improvement performance of polyacrylamide microspheres in reservoir porous media.17-19 Most research involved traditional single- and/or parallel-column experiments. The results do not clearly distinguish the essential 3 ACS Paragon Plus Environment


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ways that reservoir conditions affect polyacrylamide microspheres transport and retention. Few studies have focused on the pore-scale transport and retention of polyacrylamide microspheres in porous media. Their stabilities in suspensions have been extensively investigated with particular emphasis on temperature, pressure, ionic strength (IS), and pH for affecting their morphology, structure, particle size, thermal degradation, and viscosity.20-22 Each of these factors influences the surface interactions of polyacrylamide microspheres with the sand grains. Especially, high IS condition dramatically increases the retention of polyacrylamide microspheres in reservoir porous media.23 Little is known about how these particles are retained in porous media, how IS affects their transport-retention-release processes, or the key retention mechanisms during reservoir water shutoff treatment.24 In this work, laboratory experiments were performed at various ISs of 0.001, 0.01, 0.04, 0.10, and 0.20 M to quantitatively investigate the effect of ionic strength on the transport and retention of polyacrylamide microspheres in reservoir water shutoff treatment. Hydrodynamic diameter, zeta potential, and dynamic viscoelasticity were measured to characterize the physical and chemical properties of polyacrylamide microspheres in solutions of matching IS. The retention and release experiments were conducted in a transparent micromodel packed with translucent quartz sand in order to provide pore-scale visual information about the transport-retentionrelease processes, the retention mechanisms, and the spatial distributions during the proposed water shutoff treatment. DLVO interaction profiles between the surfaces of polyacrylamide microspheres and sand grains were calculated to analyze their retention and release trends in porous media. Chamber dissections and mass balance calculations were also used to estimate the fraction of injected polyacrylamide microspheres retained by different retention mechanisms. Therefore, the objectives of this research were to investigate the pore-scale transport and


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retention properties of polyacrylamide microspheres at each IS level tested, evaluate the effect of IS on their transport, retention, and release in porous media, and elucidate the key retention mechanisms during reservoir water shutoff treatment. 2. MATERIALS AND METHODS 2.1. Polyacrylamide Microspheres and Porous Media.

Figure 2. Morphology (a) and particle size distribution (b) of polyacrylamide microspheres. The polyacrylamide microspheres used in this study were prepared through the methodology reported by Yao et al.25 The sample of polyacrylamide microspheres has a density of 1.0 g/cm3. As shown in Figure 2, the polyacrylamide microspheres are spherical particles with a narrow particle size distribution. The initial particle size in powder is 17.00–47.46 µm, and the average value (da) is 30.55 µm. Prior to experimentation, the sample of polyacrylamide microspheres was washed with deionized (DI) water to remove the dispersing stabilizer added in the preparation process. Stabilizer-free polyacrylamide microspheres were dispersed at a mass concentration of 1000 mg/L in NaCl solutions at 0.001, 0.01, 0.04, 0.10, or 0.20 M IS. The viscosity of polyacrylamide microspheres in aforementioned suspensions was less 1.0 mPa·s and thus almost no additional flow resistance would be generated in pipeline and equipment during injection.



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Additional microsphere-free NaCl solutions of matching IS were prepared, and DI water was used as the background solution of 0 M IS. Translucent quartz sand supplied by AGSCO Corporation (Hasbrouck Heights, NJ, USA) was selected as the porous media. The sand is composed of > 98% SiO2, with minor Al2O3, Fe2O3, and K2O. The grain size range of the sand is 0.42–0.84 mm, the density is 2.65 g/cm3, and the pH value in DI water is 7.0. Prior to use, the sand was acid-washed, rinsed in DI water, and ovendried to remove surface impurities and to minimize chemically attractive microsites on the sand.26 2.2. Hydrodynamic Diameter and Zeta Potential Measurements. Hydrodynamic diameter (dh) of polyacrylamide microspheres in NaCl solutions at 0, 0.001, 0.01, 0.04, 0.10, or 0.20 M IS was measured at regular time intervals by the laser particle size analyzer (BT-2002, Dandong Better Instrument Ltd., Liaoning, China). The zeta potential (ζ) of polyacrylamide microspheres and sand grains in suspensions at each IS tested was measured by the Zetasizer (Nano-ZS, Malvern Instruments Ltd., Worcestershire, UK). All measurements were conducted at 60 °C. 2.3. Dynamic Viscoelastic Measurements. Dynamic viscoelasticity of polyacrylamide microspheres in suspensions at each IS (0, 0.001, 0.01, 0.04, 0.10, or 0.20 M) was characterized using dynamic modulus, i.e., the storage modulus (G') and the lose modulus (G"). The dynamic modulus was measured by the rheometer (M5600, Grace, USA) at a fixed frequency of 1.0 Hz. All measurements were conducted at 60 °C. 2.4. Retention and Release Experiments. A transparent micromodel with an interior chamber of 8.0 cm length, 1.5 cm width, and 0.2 cm depth, as shown in Supporting Information (SI) Figure S1, was used to investigate the retention and release of polyacrylamide microspheres in porous media. The translucent quartz sand of


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0.42–0.84 mm was packed into the chamber to a porosity of ~0.40 m3/m3. The influent was injected into the micromodel with a peristaltic pump at a flow velocity of 0.02 cm/s for all the experiments. Microscopic images were collected throughout the injection process to investigate the retention-release phenomena and the deposition mechanisms of polyacrylamide microspheres in porous media. The injection pressure was measured with a small piezometer filled with water at the model inlet. A transparent jacket-heater, equipped with the circulating water bath, was used to maintain the temperature of the fluids in the model chamber. Figure 3 schematically illustrates the experimental apparatus.

Figure 3. Schematic of retention and release experiments. Before each experiment, the micromodel was saturated with DI water. Then fluids were added in four phases. The first three phases were performed at IS of 0.001, 0.01, 0.04, 0.10, or 0.20 M and DI water for phase 4. In phase 1, one pore volume (PV) of microsphere-free NaCl solutions at 0, 0.001, 0.01, 0.04, 0.10, or 0.20 M IS was injected into the chamber. For phase 2, 2.0 PVs of polyacrylamide microspheres of 1000 mg/L suspended in NaCl solutions of matching IS were supplied to the chamber. In phase 3, microspheres-free NaCl solutions of same IS were injected until the effluent polyacrylamide microspheres were negligible (i.e., 1.5 PVs). In phase 4, the 7 ACS Paragon Plus Environment


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chamber of the micromodel was flushed with 2.0 PVs of DI water. The effluent samples were collected at a regular time interval. The effluent concentrations of polyacrylamide microspheres were determined by spectrophotometry using a Spectronic 501 (Milton Roy, Ivyland, PA, USA) at a wavelength of 590 nm. Known concentrations of polyacrylamide microspheres standard suspensions were used to produce a calibration curve prior to experimentation. All experiments were conducted at 60 °C. For mass balance comparisons, the relative mass of polyacrylamide microspheres recovered in phase 2-3 (MR1, %) or released in phase 4 (MR2, %) in the effluent was calculated as eq. (1).27

MRj =

∑ ( C + C ) ∆t ×100% i

i +1

i

2C0 tc

(1)

where j is the subscript number of 1 or 2; Ci is the concentration of polyacrylamide microspheres in the ith effluent sample; ∆ti is the sampling interval; C0 is the concentration of polyacrylamide microspheres injected; tc is the injection duration of polyacrylamide microspheres. At the end of phase 4, the deposition profiles of polyacrylamide microspheres in the chamber of the micromodel was obtained from the chamber dissections following the similar method and procedure reported by Lanphere et al.28 In brief, once the transport experiments were completed, the cover plate of the micromodel was removed and the sand was carefully excavated from the chamber and segmented into one-centimeter portions. All segments were collected with 10 mL glass centrifuge tubes. The collected samples, comprising of sand and retained polyacrylamide microspheres, were rinsed by adding 5 mL of DI water into the tubes, respectively. These tubes were gently shaken for 1 hour to liberate and resuspend polyacrylamide microspheres that were retained by the pore structure of sand-packed porous media. The concentration of polyacrylamide microspheres in the supernatant (decanted from the tubes) was measured using the same protocol and calibration curve for the effluent samples mentioned previously. The remaining sand in each 8 ACS Paragon Plus Environment

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centrifuge tube was then acid-washed and oven-dried to obtain the mass of sand in each centimeter. SI Figure S2 illustrates the chamber dissection protocol. Results were then plotted as the mass of polyacrylamide microspheres retained per unit mass of sand in each centimeter dissected as a function of dimensionless distance from the inlet of micromodel. The mass of polyacrylamide microspheres irreversibly attached to the sand grains in each experiment was determined from mass balance. 2.5. Calculation of DLVO Interaction Energy. Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, which is named after Boris Derjaguin and Lev Landau29, Evert Verwey and Theodoor Overbeek30, explains the aggregation of particles suspended in solutions quantitatively and describes the total interaction energy between charged particles or/and surfaces.31,32 As reported by Khilar and Fogler,33 this total interaction energy includes electric double layer repulsion (DLR), London-van der Waals attraction (LVA), Born repulsion (BR), acid-base interaction energy (AB), and hydrodynamic forces (HR). The first four are also known as colloidal forces. The transport and retention of the suspended particles depend on whether the resultant force is attractive or repulsive.34,35 In this study, the polyacrylamide microspheres have a particle size range of 17.00–47.46 µm, which are approximately 0.9 to 4.9 orders of magnitude less than the packed sand grains. Thus the sphere-plate geometry was used to describe the interaction of polyacrylamide microspheres and sand grain surfaces.36 In addition, DI water and clean sand were used in all experiments, and thus the acid-base interaction energy was negligible compared with the London-van der Waals attraction.37 In general, the hydrodynamic forces can become comparable with the colloidal forces only when the flow velocity is greater than 0.27 cm/s (1000 cm/h).38 Actually, the flow velocity in this study is 0.02 cm/s (72 cm/h), which is much lower than the aforementioned value, and



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thus the hydrodynamic forces are neglected. Therefore, DLR, LVA and BR dominate the total interaction energy between the surfaces of polyacrylamide microspheres and sand grains. DLR is a long-range repulsive force which acts over distances as large as one to few tens of nanometers. When the diffusive layer surrounding the polyacrylamide microspheres overlaps the diffusive layer surrounding the sand grains, DLR will develop between these two surfaces.39 This force is adverse for the deposition of polyacrylamide microspheres on sand grain surfaces. The DLR can be obtained by solving the Poisson-Boltzmann equation with appropriate boundary condition and geometry.40 Constant-potential boundary condition and sphere-plate geometry are considered in this study and the calculative expression for DLR is given in eq. (2).33

VDLR =

ε rε 0 rp  4

  1 + e −κ x  2 ln + ψ 2 +ψ s2 ) ln (1 − e−2κ x )  ψ ψ  p s  −κ x  ( p  1− e   

(2)

where VDLR is the electric double layer repulsion, J; εr is the dielectric constant for water and is equal to 66.82 at 333 K (60 °C); ε0 is the dielectric permittivity of free space (vacuum) and is equal to 8.854×10-12 C2/(J·m); rp is the mean radius of polyacrylamide microspheres, m; ψp, ψs are the surface potentials of polyacrylamide microspheres and sand grains, V; κ is the thickness of diffusive electric double layer, i.e., the reciprocal of Debye length, m-1; x is the separation distance between a polyacrylamide microsphere and the sand grain surface, m. The mean radius (rp) and surface potentials (ψp and ψs) can be replaced by the measured hydrodynamic radius (dh/2) and zeta potentials (ζ), respectively. The Debye length (κ-1) can be calculated by eq. (3).41

κ −1 =

ε rε 0 K BT

(3)

2 N A e2 I

where KB is the Boltzmann constant and is equal to 1.380×10-23 J/K; T is the absolute temperature, K; NA is the Avogadro constant and is equal to 6.022×10-23 mole-1; e is the elementary charge and is equal to 1.602×10-19 C; I is the ionic strength of the solutions, mole/m3. 10 ACS Paragon Plus Environment

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LVA is a long-range attractive force which acts over distances of up to few tens of nanometers. Unlike the DLR, LVA is mainly affected by the overlap of electron orbits, spatial arrangement of atoms and molecular structure of water.42 This force is beneficial to the deposition of polyacrylamide microspheres on sand grain surfaces The LVA for the sphere-plate geometry can be determined with eq. (4).33 VLVA = −

A132 6

 2 (1 + H )  H + ln   2+H  H (2 + H )

  

(4)

where H is the dimensionless distance, H = x/rp; VLVA is the London-van der Waals attraction, J; A132 is the Hamaker constant and is estimated as 10-21 J according to the method presented by Khilar and Fogler.40 BR is a short-range repulsive force arising from the overlapping of the electron clouds when a polyacrylamide microsphere is very close to the sand grain surface. Eq. (5) gives the calculative expression of BR for the sphere-plate geometry.33

VBR

A D = 132  σ 7560  rp

  

6

 8+ H 6−H  +   7 H 7   ( 2 + H )

(5)

where VBR is the Born repulsion, J; Dσ is the atomic collision distance and is equal to 0.50 nm. Afterward, the total interaction energy (VTOT) between a polyacrylamide microsphere and the sand grain surface can be obtained by the numerical summation of VDLR, VLVA, and VBR. In order to better analyze the results, the total interaction energy (VTOT) is normalized with KBT and the dimensionless form [VTOT (KT)] of this total interaction energy is shown in eq. (6). VTOT ( KT ) =

VDLR + VLVA + VBR K BT

(6)

3. RESULTS AND DISCUSSION



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3.1. Swelling Properties of Polyacrylamide Microspheres. Polyacrylamide microspheres can swell by adsorbing water, which is directly related to the transport and plugging capacity of the polyacrylamide microspheres in reservoir water shutoff treatment. SI Figure S3 gives the mean hydrodynamic diameter (dh) of polyacrylamide microspheres in NaCl solutions of 0, 0.001, 0.01, 0.04, 0.10 and 0.20 M IS versus swelling time at 60 °C. It can be clearly seen that the mean hydrodynamic diameter of the polyacrylamide microspheres increases with the increase of swelling time. Initially, the mean hydrodynamic diameter increases rapidly, and then tends to equilibrium. The equilibrium values are achieved after 180 minutes. Actually, the polyacrylamide microspheres are cross-linked gel particles with a three-dimensional network structure.43 There are many hydrophilic groups (–CONH2) within the three-dimensional network.44,45 When the polyacrylamide microspheres contact with water, these hydrophilic groups can combine with the water molecules easily and thus the hydration layer develops on the surface of polyacrylamide microspheres. Simultaneously, the three-dimensional network extends outward. This process is fast and corresponds to the sharp increase section of the hydrodynamic diameter. These water molecules are the hydrate water. Afterward, the water molecules continue to enter the internal network due to the osmotic pressure difference inside and outside the network. Therefore the volume of polyacrylamide microspheres increases slowly and gradually. Initially, the osmotic pressure difference is relatively larger causing a faster swelling rate. With the water adsorption going on, the osmotic pressure difference becomes smaller and smaller. Thus, the swelling rate slows down and finally tends to steady. The water molecules for this process are the free water. The results indicate that the polyacrylamide microspheres have good swelling property. Furthermore, inorganic ions can compress the hydration layer on the surface of polyacrylamide microspheres and restrain the outward extension of the three-dimensional network. As indicated


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by SI Figure S4, the swelling ratio (dh/da) of the polyacrylamide microspheres decreases with the increase of IS. Adding only a small amount of NaCl can distinctly decrease the swelling ratio of polyacrylamide microspheres. However, with the increase of IS, the swelling ratio begins to level off. Eventually, the swelling ratio stays at a high level when the ionic strength is up to 0.20 M, indicating a good swelling property of polyacrylamide microspheres at high IS levels. The swelling property reflects the capacity of polyacrylamide microspheres to change their particle size by adsorbing or losing water, which is beneficial to the transport and plugging performance of the polyacrylamide microspheres in reservoir water shutoff treatment.

3.2. Viscoelastic Properties of Polyacrylamide Microspheres. Actually, the dynamic modulus of polyacrylamide microspheres in suspensions is not only attributed to the electrostatic repulsive-force between the carboxyl groups, the hydrogen-bond interaction between the amide groups and water molecules, and the van der Waals force between the molecules, but also due to their cross-linked structure.46 The un-hydrolyzed multi-branched chains together with the steric hindrance of this cross-linked structure can effectively restrain the attack of positive ions on electric double layer. SI Figure S5 gives the dynamic modulus (G', G") of polyacrylamide microspheres in NaCl solutions of different IS at 60 °C, 1.0 Hz. It can be seen that IS has little effect on the dynamic modulus of polyacrylamide microspheres as expected. The lose modulus of polyacrylamide microspheres is always greater than the storage modulus, indicating a more significant viscous-characteristic at different IS. Additionally, the storage modulus and lose modulus all stay at a high level when IS reaches to 0.20 M, indicating a good viscoelasticity for polyacrylamide microspheres at high IS. The viscoelastic property reflects the capacity of polyacrylamide microspheres to remigrate by deformation under pressure when they are captured by pore-throats (i.e., grain-to-grain contacts) in reservoir water shutoff treatment.



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3.3. DLVO Interaction Energy Profiles. In order to calculate the total DLVO interaction energy for the polyacrylamide microspheres interacting with the sand grain surfaces, the zeta potential of the polyacrylamide microspheres and the sand grains in NaCl solutions at each IS (0, 0.001, 0.01, 0.04, 0.10, or 0.20 M) was measured by the Nano-ZS. As shown in SI Figure S6, negatively charged surfaces are maintained for all IS conditions, and the sand grains are more negative than the polyacrylamide microspheres. In addition, the surface charge becomes less negative with the increase of IS. This is mainly caused by the compression effect of positive ions on diffusive electric double layer,47 and accordingly affects the total DLVO interaction energy existing between the polyacrylamide microspheres and the sand grain surfaces.

Figure 10 plots the total DLVO interaction energy of polyacrylamide microspheres interacting with the sand grain surfaces. Generally, the DLVO interaction energy profiles are characterized by deep primary energy minima wells at small seperation distances, large energy barriers of 103 KT magnitude and shallow secondary energy minima wells at large seperation distances.48 In addition, at very small separation distances, there still exists a strongly repulsive energy barrier, which is mainly due to the Born repulsion.49 In this case, the polyacrylamide microsphere cannot continue to approach the sand grain surfaces. As indicated by Figure 4 and Table 1, the height of energy barriers decreases and the depth of secondary energy minima wells increases with the increase of IS. For all IS conditions investigated, the energy barriers disappear on the profiles at high IS levels of 0.10 M and 0.20 M, and the secondary energy minima wells vanish at the lowest IS level (DI water), as shown in Table 1. These results are mainly attributed to two aspects, i.e., the change of particle size of the polyacrylamide microspheres, and the variation of surface potentials of the polyacrylamide microspheres and sand grains. These two aspects are all


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caused by IS change. With the increase of IS, the particle size of polyacrylamide microspheres decreases and the surface charge of polyacrylamide microspheres and sand grains becomes less negative. In these cases, DLR decreases while LVA increases with the increase of IS. Therefore, the DLVO interaction energy profiles present the variation characteristics mentioned previously.

Figure 4. Total DLVO interaction energy of polyacrylamide microspheres interacting with the sand grain surfaces: (a) energy barrier and (b) secondary energy minima.

Table 1 DLVO primary energy minima, energy barrier, and secondary energy minima of polyacrylamide microspheres interacting with the sand grain surfaces. Ionic strength Primary energy minima (IS)/M Distance/nm Depth/KT

a

Energy barrier

Secondary energy minima

Distance/nm

Height/KT

Distance/nm

Depth/KT

0.00

NAa

NA

14.0

5673.49

NEb

NE

0.001

NA

NA

2.30

4896.06

81.52

-13.73

0.01

0.280

-141.58

1.38

2438.23

19.96

-52.16

0.04

0.281

-2181.88

1.67

106.22

6.33

-130.60

0.10

0.282

-2739.62

NE

NE

NA

NA

0.20

0.283

-2819.86

NE

NE

NA

NA

NA: not applicable; bNE: non-existent.



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Actually, polyacrylamide microspheres can be easily retained in the attractive secondary energy minima wells, as shown in Figure 5a. This condition is known as the secondary energy minima retention.50 As expected, a greater energy is necessary to push the polyacrylamide microspheres through the large energy barrier and into the attractive primary energy minima wells, as shown in

Figure 5b. This condition is referred to the primary energy minima retention.51 The former is reversible while the latter is irreversible. In addition, due to the high IS, the energy barrier is eliminated, and a favorable condition of chemical retention develops,52 as shown in Figure 5c. In this condition, no additional energy is required for the retention of polyacrylamide microspheres on sand grain surfaces. In the next section, we will consider the DLVO interaction energy profiles to explain the retention and release of polyacrylamide microspheres in sand-packed porous media.

Figure 5. Diagram of interaction energy for (a) secondary energy minima retention, (b) primary energy minima retention, and (c) favorable condition of chemical retention.

3.4. Transport and Retention of Polyacrylamide Microspheres in Porous Media. 3.4.1. Transport and Retention at Steady Ionic Strength. During injection of polyacrylamide microspheres in phase 2, the polyacrylamide microspheres first transported with water flow in large pore channels with low flow resistence, and then some of them were deposited on the sand grain surfaces or captured by the pore structure of sand grains, as indicated by the microscopic images collected in phase 2 at 0.10 M IS in Figure 6. Accordingly, the pressure at the inlet increased gradually (Figure 7) due to the reduction of the porous media conductivity (i.e., the blocking of large pore channels, Figure 6). The fluctuation


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change of injection pressure indirectly indicates the migration, plugging, remigration through elastic deformation, and replugging characteristics of polyacrylamide microspheres in reservoir water shutoff treatment.

Figure 6. Microscopic images for the transport and retention of polyacrylamide microspheres in porous media during phase 2 at 0.10 M IS. Transport in large pore channels: (a) 0.5 cm and (b) 3.7 cm from the inlet; retention on sand grain surfaces and in pore-throats: (c) 4.6 cm and (d) 6.3 cm from the inlet.

Figure 7. Effect of ionic strength on the injection pressure (water column height) throughout the injection process at 60 °C. Phase 1 and phase 3: injection of microsphere-free NaCl solutions at IS of 0.001, 0.01, 0.04, 0.10, or 0.20 M; phase 2: injection of polyacrylamide microspheres of



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1000 mg/L suspended in NaCl solutions of matching IS; phase 4: elution with DI water. A flow velocity of 0.02 cm/s was set for all phases and all experiments. Observed breakthrough curves (BTCs) of polyacrylamide microspheres for all phases and all experiments are shown in Figure 8. In phase 2, polyacrylamide microspheres were injected and then in phase 3 flushed with an microsphere-free background NaCl solution of same IS. It can be clearly observed that the polyacrylamide microspheres arrived at the outlet until around 1.6 PVs of fluids were injected into the chamber. This observation conflicts with the breakthrough time of nonadsorbed solutes transport in porous media.53 The retardation is mainly due to the straining effect of pore structure on polyacrylamide microspheres. At the end of phase 3, the recovery of polyacrylamide microspheres in the effluent decreased from 19.06% to 6.12% of the total mass of injected polyacrylamide microspheres with the increase of IS (Figure 8 and WR1 values in

Table 2). As expected, due to the elimination of energy barrier at high IS levels (0.10 M and 0.20 M), over 93% of injected polyacrylamide microspheres are retained in porous media (MT values in Table 2), almost 13% more than that at low IS (0.001 M) condition.

Figure 8. Effect of ionic strength on the retention and release of polyacrylamide microspheres in porous media at 60 °C. Phase 1 and phase 3: injection of microsphere-free NaCl solutions at IS of 0.001, 0.01, 0.04, 0.10, or 0.20 M; phase 2: injection of polyacrylamide microspheres of 1000 18 ACS Paragon Plus Environment

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mg/L suspended in NaCl solutions of matching IS; phase 4: elution with DI water. A flow velocity of 0.02 cm/s was set for all phases and all experiments.

Table 2. Results for the Retention, Release and Recovery of Polyacrylamide Microspheres in Porous Media under Different IS Conditions. Recovery in effluent (phase 2-3)

Release in effluent (phase 4)

Retention by straining (dissection)

Retention by attachment (mass balance)

Total retention (mass balance)

MR1/%

MR2/%

MS/%

MA/%

MT/%

0.001

19.06

0.07

80.50

0.37

80.94

0.01

15.12

0.16

83.14

1.58

84.88

0.04

12.23

0.37

85.01

2.39

87.77

0.10

6.71

0.77

88.18

4.34

93.29

0.20

6.12

0.96

88.27

4.65

93.88

Ionic strength (phase 1-3) (IS)/M

From the microscopic images collected during phase 2 and 3, four mechanisms were clearly observed to contribute to the retention of polyacrylamide microspheres in porous media. Three retention mechanisms participating at low IS level of 0.001 M were deposition at the upstream stagnant points, straining by the pore-throats, and interception by the particulate filter of polyacrylamide microspheres in Figure 9abc. At high IS levels, one additional retention mechanism can be observed and it was attachment on the sand grain surfaces in Figure 9d. Actually, the deposition at the upstream stagnant points and the attachment on the sand grain surfaces made little contribution to the reduction of porous media conductivity. As indicated by

Figure 7, compared to the low IS level of 0.001 M, although 13% more polyacrylamide microspheres were retained at high IS of 0.1 or 0.2 M due to these two retention mechanisms, the pressure increased similarly at the inlet. Therefore, the polyacrylamide microspheres retained in 19 ACS Paragon Plus Environment


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these two retention mechanisms are not beneficial to the plugging performance of polyacrylamide microspheres in reservoir water shutoff treatment. In addition, the same level of the injection pressure at the end of phase 3 (Figure 7) indicates an equivalent straining by the pore-throats for all IS conditions investigated. Anyway, most of the injected polyacrylamide microspheres can be retained in porous media to potentially achieve an effective performance in reservoir water shutoff treatment.

Figure 9. Microscopic images for the retention of polyacrylamide microspheres in porous media: (a) deposition at the upstream stagnant points in phase 2 at 0.001 M IS (4.3 cm from the inlet); (b) straining by the pore-throats in phase 3 at 0.001 M IS (4.9 cm from the inlet); (c) interception by the particulate filter of polyacrylamide microspheres in phase 2 at 0.001 M IS (1.0 cm from the inlet); (d) attachment on the sand grain surfaces in phase 3 at 0.10 M IS (3.8 cm from the inlet).

3.4.2. Release by Ionic Strength Reduction. As discussed above, the reduction of IS can cause the height of energy barrier to increase and the deposition state of polyacrylamide microspheres will be changed accordingly. After phase 3, DI water was injected in phase 4 to set a chemical transient condition of ionic strength reduction. As indicated by the BTCs of phase 4 in Figure 8 and MR2 values in Table 2, more polyacrylamide microspheres released in the effluent in those experiments of higher IS in phase 1-3. Nonetheless, 20 ACS Paragon Plus Environment

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only less than 1% of the total injected polyacrylamide microspheres released in the effluent. Most of the polyacrylamide microspheres were still retained in the porous media. Actually, the release of polyacrylamide microspheres by ionic strength reduction (chemical transient) in phase 4 was attributed to the elimination of secondary energy minima wells (Figure 4). After changing to DI water in phase 4, the polyacrylamide microspheres loosely retained by the secondary energy minima at nonzero IS levels in phase 2-3 were released from the sand grain surfaces, as indicated by the microscopic images in Figure 10ac. Afterward, the released polyacrylamide microspheres were captured by the pore-throats or intercepted by the existing particulate filter of polyacrylamide microspheres (Figure 10bd). The slight variation of the injection pressure in phase 4 also indicates the secondary retention of released polyacrylamide microspheres in porous media. The injection pressure in those experiments of initially higher IS in phase 1-3 was slightly higher after changing to DI water in phase 4 (Figure 7).

Figure 10. Microscopic images for the release and secondary retention of polyacrylamide microspheres in porous media. 0.20 M IS: (a) primary retention in phase 2 and (b) release and secondary retention in phase 4 (5.8 cm from the inlet); 0.10 M IS: (c) primary retention in phase 3 and (d) release and secondary retention in phase 4 (5.6 cm from the inlet).

3.4.3. Spatial Retention by Straining. 21 ACS Paragon Plus Environment


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The retention of polyacrylamide microspheres by straining was attributed to the pore structure of sand-packed porous media.54 After chamber dissections, the spatial distirbution profiles of the polyacrylamide microspheres retained by straining are given in Figure 11. It can clearly observed that the mass fraction of polyacrylamide microspheres with sand grains all decreased with the increase of the distance from the chamber inlet. Generally, the mass fraction in those experiments of initially higher IS in phase 1-3 was greater at the inlet, but almost stayed at an equivalent level near the outlet. This is mainly caused by the secondary retention of released polyacrylamide microspheres. As indicated by MS values in Table 2, more polyacrylamide microspheres were retained by straining in phase 4 with the increase of initial IS in phase 1-3. This is in agreement with the aforementioned results. Over 80% of the injected polyacrylamide microspheres can be retained in porous media by straining, which confirms the effective performance of polyacrylamide microspheres in reservoir water shutoff treatment at high IS levels.

Figure 11. Spatial distirbution profiles of polyacrylamide microspheres retained by straining after chamber dissections.

3.4.4. Irreversible Retention by Attachment. The mass fraction of polyacrylamide microspheres irreversibly attached to the sand grains in each experiment was determined from mass balance of the total mass fraction (100%), minus the 22 ACS Paragon Plus Environment

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relative mass of polyacrylamide microspheres recovered in the effluent (MR1 in phase 2-3 and MR2 in phase 4), and that retained by straining after chamber dissections (MS). Figure 12 visually demonstrates the irreversible attachment of polyacrylamide microspheres to the sand grains by primary energy minima retention (i.e., attachment). The amount of polyacrylamide microspheres retained by attachment slightly increased with initial IS. As indicated by MA values in Table 2, for all IS (0.001, 0.01, 0.04, 0.10, and 0.20 M), the retention by attachment ranged between 0.37% and 4.65% of the total injected polyacrylamide microspheres. These phenomena are possibly attribured to the roughness of sand grains, the article size of polyacrylamide microspheres, and the flow velocity of fluid in porous media.27,55-57 In general, at low flow velocities (which are usually associated with low hydrodynamic drag forces), favorable microsites on the sand grain surfaces would push the polyacrylamide microspheres with small particle size through energy barrier and into primary energy minima. The elimination of energy barrier at high IS conditions can enhance this process. Nevertheless, for the polyacrylamide microspheres used in this study, only less than 4.65% of the total injected polyacrylamide microspheres was irreversibly attached to the sand grains. Therefore, polyacrylamide microspheres can achieve an effective performance in water shutoff treatment for high IS reservoirs.

Figure 12. Microscopic images for the irreversible retention of polyacrylamide microspheres by attachment in phase 4: (a) 0.20 M of initial IS in phase 1-3 (5.8 cm from the inlet), (b) 0.10 M of initial IS in phase 1-3 (5.6 cm from the inlet), (c) 0.04 M of initial IS in phase 1-3 (5.7 cm from



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the inlet), (d) 0.01 M of initial IS in phase 1-3 (6.1 cm from the inlet), and 0.001 M of initial IS in phase 1-3 (5.9 cm from the inlet),

4. CONCLUSIONS In this work, laboratory experiments were performed at various ionic strengths (ISs) of 0.001, 0.01, 0.04, 0.10, and 0.20 M to quantitatively investigate the effect of IS on the transport and retention of polyacrylamide microspheres in porous media, and elucidate the key retention mechanisms during reservoir water shutoff treatment. The major conclusions are as follows: (1) The recovery of polyacrylamide microspheres in the effluent ranged between 19.06% and 6.12% of the total mass of injected polyacrylamide microspheres as IS increased. The retention of polyacrylamide microspheres increased accordingly due to the diminution/elimination of energy barrier at high IS levels. More than 80% retention of the polyacrylamide microspheres effectively reduced the conductivity of porous media, which contributed to their potentially good water shutoff performance. (2) Straining at pore-throats accounted for over 80% of the total mass of injected polyacrylamide microspheres and was the largest contributor to the retention of polyacrylamide microspheres for all IS conditions. IS reduction was beneficial to the release of polyacrylamide microspheres that were loosely retained on sand grain surfaces by secondary energy minima. Nevertheless, most of the released polyacrylamide microspheres were once again retained at pore-throats by straining which would further enhance the water shutoff performance. (3) The irreversible retention by primary energy minima ranged between 0.37% and 4.65% with increasing IS. At low flow velocities, favorable microsites on the sand grain surfaces would push the polyacrylamide microspheres with small particle size through energy barrier and into primary energy minima. The elimination of energy barrier at high IS conditions could enhance this 24 ACS Paragon Plus Environment

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process. However, more irreversible retention at high IS conditions was not beneficial to the water shutoff performance of polyacrylamide microspheres.

SUPPORTING INFORMATION Supporting Information contains the following: Figure S1. Diagram of transparent micromodel; Figure S2. Diagram of chamber dissection protocol; Figure S3. Mean hydrodynamic diameter (dh) of polyacrylamide microspheres in NaCl solutions of 0, 0.001, 0.01, 0.04, 0.10, and 0.20 M IS versus swelling time at 60 °C; Figure S4. Swelling ratio (dh/da) of polyacrylamide microspheres in NaCl solutions of different IS at 60 °C; Figure S5. Dynamic modulus (G', G") of polyacrylamide microspheres in NaCl solutions of different IS at 60 °C, 1.0 Hz; Figure S6. Zeta potential (ζ) of polyacrylamide microspheres and sand grains in NaCl solutions of different IS at 60 °C. This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (C.Y.). *E-mail: [email protected] (T.S.S.).

ORCID Chuanjin Yao: 0000-0002-6125-8991 Jing Wang: 0000-0002-0465-3143 Guanglun Lei: 0000-0002-8209-8974

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT 25 ACS Paragon Plus Environment


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This research was supported by the National Natural Science Foundation of China (Grant No. 51604291), the Natural Science Foundation of Shandong Province (Grant No. ZR2016EEB05), the Applied Fundamental Research Project Funded by Original Innovation Program of Qingdao City (Grant 17-1-1-34-jch), the Fundamental Research Funds for the Central Universities (Grant No. 17CX02010A, 15CX08004A), the China Scholarship Council for Chuanjin Yao (Grant No. 201306450015), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT1294).

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