Development of Greener D-Metal Inorganic Crosslinkers for Polymeric Gels Used in Water Control in Oil and Gas Applications

Crosslinkable polymers, such as polyacrylamide (PAM), are widely applied for water control in oil and gas reservoirs. Organic and inorganic crosslinkers are used to formulate a gel with PaM. Although chromium has a high level of toxicity, it has been implemented as an effective crosslinker combined with carboxylates because of the controllability of crosslinking time at low temperatures. The objective of this work was to develop greener d-metal inorganic crosslinkers based on cobalt, copper, and nickel to replace chromium for application at reservoir conditions. The obtained results showed that the gelation chemistry of the developed systems depends on the metal charge density. The gelation of PaM with d-metals depends on pH and temperature for lowand high-charge density, respectively. Cobalt (II) acetate (CoAc) was effective at high temperatures (130–150 ◦C) and forms (4% CoAc + 9%PAM) stable, and strong gels at a pH > 7 with a storage modulus exceeding 4300 Pa. However, Nickel Acetate and Cupper Acetate formed stable weak gels at low temperatures (50–70 ◦C) and a pH > 6 and gel decomposition was observed upon increasing the temperature. The developed formulations were compatible with low-salinity water (1000 ppm NaCl).


Introduction
The productivity of oil and gas wells is highly affected by the coproduction of water from water zones as well as the heterogeneity of the formations [1]. Sandstone reservoirs are associated with high permeability zones, whereas carbonates are naturally fractured. Crosslinked polymers have shown high efficiency in solving theses conformance control problems compared to conventional treatments such as mechanical sealing using Packers or cement squeezing [2][3][4][5]. Polyacrylamide (PAM) crosslinked by organic or inorganic crosslinkers is the most commonly applied type of polymer because of the ability to withstanding reservoir salinity and high-temperature conditions as well as its relatively low cost for field applications [2,[6][7][8][9][10].
Inorganic crosslinkers are preferred for the low-temperature range because of their fast gelation at high temperatures (<2 h at temperature > 90 • C) compared to organic crosslinkers such as polyethyleneimine (PEI), which results in stable gel after crosslinking with PaM at high temperatures (>90 • C) [11]. Aluminum (III), chromium (III), zirconium (IV), and titanium (IV) have been used to crosslink polyacrylamides and polysaccharides [12,13].
The crosslinking between PaM and inorganic metals is established through ionic bonds [14][15][16]. The inorganic crosslinkers form a 3D gel structure with PaM through the formation of coordination The metal carboxylate dissolves in water to form aqua complexes with d-metal. These aqua complexes, exemplified by the octahedral complex, [M(H2O)6] n+ , undergo substitution reactions in solution with the available ligands, such as amide, carboxylates moieties in the HPAM, and chloride ions in saline conditions. Ligand substitution reactions are among the most fundamental reactions a complex can undergo. This class of reaction comprises complex formation, in which the leaving group, the displaced ligand, X, is a solvent and the entering group the displacing ligand, Y, is some other ligand as in Equation (1): The rate of substitution reaction spans a very wide range and correlates with the structures of the complexes. In general, complexes of metal ions that have no extra factors to obtain additional stability, such as strong chelate effect and/or LFSE effect, are among the most labile with respect to substitution reactions. Complexes that show fast substitution are known as labile, while those associated with slow rates are known as inert or nonlabile. Any additional stability of the complex results in an increase in activation energy for a ligand replacement reaction and hence reduces the lability of the complex. Generally, very small ions are often less labile because they have greater M-L bond strengths, which makes it very difficult for incoming ligands to approach the metal atom closely [22,27].
Across the 3d series, complexes of d-metal M (II) ions are generally moderately labile, with distorted Cu (II) complexes among the most labile. Complexes of d-metal, M (III) ions are distinctly less labile than d-metal, M (II) ions. The d-Metal complexes with d 3 and low-spin d 6 configurations (for example, Fe (II), Cr (III), and Co (III)) are mostly nonlabile. Figure 2 reveals the crosslinking mechanism through complexation explained by formation of octahedral complex.  The metal carboxylate dissolves in water to form aqua complexes with d-metal. These aqua complexes, exemplified by the octahedral complex, [M(H 2 O) 6 ] n+ , undergo substitution reactions in solution with the available ligands, such as amide, carboxylates moieties in the HPAM, and chloride ions in saline conditions. Ligand substitution reactions are among the most fundamental reactions a complex can undergo. This class of reaction comprises complex formation, in which the leaving group, the displaced ligand, X, is a solvent and the entering group the displacing ligand, Y, is some other ligand as in Equation (1): The rate of substitution reaction spans a very wide range and correlates with the structures of the complexes. In general, complexes of metal ions that have no extra factors to obtain additional stability, such as strong chelate effect and/or LFSE effect, are among the most labile with respect to substitution reactions. Complexes that show fast substitution are known as labile, while those associated with slow rates are known as inert or nonlabile. Any additional stability of the complex results in an increase in activation energy for a ligand replacement reaction and hence reduces the lability of the complex. Generally, very small ions are often less labile because they have greater M-L bond strengths, which makes it very difficult for incoming ligands to approach the metal atom closely [22,27].
Across the 3d series, complexes of d-metal M (II) ions are generally moderately labile, with distorted Cu (II) complexes among the most labile. Complexes of d-metal, M (III) ions are distinctly less labile than d-metal, M (II) ions. The d-Metal complexes with d 3 and low-spin d 6 configurations (for example, Fe (II), Cr (III), and Co (III)) are mostly nonlabile. Figure 2 reveals the crosslinking mechanism through complexation explained by formation of octahedral complex.  The metal carboxylate dissolves in water to form aqua complexes with d-metal. These aqua complexes, exemplified by the octahedral complex, [M(H2O)6] n+ , undergo substitution reactions in solution with the available ligands, such as amide, carboxylates moieties in the HPAM, and chloride ions in saline conditions. Ligand substitution reactions are among the most fundamental reactions a complex can undergo. This class of reaction comprises complex formation, in which the leaving group, the displaced ligand, X, is a solvent and the entering group the displacing ligand, Y, is some other ligand as in Equation (1): The rate of substitution reaction spans a very wide range and correlates with the structures of the complexes. In general, complexes of metal ions that have no extra factors to obtain additional stability, such as strong chelate effect and/or LFSE effect, are among the most labile with respect to substitution reactions. Complexes that show fast substitution are known as labile, while those associated with slow rates are known as inert or nonlabile. Any additional stability of the complex results in an increase in activation energy for a ligand replacement reaction and hence reduces the lability of the complex. Generally, very small ions are often less labile because they have greater M-L bond strengths, which makes it very difficult for incoming ligands to approach the metal atom closely [22,27].
Across the 3d series, complexes of d-metal M (II) ions are generally moderately labile, with distorted Cu (II) complexes among the most labile. Complexes of d-metal, M (III) ions are distinctly less labile than d-metal, M (II) ions. The d-Metal complexes with d 3 and low-spin d 6 configurations (for example, Fe (II), Cr (III), and Co (III)) are mostly nonlabile. Figure 2 reveals the crosslinking mechanism through complexation explained by formation of octahedral complex.  The nature of associations depends on the characteristics of the cation used. Group II metals show a mainly electrostatic interaction, while d-block metals can complex with carboxylate ligands leading to more stable, pseudo-covalent bonds with reversible character. The bond stability of complexes formed follows the Irving Williams series, which basically scales with decreasing cation size [22]. The decrease in cation size indicates an increase in cation charge density or charges over radius ratio. However, the trend is further explained by the effect of electronic configuration through ligand field stabilization energies (LFSE) up to d 8 metals. For the ions used in this study, this trend follows Co (II) < Ni (II) < Cu (II). Copper (II) has lower LFSE energies than Ni (II), which can be attributed to the geometry of the complex. Cu (II) complexes reveal tetragonal distortion of the octahedral geometry because of the Jahn-Teller effect [12]. Table 1 illustrates the charge/radius ratio (charge density) of different metals. Cr (III) and Al (III) have a higher charge/radius ratio (charge density), which gives a clear explanation of the behavior of their stronger gelation properties compared to 3d metal M (II) cation in this study. The gelation properties for the M (II) [M = Co, Ni, Cu] decreased from cobalt to copper, which correlates very well with the drop in charge density. The effect of metal charge to size ratio has been addressed for hydrogels in previous studies [28]. All these complexes for M (III) or M (II) are affected by pH because at lower pH, the proton H + competes with the metal ion in the solution and facilitates hydrolysis and complex instability. Whereas, in basic solution, the hydroxyl group acts as ligand as well, and eventually lead to hydrolysis of HPAM-Metal complex, as confirmed in case of Ti due to ligand competition and formation of polynuclear Ti oxo/hydroxo-complexes [13].
All solutions were prepared using deionized water, except high salinity experiments, in which seawater was mixed with the polymer. The preparation steps were as follows: The water phase was added to PaM first, followed by additives such as salts and nanosilica. Then, the crosslinking metal (Co (II), Cu (II), or Ni (II)) acetate was added as drops, and pH was adjusted using KOH, if necessary, in the final step. The formulations containing nanosilica were subjected to sonication for 10 min to assure good distribution of nanosilica Particles in the solution.

Characterization
Thermogravimetric analysis (TGA) was conducted for Cobalt (II) acetate to investigate the effect of temperature on the sample up to 200 • C at a scanning rate of 5 • C/min with a waiting period at 105 • C and 130 • C for 15 min at each point. Ultraviolet-visible spectroscopy (UV-Vis) and Fourier-transform infrared spectroscopy (FTIR) were done to evaluate the bonds of the PaM and polymeric gels formulated by cobalt acetate.

Rheology Measurements
The effect of temperature was investigated by immersing the polymeric solutions within test GL-80 tubes in an oil bath for one day at room temperature, 70 • C and 130 • C, to simulate the conditions of oil and gas reservoirs. The Sydansk [30] gel coding system was applied to visually evaluate each formed gel. The rheological properties such as storage modulus and loss modulus, were studied using an Anton Par rheometer (MCR 302) using Parallel plate geometry with a 2-mm gap and fixed strain 1% at room temperature (25 • C). Additionally, the effect of initial pH on the rheological characteristics was investigated at different temperatures of 25 • C, 70 • C, and 130 • C.

Rheological Behavior and Viscoelastic Properties of the Inorganic Gels
Cobalt (II), copper (II), and nickel (II) acetates were used to crosslink 9% PaM. Each metal ion showed different behavior during the crosslinking. However, all metal ions were influenced by pH. Cobalt crosslinked with PaM at a high temperature (130 • C) and a pH of 7.3, while copper and nickel were able to crosslink at room temperature and a pH of higher than 6. Nevertheless, nickel showed better stability range (up to 70 • C) in comparison with copper (up to 50 • C). The minimum amount used to crosslink 9 wt.% PaM was about 10,000 ppm of Co (II). However, using a similar concentration for Cu (II) and Ni (II) did not crosslink PaM at the same conditions (130 • C). Gel formation as a result of the crosslinking of PaM and cobalt acetate was achieved in the range of 3-4.5 wt.% CoAc. Depending on the Co (II) concentration in the formulation, the formed gels showed different gel strength, as characterized by Sydansk codes [30]. Table 2 reveals the Sydansk codes after crosslinking PaM with CoAc. CoAc formed a gel at high temperature (130-150 • C) only, while no gel was detected at lower temperatures. The minimum concentration of CoAc that provided a gel was 10,000 ppm (3wt.) (Sydansk code "E-D"), which showed a barely flowing gel (Figure 3). Increasing the concentration up to 13,300 ppm (4.5 wt.%) enhanced the gel strength. After that, no gel was detected. This could tentatively Oxo/or hydroxide ligand substitution equilibria that decrease the strength of the desired PAM-Co complex. Increasing the concentration of CoAc from 10,000 ppm to 13,300 ppm (3 to 4.5 wt.%) increased the storage modulus of the gels from 24 Pa (weak gel) to 4300 Pa (strong gel) at a pH of 7.3 ( Figure  4). The obtained storage modulus using 4.5 wt.% cobalt (II) acetate with 9 wt.% PAM was comparable with the currently used organic and inorganic crosslinkers such as PEI and chromium acetate. The storage modulus of 9 wt.% PAM/1 wt.% PEI was around 1700 Pa at 130 °C [31]. Chromium (III) acetate at lower concentration (50-500 ppm) compared to cobalt (II) can crosslink PAM at temperature > 90°C. In another study by our group, 0.5 wt.% chromium acetate was used to crosslink 9 wt.% polyacrylamide-co-tert-butyl acrylate (PAtBA) at 90°C, providing a gel with a storage modulus of 1280 Pa. PAM/chromium (III) showed a high crosslinking rate at high temperatures (90-135 °C) [32,33]. Therefore, cobalt can be a green crosslinker alternative to PEI and chromium (III) at high temperature conditions, especially for gas reservoirs, where the temperature is in the range between 130-150 °C.
In contrast, CuAc based gels were formed at a minimum pH of 6. Figure 5 shows the flow behavior of the 11,145 ppm Cu (II) (3.5 wt.% CuAc)/9% PAM gel formed at room temperature at pH of 10.4. The decreasing trend of viscosity as the shear rate increased depicts that the flow behavior of this type of gels follows non-Newtonian shear-thinning behavior with yield stress at rest. The developed weak gels by crosslinking of PAM/CuAc could be useful for other applications such as enhanced oil recovery. Increasing the concentration of CoAc from 10,000 ppm to 13,300 ppm (3 to 4.5 wt.%) increased the storage modulus of the gels from 24 Pa (weak gel) to 4300 Pa (strong gel) at a pH of 7.3 ( Figure 4). The obtained storage modulus using 4.5 wt.% cobalt (II) acetate with 9 wt.% PaM was comparable with the currently used organic and inorganic crosslinkers such as PEI and chromium acetate. The storage modulus of 9 wt.% PaM/1 wt.% PEI was around 1700 Pa at 130 • C [31]. Chromium (III) acetate at lower concentration (50-500 ppm) compared to cobalt (II) can crosslink PaM at temperature > 90 • C. In another study by our group, 0.5 wt.% chromium acetate was used to crosslink 9 wt.% polyacrylamide-co-tert-butyl acrylate (PAtBA) at 90 • C, providing a gel with a storage modulus of 1280 Pa. PaM/chromium (III) showed a high crosslinking rate at high temperatures (90-135 • C) [32,33]. Therefore, cobalt can be a green crosslinker alternative to PEI and chromium (III) at high temperature conditions, especially for gas reservoirs, where the temperature is in the range between 130-150 • C.   In contrast, CuAc based gels were formed at a minimum pH of 6. Figure 5 shows the flow behavior of the 11,145 ppm Cu (II) (3.5 wt.% CuAc)/9% PaM gel formed at room temperature at pH of 10.4. The decreasing trend of viscosity as the shear rate increased depicts that the flow behavior of this type of gels follows non-Newtonian shear-thinning behavior with yield stress at rest. The developed weak gels by crosslinking of PaM/CuAc could be useful for other applications such as enhanced oil recovery.  All the measurements were done at least twice to confirm the repeatability of the obtained results. On average, the uncertainty in the rheological measurements is 5%. Proper mixing of the polymer/crosslinkers and adjusting the pH are vital for developing homogenous gels and obtaining consistent results.

Investigation of The Effect of Temperature on Gel Strength
CoAc with Co (II) concentration in the range of 10,000 ppm to 13,300 ppm (3-4.5 wt.% CoAc) formed gels with 9% PAM at 130 °C to 150°C, whereas both CuAc and NiAc failed to compose a gel at these conditions. Black precipitation of copper oxide was observed when 9% PAM/11,145 ppm Cu (II) heated up to 130 °C ( Figure 6). For NiAc, the formed gel was able to withstand temperature up to 70 °C, whereas the CuAc gel decomposed at that level at a similar pH (>6). All the measurements were done at least twice to confirm the repeatability of the obtained results. On average, the uncertainty in the rheological measurements is 5%. Proper mixing of the polymer/crosslinkers and adjusting the pH are vital for developing homogenous gels and obtaining consistent results.

Investigation of the Effect of Temperature on Gel Strength
CoAc with Co (II) concentration in the range of 10,000 ppm to 13,300 ppm (3-4.5 wt.% CoAc) formed gels with 9% PaM at 130 • C to 150 • C, whereas both CuAc and NiAc failed to compose a gel at these conditions. Black precipitation of copper oxide was observed when 9% PaM/11,145 ppm Cu (II) heated up to 130 • C ( Figure 6). For NiAc, the formed gel was able to withstand temperature up to 70 • C, whereas the CuAc gel decomposed at that level at a similar pH (>6).        FTIR analysis (Figure 9) of 9 wt.% PAM and 9 wt.% PAM with 12,000 ppm Co (II) gel sample revealed the main bonds associated with the peaks and the change in wavenumber after the crosslinking. Table 3 illustrates the interpretation of the peaks assigned for the peaks of 9 wt.% PAM and mature gel composed of crosslinking 9 wt.% PAM with 12,000 ppm Co (II). The peaks were assigned as follows: 3317-3262 cm −1 for secondary amide N-H stretching, 2871 cm −1 for C-H bond stretching, 1550-1636 cm −1 for primary amide C=O stretching, and 475-491 cm −1 C-C and 413-460 O-C-N bending, respectively [34][35][36]. The significant change was observed in the carbonyl group bond, which confirms the crosslinking mechanism. FTIR analysis (Figure 9) of 9 wt.% PaM and 9 wt.% PaM with 12,000 ppm Co (II) gel sample revealed the main bonds associated with the peaks and the change in wavenumber after the crosslinking. Table 3 illustrates the interpretation of the peaks assigned for the peaks of 9 wt.% PaM and mature gel composed of crosslinking 9 wt.% PaM with 12,000 ppm Co (II). The peaks were assigned as follows: 3317-3262 cm −1 for secondary amide N-H stretching, 2871 cm −1 for C-H bond stretching, 1550-1636 cm −1 for primary amide C=O stretching, and 475-491 cm −1 C-C and 413-460 O-C-N bending, respectively [34][35][36]. The significant change was observed in the carbonyl group bond, which confirms the crosslinking mechanism.
Energies 2020, 13, 4262 9 of 15 revealed the main bonds associated with the peaks and the change in wavenumber after the crosslinking. Table 3 illustrates the interpretation of the peaks assigned for the peaks of 9 wt.% PAM and mature gel composed of crosslinking 9 wt.% PAM with 12,000 ppm Co (II). The peaks were assigned as follows: 3317-3262 cm −1 for secondary amide N-H stretching, 2871 cm −1 for C-H bond stretching, 1550-1636 cm −1 for primary amide C=O stretching, and 475-491 cm −1 C-C and 413-460 O-C-N bending, respectively [34][35][36]. The significant change was observed in the carbonyl group bond, which confirms the crosslinking mechanism.

Effect of PH
The working range of pH to initiate the gelation of the inorganic crosslinkers was above 6. Copper and nickel acetate composed gels with 9% PaM at pH > 6. At basic conditions, gels were observed even at room temperature, indicating the impact of pH on the crosslinking reaction for these two metals. Increasing the pH enhanced gel strength and stability. On the Sydansk gel coding system, the formed gels based on Cu (II) and Ni (II) acetate can be classified as class "E" and "F" at a pH of 8 and 11, respectively. However, the formed gels were not homogeneous and liquid phase could be observed in the samples. Table 4 and Figure 10 depict the influence of pH on the formed gels by CuAc at different concentrations.   The effect of pH on the storage modulus after crosslinking 9% PaM with 3.5% CuAc at different pH values is shown in Figure 11. It can be clearly seen that there was an optimum for the pH, since at a low pH of 5.23, no gel was formed, while increasing the pH to 7 and 11 raised the storage modulus to 21 Pa and 179 Pa, respectively. In contrast to CuAc, NiAc showed a decreasing trend for the storage modulus with an increase in pH ( Figure 12). However, the gels were not homogeneous and this can be seen visually in Figure 13. Therefore, at a neutral pH of 7, the formed Particles of the gels were stronger than the more homogeneous gels at a strong basic pH of 12. This heterogeneous behavior could be useful in other applications such as forming microgels. Both carboxylate ligands and the charged states of metallic cations are affected by pH. Complex formation and binding modes in these experiments are highly pH-dependent. Little complexation exists under basic conditions because of hydroxide formation, which diminishes the polyvalent state of a hydrated cation [37]. The effect of pH on the storage modulus after crosslinking 9% PAM with 3.5% CuAc at different pH values is shown in Figure 11. It can be clearly seen that there was an optimum for the pH, since at a low pH of 5.23, no gel was formed, while increasing the pH to 7 and 11 raised the storage modulus to 21 Pa and 179 Pa, respectively. In contrast to CuAc, NiAc showed a decreasing trend for the storage modulus with an increase in pH ( Figure 12). However, the gels were not homogeneous and this can be seen visually in Figure 13. Therefore, at a neutral pH of 7, the formed particles of the gels were stronger than the more homogeneous gels at a strong basic pH of 12. This heterogeneous behavior could be useful in other applications such as forming microgels. Both carboxylate ligands and the charged states of metallic cations are affected by pH. Complex formation and binding modes in these experiments are highly pH-dependent. Little complexation exists under basic conditions because of hydroxide formation, which diminishes the polyvalent state of a hydrated cation [37].

Gel Compatibility with Additives
Generally, salts work as retarders to the gelation of the crosslinked gels [26,38]. Adding 0.1% NaCl decreased the storage modulus of the gel formed of 3.5% CoAc/9% PAM by 70% to about 360 Pa (Figure 14), whereas no gels were identified at higher salt concentrations and when seawater was used. For copper and nickel, no gels were detected when NaCl or seawater were used. Nanosilica is generally used to reinforce the crosslinked gel [9,[39][40][41]. However, no gels were formed for all used types of d-metal crosslinkers when nanosilica (1000-20,000 ppm) to reinforce the inorganic gels. This can be explained by the competitive behavior between the silanol group in the nanosilica and the metals to form a bond with PAM.

Gel Compatibility with Additives
Generally, salts work as retarders to the gelation of the crosslinked gels [26,38]. Adding 0.1% NaCl decreased the storage modulus of the gel formed of 3.5% CoAc/9% PaM by 70% to about 360 Pa (Figure 14), whereas no gels were identified at higher salt concentrations and when seawater was used. For copper and nickel, no gels were detected when NaCl or seawater were used. Nanosilica is generally used to reinforce the crosslinked gel [9,[39][40][41]. However, no gels were formed for all used types of d-metal crosslinkers when nanosilica (1000-20,000 ppm) to reinforce the inorganic gels. This can be explained by the competitive behavior between the silanol group in the nanosilica and the metals to form a bond with PAM.
used. For copper and nickel, no gels were detected when NaCl or seawater were used. Nanosilica is generally used to reinforce the crosslinked gel [9,[39][40][41]. However, no gels were formed for all used types of d-metal crosslinkers when nanosilica (1000-20,000 ppm) to reinforce the inorganic gels. This can be explained by the competitive behavior between the silanol group in the nanosilica and the metals to form a bond with PAM.

Environmental Impact of Inorganic Crosslinkers
The health and environmental impact of the chemical materials was generally evaluated using a lethal dose (LD50). A LD50 can be explained as the amount of substance taken all at once that can cause the death of half of the tested group. Table 5 reveals the lethal dose of d-metals versus chromium and titanium. The average lethal dose of the transitional metals used in this study was higher than that of chromium, which is widely used in the oil industry. Therefore, this an indication that cobalt, nickel, and copper are safer than chromium, with less environmental impact on the environment than chromium.

Environmental Impact of Inorganic Crosslinkers
The health and environmental impact of the chemical materials was generally evaluated using a lethal dose (LD 50 ). A LD 50 can be explained as the amount of substance taken all at once that can cause the death of half of the tested group. Table 5 reveals the lethal dose of d-metals versus chromium and titanium. The average lethal dose of the transitional metals used in this study was higher than that of chromium, which is widely used in the oil industry. Therefore, this an indication that cobalt, nickel, and copper are safer than chromium, with less environmental impact on the environment than chromium. Table 5. Lethal dose of inorganic crosslinkers.

Conclusions
In this study, inorganic crosslinked polymeric gels were formulated using cobalt (II), copper (II), and nickel (II) acetates at high (130 • C) and low (up to 70 • C) temperatures and at intermediate to high pH. The following conclusions can be made: The crosslinking of PaM with d-metals depends mainly on metal charge density.
Cobalt (II) acetate was able to crosslink with 9 wt.% PaM at 130 • C, showing a strong gel with a storage modulus exceeding 4300 Pa when 13,300 ppm Co (II) (4.5 wt.% CoAc) was used.
Copper (II) acetate and nickel (II) acetate were affected by pH and gels formed at lower temperatures between 50 • C-70 • C. However, these gels decomposed at higher temperatures.
For nickel (II) acetate solutions, the gel strength decreased as the pH increased and vice versa for copper (II) acetate samples.