Introduction

Alkaline-Surfactant-Polymer flooding (ASP) is a tertiary recovery method designed to lower interfacial tension (IFT) and decrease water mobility to produce residual oil (Wyatt et al. 2002). ASP flood uses a combination of alkali, surfactant, and polymer to achieve these results. The purpose of the surfactant in an ASP flood is to lower the IFT between the residual oil and the injected fluids. However, the alkali reacts with acidic components of the oil to form additional surfactant within the formation to further lower the IFT. The use of alkali is much less costly than equivalent levels of surfactant, allowing for a more cost-efficient flood (Demin et al. 1997). The polymer is used for better sweep of the reservoir due to its ability to increase the viscosity of the fluids. This allows for better mobility control. The use of these three fluid injection additives offers great synergistic effects in terms of oil recovery and sweep efficiency (Huang and Dong 2004).

The ASP flood has high pH of 11 or above. As it moves through the reservoir, quartz silica is dissolved (Arensdorf et al. 2010) and the dissolved silica becomes stable in the high pH alkaline flood. However, the ASP flood encounters neutral pH connate water either near the wellbore or in the well as it flows to the production well. This, therefore, neutralizes the high pH alkaline water. The decreased pH of the mixed waters dramatically lowers the solubility of the monomeric silica. Besides, silicate ions complex with metal ions in the formation water pose a scale mitigation challenge. Systems with pH levels greater than 8.5 and increasing temperature may experience magnesium silicate scaling. Magnesium have been shown to aggravate the scaling tendency by complexing silica and providing the hydroxide ion in which silica precipitation is catalyzed (Demadis et al. 2007). Other metal hydroxides such as calcium hydroxide may also interact with silica, but these metal silicates have higher solubility than magnesium silicate (Amjad and Zuhl 2008). Furthermore, silica deposition may also be affected by the formation of calcium carbonate. Calcium carbonate scale does not provide nucleation sites, but it provides a matrix in which silica may be entrapped (Gill 1998). In accordance with this theory, co-precipitation and deposition of silica and silicate may also occur in this manner. The presence of both silica and magnesium creates a predicament, due to the converse solubilities of colloidal silica and magnesium silicate. Both scale species tend to form concurrently in ASP flood production systems.

Scale prevention, in principle, can be achieved by use of scale inhibitors. Scale inhibitors are chemicals that delay, reduce, or prevent scale formation when added into scaling water. Most of the scale inhibitors work by absorbing onto the crystal surface to prevent further growth of precipitate or by preventing the precipitate from adhering to solid surfaces such as pipes and vessels. The most common scale inhibitors used in the oil industry are inorganic phosphates, organophosphorous compounds, and organic polymers. Unfortunately, these “traditional” scale control methods applied to crystalline mineral salt precipitates do not apply to silica because it is amorphous (Ehrlich et al. 2010). In addition, the inherent and consequent environmental hazards of using toxic and non-biodegradable scale inhibitors have hindered the use of phosphonates due to their poor ecotoxicity. Moreover, many polymers fail to meet minimum biodegradation requirements (Holt et al. 2009). Therefore, the call for going green with scale inhibitors has become necessary. Though the use of green scale inhibitors to inhibit scale in oil and gas wells is relatively an unexplored area (Kumar et al. 2010), there have been several works on this “promising alternative” (Kohler et al. 2004).

Silicate scale formation

Solubility of silica depends upon several factors including pH, temperature, other ions present, and the silica form(s) present. The solubility of silica increases with pH varying from 120 mg/L at pH 6–140 mg/L at pH 9 and increases rapidly as pH is increased from 9.5 to 10.5 (Iler 1979). This could suggest that adjusting the pH may solve the problem of silica scale deposition. Also, solubility of silica increases with increasing solution temperature (Iler 1979). Because silica solubility increases with increasing pH, operating systems at high pH may potentially reduce silica scaling problems; the presence of various multivalent ions also influences the solubility of amorphous silica in aqueous solution (Chan 1989).

Summary of the silicate scale formation.

  1. 1.

    Silica dissolution The alkaline flood typically has pH 11 or higher as it sweeps the reservoir. The high pH water dissolves quartz in the formation, which results in dissolved monomeric silica (Si(OH)3O− Na+) flowing with the water flood.

  2. 2.

    Silica polymerization As the ASP water flows to the production well, it encounters neutral pH connate water near the wellbore or in the well. As the high pH ASP water is partially neutralized by the connate water, dissolved silica begins to polymerize and forms colloidal silica nanoparticles. Colloidal silica forms when the solubility level of monomeric silica is exceeded. The solubility of monomeric silica is pH dependent and decreases significantly below pH 10.5 (Amjad and Zuhl 2008).

  3. 3.

    Silica scale formation Magnesium, if present, can bridge the colloidal silicate particles and form an amorphous magnesium silicate scale. The ASP water is typically softened, and any residual magnesium would precipitate as Mg(OH)2 in the ASP. Magnesium is introduced in the neutral pH connate water. Magnesium silicate scale typically has non-stoichiometric ratios of magnesium to silicate. Similar interactions are possible with other polyvalent metal ions (iron, aluminum, and calcium), but magnesium silicate has a higher scaling index than the other metal silicates. In the absence of divalent cations, the polymerized silicate may continue to grow and form an amorphous “silica scale.” In the oilfield, different ratios of the two scales are likely forming in various wells as pH and cation concentrations differ.

  4. 4.

    Co-precipitation of silicate scale with other mineral scales, e.g., calcium carbonate If calcium is introduced in the connate water, the high pH of the ASP water mixing in the well will promote calcium carbonate scale. Calcium carbonate may provide nuclei for the development of silicate scales (Gill 1998). In industrial water, it has been observed that if carbonate scale is prevented, then silica can be tolerated at higher levels without generating scale.

Handling of silicate scale

The inhibition program using calcium carbonate inhibitor such as phosphonates or phosphates-based was put in place in early 2008, and was very successful for the first 6 months (Hunter et al. 2013). The program significantly increased pump run life and resulted in minimal or no scale buildup in pipelines or facilities. In late 2008 serious scale problems returned, indicated by numerous well failures and significant buildup of scale on downhole equipment. It was clear that the calcite inhibitor used was no longer effective. Wohlever et al. (2001) found that under stressed conditions (i.e., high pH, high temperature, high hardness, etc.), these phosphorus-containing inhibitors frequently react stoichiometrically with calcium ions leading to calcium-polyphosphate/phosphonate precipitation (Wohlever et al. 2001). This is because the amorphous nature of silica renders crystal modifiers phosphonates or mixed phosphonates/carboxylates ineffective (Agnihotri et al. 1999). Therefore, to overcome this problem, Arensdorf et al. (2010) and (2011) have developed new scale inhibitors in mitigating silicate scale during ASP flooding (Arensdorf et al. 2010, 2011). Even though the developed scale inhibitors could not completely prevent the silicate scale formation, they could delay it. After initial trials, the primary conclusion was that the same scale inhibitor had significantly different levels of effectiveness, depending on the inhibitor concentration and water chemistry of the individual wells. From the study, none of the silicate inhibitors acted as threshold inhibitors and completely prevented scaling at low doses. It was found that the effective delay process of scaling required high doses of inhibitors about 500 ppm (Arensdorf et al. 2010, 2011).

Therefore, much more well-thought inhibition approaches have to be utilized for controlling silica scale. Increasing environmental concerns and discharge limitations have imposed additional challenges. Therefore, the discovery and successful application of low doses chemical additives that have mild environmental impact has been the focus of several researchers (Quraishi et al. 1999; Demadis et al. 2007, 2005, 2004; Neofotistou and Demadis 2004a, b; Mavredaki et al. 2005). Recently, the use of cationic polymer as silica polymerization inhibitors has been the subject of numerous investigations. Amjad and Yorke in their evaluation of polymers reported that cationic-based copolymers are effective silica polymerization inhibitors (Amjad and Yorke 1985). Similar conclusions were also reported by Harrar et al. (1982) in their investigation on the use of cationic polymers and surfactants in inhibiting silica polymerization under geothermal conditions (Harrar et al. 1982). It is now certain that effective silica scale inhibition is dependent on the cationic charge on the polymer backbone (Demadis 2004a, b, 2005; Mavredaki et al. 2005; Demadis and Mavredaki 2005; Demadis and Stathoulopoulou 2006; Demadis and Neofotistou 2004; Neofotistou and Demadis 2004a, b).

Aside from that, extensive silica inhibition also have been done using two dendrimer inhibitors, Poly(amido amine)-1 and Poly(amido amine)-2 (Demadis and Oner 2009; Demadis 2005, 2008; Demadis and Neofotistou 2007, 2004a, b; Demadis and Stathoulopoulou 2006; Demadis et al. 2005; Neofotistou and Demadis 2004a, b). Poly(amido amine) dendrimers (PAMAM) backbone are composed of amide bonds rendering them biodegradable. Therefore, they are undoubtedly benign molecules. The dendrimer generation number indicates its degree of growth and branching. More specifically, PAMAM dendrimers of generations 0.5, 1.5, 2.5 possess –COOH termini, and those of generations 1 and 2 have –NH2 termini. From the previous studies, it was reported that the –COOH terminated dendrimers (generations 0.5, 1.5, and 2.5) showed virtually no activity as silica inhibitors (Neofotistou and Demadis 2004a, b; Demadis 2005). In contrast, the –NH2 terminated analogs (generations 1 and 2) are potent SiO2 scale inhibitors (Fig. 1).

Fig. 1
figure 1

Cationic polymer attachment on a single silica nanoparticle (Demadis 2008)

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Schematic structures of PAMAM-1 and 2 dendrimers are shown in Fig. 2.

Fig. 2
figure 2

Structures of PAMAM-1 and PAMAM-2 (Mavredaki et al. 2007)

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Despite the excellent performance of PAMAM-1 and 2 as colloidal silica growth inhibitors, these dendrimers suffer from a serious disadvantage: the silicates that are not inhibited lead to formation of large colloidal silica particles that entrap the dendrimers (Demadis 2008). This also leads to inhibitor depletion from solution, resulting in drop of inhibitory activity in the bulk. Formation of SiO2-PAMAM precipitates occurs due to association of anionic silica particles and cationic dendrimers as illustrated in Fig. 3.

Fig. 3
figure 3

Inhibitor entrapment within the colloidal silica matrix because of polycation (polymer)-polyanion (silica) interactions (Demadis 2008)

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Additionally, from the literature it was found that scale inhibition can be achieved by use of scale inhibitors in combination with dispersant polymers (Barouda et al. 2007). Therefore, to combat this problem some researchers resorted to use anionic polymer additives that could work with the dendrimer inhibitors (Neofotistou and Demadis 2014; Mavredaki et al. 2005, 2007). However, if the dosage of anionic polymer used is high, the activity of the dendrimers drops dramatically. It was found that the negative charge of polymer “overwhelms” the dendrimer and poisons its inhibitory ability (Demadis 2008; Mavredaki et al. 2005). Plus, most anionic polymers used are not green and/or have mild environmental impact. Based on the previous study, Mavredaki et al. 2005 have proposed a possible mechanism for the dissolution of colloidal silica by such zwitterions (Mavredaki et al. 2005). It is believed that an effective silica inhibition should be based on a delicate balance structure of cationic–anionic charges. In the study, a zwitterion compound that contains a positive and a negative charge on its backbone is used in synergistic action with cationic polymer. The possible mechanism of the dissolution of colloidal silica with zwitterions is shown below:

From the Fig. 4, it can be seen that the first silica–zwitterion additive interaction is an electrostatic association between the negatively charged silica particle and the cationic moiety of the zwitterion additives. This cationic group would minimize the effect of “overwhelms” on the cationic polymer and thus maintain its inhibitory ability. The positioning of the zwitterion additives in such way cause deprotonated and negatively charged carboxylate group can “swing” and attach to surface of Si center. Once OH−forms a Si–OH bond with surface Si, the Si–O network that connects the surface Si atoms with internal Si centers starts to collapse, thus exposing additional Si sites that become susceptible to attack. This function is in a way “mimicking” the action of HO− anions in the hydrolysis of the Si–O–Si network.

Fig. 4
figure 4

Possible mechanism of dissolution enhancement of colloidal SiO2 in the presence of zwitterion additives (Mavredaki et al. 2007)

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Conclusion and future directions

Despite the excellent performance cationic polymer as colloidal silica growth inhibitors, these polymers have some limitations which require improvement. Cationic charge on the polymer backbone, although necessary for inhibition, can cause inhibitor entrapment within the colloidal silica matrix because of polycation–polyanion interactions. Inhibitor entrapment causes its depletion from solution and its deactivation. Therefore, some researchers resorted to use anionic polymer additives that could work together with the cationic inhibitors. Nevertheless, increase of anionic polymer dosage above a certain threshold “overwhelms” the cationic charge of the inhibitor and poisons its inhibition ability. Therefore, an effective silica inhibition is based on a delicate balance of cationic–anionic charges on the polymer backbone. Thus, a green zwitterion compound that contains a positive and a negative charge on its backbone is proposed. This zwitterion compound would assist the inhibitors to operate more effectively. However, the details of such a dissolution mechanism are still under study and required further investigation. Besides, these dendrimers are widely used in water treatment and do not yet apply in mitigating silicate scales during ASP flooding. This previous fundamental research will be used as a guideline to improve our knowledge on the state of the anionic–cationic charges in the optimized inhibitor toward the presence of silica colloidal particles. This added knowledge would be a new information for developing and enhancing new and improved chemical additives for silicate scale inhibition during ASP flooding. Besides, it is anticipated that the results ensuing from this study will assist the oil and gas service industry in applying the optimized inhibitor at reservoir conditions. Moreover, green formulations have become the goal of most inhibitor developers. The diversified use of green scale inhibitors has been sporadic and evolutionary, and the trend seems to adopt a rather reactionary response to the present and potential environmental regulations and to support economic activities of oil and gas industry in the future.