Research Progress on eco-friendly Scale Inhibitors for Oilfield Development

The Development of Scale Inhibitors:

Scale inhibitors inhibit scale formation by disrupting one or more steps in the scale formation process. In the 1930s, natural polymer scale inhibitors such as lignin and tannin were used to prevent scale formation, but they suffered from unstable performance and required high dosages. In the 1950s, to enhance their scale inhibition, natural polymer scale inhibitors were gradually replaced by inorganic phosphates, such as sodium tripolyphosphate. In the 1960s, organic phosphates, such as aminotrimethylene phosphoric acid, were developed. Phosphate scale inhibitors offer excellent and stable scale inhibition, and for a long time dominated the scale inhibitor rankings. However, phosphate scale inhibitors contain large amounts of phosphorus, which can easily cause eutrophication, severely damaging the aquatic environment and endangering human survival and development. Therefore, in the 1970s, polymer scale inhibitors, such as hydrolyzed polymaleic anhydride, were synthesized. They offer better scale inhibition performance but have a lower tolerance for calcium. In the 1990s, sulfonic acid-based scale inhibitors with enhanced stability and temperature resistance, such as AA-methacrylic acid sulfonic acid copolymer, were developed. Because traditional scale inhibitors are difficult to degrade, people gradually realized their potential threat to the ecological environment. At the end of the 20th century, green and environmentally friendly scale inhibitors, such as polyepoxysuccinic acid and polyaspartic acid, were developed. These phosphorus- and nitrogen-free, biodegradable, and water-safe scale inhibitors have long been a research hotspot in the scale inhibitor field. In 2015, the mandatory national standard GB 31570-2015, "Petroleum Refining Industry Pollutant Emission Standard," was promulgated, stipulating that the total phosphorus content in discharged water must not exceed 1.0 mg/L. Therefore, scale inhibitor research should focus on reducing phosphorus emissions at the source and protecting the ecological environment, aiming to develop non-toxic and non-polluting green scale inhibitors.

Eco-friendly Scale Inhibitors:

Developed in response to the trend of building an environmentally friendly society, green scale inhibitors offer advantages such as high scale inhibition efficiency, biodegradability, and environmental friendliness. Currently, the most common green scale inhibitors used in oilfields are polyepoxysuccinic acid (PESA) and polyaspartic acid (PASP). To improve the effectiveness of these two green scale inhibitors in oilfields, many researchers have recently introduced into PESA and PASP groups with high electronegativity that facilitates chelation with metal cations such as Ca2+, Mg2+, and Ba2+, or increases the solubility of metal cations in solution.

Table 1 summarizes several commonly used functional groups for modifying PESA and PASP and their biodegradability. 

2.1 Polyepoxysuccinic Acid Scale Inhibitor

Polyepoxysuccinic acid scale inhibitor (PESA) is a phosphorus- and nitrogen-free green scale inhibitor developed in the United States in the 1990s. It exhibits excellent biodegradability and relatively good environmental adaptability, making it suitable for water environments with high alkali and metal concentrations. PESA molecules contain multiple carboxyl groups, which ionize upon dissolution to produce carboxyl anions. Under alkaline conditions, these carboxyl anions can transform the chain structure of the scale inhibitor molecule from curved to straight, exposing more negatively charged groups. This makes it easier for them to adsorb and become entangled in scale crystals, deforming them and affecting their growth or changing their crystal form, thereby inhibiting scale formation. Furthermore, the carboxyl groups can chelate with calcium and magnesium ions to form soluble chelates, increasing their solubility and achieving scale inhibition. The general synthesis route for PESA is as follows: maleic anhydride is hydrolyzed under alkaline conditions to produce sodium maleate. Then, sodium maleate is epoxidized in the presence of a catalyst (sodium tungstate) and an oxidant (hydrogen peroxide) to produce sodium epoxysuccinate. Finally, the sodium epoxysuccinate is polymerized in the presence of an initiator (calcium hydroxide) to form polyepoxysuccinic acid. PESA has good scale inhibition potential, but its high temperature resistance is poor and has a threshold effect, which limits its application range. Therefore, the research focus on PESA is to broaden the application range of PESA, including broadening the applicable temperature and increasing the upper limit of ion concentration. In order to enhance the scale inhibition performance of PESA, researchers have developed a series of polyepoxysuccinic acid derivatives by introducing some modified groups. 

2.1.1 Introduction of modified groups to enhance PESA performance 

(1) Introduction of -NH2 to enhance adsorption capacity

(2) Introduction of -COOH to enhance chelation and lattice distortion capacity

(3) Introduction of -CO-NH- to enhance biodegradation, adsorption and chelation

2.2 Polyaspartic acid scale inhibitor

Polyaspartic acid (PASP) scale inhibitor is also a green scale inhibitor that is non-polluting, non-toxic, harmless and easily biodegradable. PASP has α- and β-type structures and also contains carboxyl groups. The carboxyl groups ionize, generating negative ions that chelate with metal ions such as Ca2+ and Mg2+, thereby increasing their solubility and achieving scale inhibition. Figure 5 shows the current PASP synthesis route. First, the intermediate polysuccinimide (PSI) is synthesized, followed by alkaline hydrolysis of PSI to produce PSAP. Depending on the raw materials, PSI synthesis can be performed using L-aspartic acid (L-Asp) as a reactant or using maleic anhydride (MA) and ammonium salts as reactants.

2.2.1 Introducing modified groups to enhance PASP performance

(1) Introducing -CO-NH- to enhance biodegradation, adsorption and chelation

(2) Introducing -SO3H to enhance high temperature resistance

(3) Introducing -OH to enhance adsorption capacity

Problems and prospects

Currently, many green scale inhibitors have shown good scale inhibition performance, and even the scale inhibition rate can reach 100%. However, these green scale inhibitors are mostly targeted at one type of scale and the research on the scale inhibition mechanism is not in-depth enough. In addition, the cost control and industrialization of green scale inhibitors need to be studied. In order to obtain an oilfield scale inhibitor that is biodegradable, green, non-toxic, economical, and does not pose any threat to the environment and ecosystem, there is still a lot of work to be done:

(1) Research and development of scale inhibitors suitable for composite scales. Current green scale inhibitors can only exert scale inhibition effects on a certain type of scale, but cannot show scale inhibition effects on multiple scales at the same time.

(2) Research on the scale inhibition mechanism of green scale inhibitors. The description of the scale inhibition mechanism of green scale inhibitors is still at a qualitative level, with little specific data to support it. In particular, the exploration of the specific synergistic effects of multiple scale inhibition mechanisms is even more lacking in data support.

(3) Recycling and reuse of green scale inhibitors. In order to save resources and reduce the cost of oilfield development, it is of great significance to study the recycling and reuse of green scale inhibitors. However, there are currently few researchers conducting in-depth research on this issue.

(4) Industrialization of green scale inhibitor synthesis. The purpose of research and development is to optimize the synthesis route, use abundant, easily available and low-cost raw materials for synthesis, improve yield, accelerate industrial production, and enable green scale inhibitors to be applied to oilfield development more quickly.