Biodegradable Chelating Agents for Effective CaSO4 Scale Removal in Oil and Gas Production

Abstract

CaSO₄ mineral scale is a persistent flow assurance challenge in oil and gas production. Usually, high concentrations of chelating agent formulations are used to dissolve scale deposits formed in surface flow lines and downhole. However, many conventional aminopolycarboxylates (APCAs) could raise health and environmental concerns. On the other hand, biodegradable chelators are often regarded as less efficient and less cost-effective for scale removal. This work compares the performance of two biodegradable chelators, methylglycine diacetic acid (MGDA) and l-glutamic acid N,N-diacetic acid (GLDA), with the nondegradable diethylenetriamine pentaacetic acid (DTPA) and 1-hydroxyethane-1,1-diphosphonic acid (HEDP), in dissolving CaSO₄·2H₂O crystals at elevated temperatures. Results shows that MGDA is highly effective and has strong potential to replace traditional chelators. The scale removal efficiency followed the order of MGDA > GLDA > DTPA > HEDP across concentrations at 25 °C, 50 °C, and 100 °C, respectively. With 16% MGDA and pH 10, more than 80% CaSO₄·2H₂O was removed at 100 °C with a solid/solution ratio of 2 g/10 mL. The scale removal efficiency of MGDA exceeded that of GLDA (66%), DTPA (45%), and HEDP (25%) at the optimal pH. Through characterizations of the dispersed solids in chelator solutions using X-ray diffraction, it is concluded that the dissolving mechanism involves direct interactions between Ca²⁺ and the chelating agents, along with minor formation of Ca-(OH)₂ particles at optimized alkaline conditions. Overall, this work demonstrated MGDA and GLDA as effective biodegradable alternatives for CaSO₄ removal in the oil and gas industry.

1

Chemical structures of MGDA, GLDA, HEDP, and DTPA, respectively.

2

CaSO₄ solubility in MGDA, GLDA, DTPA, and HEDP at concentrations up to 10%, respectively. The error bars present three replications.

3

CaSO₄ scale removal efficiency at a series of concentrations of MGDA (orange line), GLDA (blue line), DTPA (gray line), and HEDP (yellow line) at 50 °C, respectively. The error bars present three replications.

4

CaSO₄ scale removal efficiency at a series of concentrations of MGDA (orange line), GLDA (blue line), DTPA (gray line), and HEDP (yellow line) at 100 °C, respectively. The error bars present three replications.

5

(a) Curves of CaSO₄ scale removal efficiency versus pH values of 16% MGDA, 16% GLDA, 15% DTAP, and 15% HEDP; (b) photos of the MGDA-treated solutions. Conditions: 2 g of CaSO₄ crystals immersed in 10 mL chelating agent solutions at various pH conditions from ∼5 to ∼13 at 100 °C for 24 h. The error bars present three replications.

6

Removal efficiency of four chelators before and after aging at 100 °C for 24 h. The scale removing tests were conducted by immersing 2 g of CaSO₄·2H₂O in 10 mL chelator solutions at 100 °C for 24 h.

7

CaSO₄ solubility of 8% MGDA and 8% GLDA at 25 °C, 50 °C, 75 °C, and 100 °C, respectively. The error bars present three replications.

8

Removal efficiency and removed mass of CaSO₄ with different initial crystal weights at 50 °C (green lines) and 100 °C (black lines), respectively. The solution is 8% MGDA fixed at 10 mL. The error bars present three replications.

9

Scale removal efficiency by adding different concentrations of K₂CO₃ in MGDA. The test condition is 2 g crystals in 10 mL chemicals for 24 h at 100 °C. The error bars present three replications.

10

Scale removal efficiency and Ca²⁺ concentration in time scale. Removal conditions: 1 g CaSO₄·2H₂O crystals in 10 mL of 8% MGDA. The error bars present three replications.

11

(a) SEM images and (b) XRD of the solid particles dispersed in solution after MGDA treatment at 100 °C for 24 h.