Iron Sulfide Scale Control using a Dispersant of Carboxymethylcellulose (CMC) in Sour Environment

When hydrogen sulfide gas is evolved in the presence of iron from various corrosion processes in downhole, iron sulfide can quickly precipitate. In the recent year, sulfide scale issues have been drawing lots of attention. Firstly, iron sulfide is one of the most or significantly unsolved deposition problem in oil and gas production. Secondly, iron sulfide has oleophilic nature so that it can be difficult to separate iron sulfide from oil phase during production processes. Polymeric dispersants have exhibited their feasibility to prevent the deposition of iron sulfide scales, but dispersants have not been widely validated to control FeS scale problems and limited numbers of trials and reports have been available. The goals of this study were (1) to develop efficient and effective technology preventing iron sulfide particle deposition on the surface as well as maintaining iron sulfide in the water phase; and (2) to understand FeS scale controlling reaction mechanism. Our studies indicate that carboxymethycellulose (CMC) displays the excellent performance of iron sulfide dispersion in pH 4.3 – 6.7, temperature 70 – 90 °C, and FeS saturation index (SI) 0.13 – 1.27. At pH 5.2, the required minimum CMC concentration to disperse FeS particles (C_crit) was 20 mg/L at 70 °C and SI(FeS_m) = 0.54 and 40 mg/L at 90 °C and SI(FeS_m) = 0.59. As pH increased to 6.7 at 70 °C, C_crit was reduced to 5 mg/L at SI (FeS_m) = 1.27. On the other hand, C_crit significantly increased to 100 mg/L at SI (FeS_m) = 0.13 and 400 mg/L at SI(FeS_m) = 0.44 at pH 4.3 and 70 °C. Hydrodynamic particle sizes remained in nano-size in different CMC concentrations in ranges of 300 to 530 nm at pH 4.3 and 170 to 335 nm at pH 5.0. The combination of DTPMP and CMC displayed synergistic effect. The greater portion of FeS particles were dispersed and kept their size smaller in the combination of DTPMP and CMC than CMC by itself. But it became less effective at 90 °C to inhibit or disperse iron sulfide solid formation than at 70 °C. FeS particles remained in water phase in the presence of CMC, while they stayed in oil phase in the absence of CMC.