Nucleation and Crystallization Kinetics of Barium Sulfate in the Hydrodynamic Boundary Layer: An Explanation of Mineral Deposition

Abstract

The initialization of barite deposition is believed to be mineral nucleation. The induction times characterizing the kinetics of nucleation have been widely studied to investigate the precipitation kinetics of barium sulfate and to estimate the risk of mineral deposition in the flowing tube. It is generally accepted that if the solution’s supersaturated solution’s induction time is longer than the pipe’s traveling time, barium sulfate will not precipitate inside the pipe. However, recent research shows that barium sulfate still deposits inside the flowing tube even though the predicted induction time is longer than the travel time. The results correlate to the field observations that the mineral scale deposition is found in the production well even at the condition that is predicted to be kinetically stable. The contradiction suggests that there is a missing step of mineral deposition in the flow. In this work, a hypothesis of nucleation in the boundary layer is proposed to explain the contradiction: the barium sulfate can nucleate inside the boundary layer at the surface where the linear flow velocity approaches zero. This slow-flow region offers enough time for the crystals to form without being flushed away. A series of experiments were conducted in the flowing tube and the microfluidic channels to support the hypothesis. The amounts of deposited barium sulfate in the pipes were measured and compared with the induction time prediction. The result shows that with barite SI (supersaturation index) = 0.5–0.9 at 120 °C, the barium sulfate can deposit all along the tube even when the predicted induction times are longer than the traveling time (311 s). On the other hand, the observed nucleation time in the microfluidic channels matches the previously reported nucleation time in the kinetic turbidity tests in beakers. This implies that the barium sulfate can stay and nucleate in the boundary layer at the surface, which further supports the proposed mechanism. To conclude, we combine the ideas of induction times, boundary layer, and precipitation kinetics to describe a new mechanism of barium sulfate deposition. Once the solution in the boundary layer reaches its induction time, the deposition begins.