Abstract
Barium sulfate is one of the most serious problem in mineral scale precipitation, the deposition of barite on the surface is a concern to flow assurance in the oil well. The scale formation is typically predicted by thermodynamic solubility limit. However, it is believed that a kinetically stable region exists where no scale will form even when the solution is supersaturated. The purpose of this study is to identify the existence of such a supersaturated region where scale does not form and to determine whether inhibitor is needed to control scale at this range of supersaturation.
Series of experiments were conducted in both batch reactor and flowing tube to investigate the range of supersaturation index (SI) where barite forms. In the batch experiments, a green laser apparatus was used to determine the induction time, which is defined as the time elapsed between the creation of supersaturation and detectable particle formation. In the flowing tubing experiments, the barium sulfate solution was continuously injected into a 92.7 cm 316 stainless steel tubing with 0.21 inch ID at SI = 0.3 to 1.2 at 120°C. The traveling time inside the tubing is 5 minutes with flow rate = 240 ml/hr. When barite SI = 0.3, no deposited barite were found in flowing tube which agrees with measured induction time in batch. However, at barite SI = 0.5 to 0.9, the deposited crystals were observed even though the predicted induction time is much longer than 5 minutes in the reactor tubing. The contradiction is, the barite deposited inside the flowing tube even though it was predicted not to precipitate in such short time according to batch experiment results. –The objective of this study is to resolve this apparent contradiction.
The presence of the hydrodynamic boundary layer may explain this phenomena. Further investigations were conducted in a microfluidic device to visually measure the time and size of deposited crystals to verify the hypothesis. The measured nucleation time corresponds with the batch reactor nucleation time. The results indicate that the supersaturated solution inside the boundary layer may have enough time to precipitate and deposit. This concept explains some of the field experience that the scaling happens even at a low SI value.
Furthermore, 0.25 ppm DTPMP can inhibit most of the barite deposition for 48 hours at 120°C with barite SI = 0.9. The result indicates that with trace amount of inhibitors in the boundary layer, the barite deposition can be prevented. On the other hand, 0.25 ppm PPCA shows partial inhibition and 1.0 ppm PPCA can completely inhibit barite deposition for 48 hours at 120 °C when barite SI equals to 0.9.
This work contributes to the verification of the kinetic stable SI range. The experimental results suggest that the deposition can take place inside the boundary layer even at a SI value predicted to be safe. Trace amounts of inhibitors can prevent the deposition at the same conditions. It is believed that these results provide a novel view of scaling risk prediction.
