The kinetics of sulphate deposition in seeded and unseeded tests

In a previous paper (Boak et al. 2005), we addressed the issue: What level of sulphate reduction is required to eliminate the need for scale inhibitor squeezing? In this earlier work, we developed the idea of a simple kinetic scheme for sulphate deposition on the basis of a “safe envelope” concept within which the system could operate. Central to the safe envelope approach is the choice of rate constant for barite deposition and this can be estimated experimentally under various conditions appropriate to the specific field application. Experimental results on kinetic barite deposition in low sulphate brine mixtures were presented previously (Boak et al. 2005). However, these previous results were carried out for unseeded solutions (i.e., no solid barite or sand particles were added to the scaling solutions). The following questions were raised

If barite particles were present, would this mean that the solution supersaturation (Sp ) would reduce to 1 very rapidly thus over-ruling any kinetic deposition effects?
Does the presence of sand particles have the same effect as barite particles?
In the case of both barite or sand particles does the size of the particles (i.e. the surface area per unit mass) have any effect on the barite deposition rate?
In addition, a very simple kinetic model for bulk deposition of barite was used for which the long time behavior was that barite is deposited until the limiting ion was totally consumed. However, it is known that barite has some solubility at higher levels of sodium chloride and the kinetic model should limit to this value. This may be an important consideration especially in lower sulphate (or lower barium) brines.

Experimental results are presented in this paper which both address and answer the above questions, and extend the analysis with an improved kinetic model for barite deposition. Even in the presence of seed materials (barite and sand of various surface areas), clear rate effects can be observed and rate constants can be derived. As expected, barite seed material of higher surface area induces faster depletion of the scaling ions to a supersaturation ratio of 1 than barite of lower surface area. However, allowing for the effect of surface area, the barite seed material shows a clearly enhanced potential to induce scale formation than the sand. In addition, the safe envelope model is extended by considering a more accurate analytical kinetic model which limits correctly to the equilibrium solubility of barite at long times.