In a previous paper1, 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 based on 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 previously1. 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 behaviour 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.
Work on the kinetics of barite deposition in low sulphate brine mixtures was reported previously. In this earlier work, we addressed the question: what level of sulphate reduction is required to eliminate the need for scale inhibitor squeezing? The example of the Marlim Leste reservoir was used in this earlier work in terms of the brine compositions and conditions,[2,3] and sensitivity studies were performed for various barium and sulphate concentrations. A series of static kinetic barite deposition experiments were conducted to determine the optimum sulphate level that - for a given set of kinetic deposition parameters - allowed us to stop the application of scale inhibitor squeeze treatments. The experimental data was analysed using a novel analytical approach1 to develop "safe envelopes" of [Ba2+] and [SO42-] within which we should work. Some estimate of likely barite deposition rates was calculated along these envelopes. This modelling approach is described in outline in a later section.
Our previous experimental results were carried out only for unseeded solutions i.e. no solid barite or sand particles were added to the scaling solutions. It has been observed in the past that adding finely powdered barite crystals to the supersaturated brine mixture accelerates the barite deposition. This is because it allows the system to deposit barite by continued growth of the seed material by a crystal growth mechanism on the high surface area of the seed crystals. The unseeded tests are thought to indicate a slower kinetic growth rate since they must first form proto-crystals by crystal nucleation that must then subsequently grow. The presence of the seed crystals avoids the necessity of the nucleation step and allows more rapid barite crystal growth. Indeed if a sufficient quantity of very fine barite particles are present, it is thought that the supersaturation may very rapidly fall to Sp =1 and kinetic effects would be minimised. Therefore, in the light of our previous experimental results, and the resulting "safe envelope" kinetic model, the questions listed above were raised. These questions are addressed in the experiments presented in this paper.