We describe the reaction which occurs when formation fluid containing Ba in solution is displaced by and mixes with injected water, containing sulphate, to form BaSO4. The model is based on the conservation of mass of both the barium and sulphate, assuming that each species is advected with the flow, and that at the interface between the two fluids, dispersive and diffusive mixing of the fluids leads to precipitation. The model equations are solved numerically, and a simple asymptotic model for the reaction, valid at long time, is developed. This identifies that in a linear geometry, the mass of solid which forms when the front has distance x from the source, decays at a rate proportional to 1/x½. The modelling approach may also be used to describe injection into a radial geometry, leading to an analogous asymptotic model for the decay of solid precipitate with distance from the injector well.

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