Design of the HCl preflush in sandstone acidizing
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Hydrochloric acid (HCl) usually precedes a mixture of hydrofluoric (HF) and hydrochloric acid in the sandstone acidizing process to reduce the consumption of HF by carbonates and to avoid the precipitation of HF reaction products. Assuming equilibrium reactions between injected HCl and calcium carbonate in a sandstone formation, a numerical model of HCl frontal movement in the radial direction from the wellbore has been developed and added to an existing sandstone acidizing simulator. The simulator predicts the distribution of HCl and HF in a multilayered formation with damage around the wellbore. Using this model, the required size of the HCl preflush has been investigated for a range of initial concentrations of HCl and carbonate. The influence of fluid diversion on the HCl frontal locations in multilayered reservoirs for multiple sequence treatments has also been studied. In particular, we simulated treatments where HCl was injected without diversion, followed by an HF/HCl sequence including diverting agent to determine whether the HF would be diverted to regions that had not been contacted by HCl. In general, only small preflush volumes are needed to prevent live HF from contacting high pH regions, with as little as 25 gal/ft of HCl preflush being adequate for this purpose. However, in almost all treatments, spent HF will penetrate beyond the live HCl front; larger HCl preflushes serve to displace the region where spent HF contacts undissolved carbonates in the formation farther from the wellbore. Lack of diversion of the preflush followed by diversion of the HF/HCl sequence will lead to HF contacting high pH regions only in cases of extremely large permeability contrast between zones.
