Design of the HCl Preflush in Sandstone Acidizing
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SummaryHCl usually precedes an HF/HCl mixture in the sandstone acidizing process to reduce HF consumption by carbonates and to avoid the precipitation of HF reaction products. Assuming equilibrium reactions between injected HCl and calcium carbonate in a sandstone formation, we developed a numerical model of HCl frontal movement in the radial direction from the wellbore and added it to an existing sandstone acidizing simulator. In general, only small preflush volumes (as little as 25 gal/ft can be adequate) are needed to prevent live HF from coming in contact with high-pH regions. In almost all treatments, however, spent HF will penetrate beyond the live HCl front; larger HCl preflushes displace the region where spent HF comes in contact with undissolved carbonates in the formation farther from the wellbore. The new simulator predicts the distribution of HCl and HF in a multilayered formation with damage around the wellbore. Using this model, we investigated the required size of the HCl preflush for a range of initial concentrations of HCl and carbonate. We also studied the influence of fluid diversion on the HCl frontal locations in multilayered reservoirs for multiple sequence treatments. 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 reached by HCl.
