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Summary

HCl 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.

Introduction

Sandstone matrix acidizing generally consists of an HF/HCl mixture aimed at reducing near-wellbore damage by dissolving silicate minerals, such as clays and feldspars. The HF/HCl mixture is usually preceded by a preflush of HCl for two reasons. First, the HCl removes carbonate minerals, preventing the consumption of the HF acid in reactions with these minerals. Second, and more importantly, the HCl is intended to prevent precipitation of HF reaction products that could redamage the formation. The HCl preflush displaces brine from the wellbore and the surrounding formation, preventing contact between sodium and potassium ions in the brine and fluosilicate reaction products, which, if mixed, would form insoluble sodium or potassium fluosilicates. Gdanski and Peavy also suggested that sufficient HCl is needed to replace all sodium and potassium ions held in exchangeable sites on clays with hydrogen ions. By removing calcium carbonate, the HCl preflush also prevents contact between calcium ions and the HF, avoiding the formation of calcium fluoride, another potential precipitate.