Abstract

The objective of matrix acidizing treatments is to remove the skin damage from the vicinity around the wellbore and enhance the permeability and the well productivity. While this is accomplished in sandstone by using reactive fluids to dissolve the clay and the surrounding cementing materials, carbonate stimulation is successful when the reactive fluids bypass the damage by creating conductive channels or “wormholes”. It is rare in both treatments that the acids are used alone without several additives. The presence of any of these additives, which is required to achieve a certain function, may impact significantly the outcome of the acid treatment either positively or negatively. The objective of this study is to examine the effect of the type and concentration of several additives, which are currently in-use in the oilfield, on the calcite dissolution rate with HCl using the rotating disk reactor. Additives such as corrosion inhibitors, synthetic polymers, mono and divalent chloride salts, mutual solvent, dissolved iron, viscoelastic surfactants, and chelating agents were investigated.

The results showed that more reliable data was obtained at lower acid concentrations. Higher acid concentrations were associated with significant surface changes and inevitable source of error. The effect of corrosions inhibitors was studied by investigating three corrosion inhibitors (CI-A, CI-B, and CI-C) obtained from three different service companies. The presence of two corrosions inhibitors was associated with a reduction in the dissolution rate (between 7 and 45%), while the presence of the third one (containing formic acid) increased the rate of dissolution up to 38%. The effect of all other additives was further examined in the presence of one of the corrosion inhibitors. The presence of a synthetic polyacrylamide polymer significantly reduced the reaction rate down to 96% compared to a blank acid solution that did not contain the polymer.

Monovalent ions (sodium and potassium) showed little effect on the calcite dissolution rate, the presence of divalent ions such as magnesium chloride was associated with a reduction in the rate. Calcium chloride showed an increase in the reaction rate for both cases of the presence and the absence of a corrosion inhibitor.

The results also showed that mutual solvent changed the dissolution rate and a minimum was observed at 5 vol%. These results were confirmed by changing the corrosion inhibitor and the same behavior was observed at the same mutual solvent concentration. Dissolved iron reduced the rate of reaction by 17% at iron concentration up to 5000 mg/l. At 17,000 mg/l, the rate was reduced by 39% and a reddish to brown precipitate of ferric hydroxide was observed on the rock samples after the reaction, indicating a ferric precipitate. The reaction of VES-based acid showed a minimum at 125°F, which matched the maximum viscosity at low shear rate. The presence of 5 wt% chelating agents increased the reaction rate of 0.1 N HCl by 12.7 and 25%, when the reactions were conducted with no corrosion inhibitors and with 1 vol% CI-A, respectively.

Additives change the reaction of HCl with calcite, and these changes should be considered when the acids are used in matrix acidizing or acid fracturing.

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