Polymer-based in-situ gelled acids have been used in the field for more than a decade. These acids are typically used for matrix applications at HCl concentrations of 3-5 wt%. Recent lab work and field data indicated that these acids are not fully understood, and that there is a need to better characterize and understand how these acid systems develop viscosity in-situ. This understanding will enhance the way these acids are used in the field. Therefore, the objective of this study is to determine the elastic and viscous properties of these acids as they react with the rock.

Experimental studies were conducted to measure the rheological properties for in-situ gelled acids using an oscillatory rheometer. To the best of our knowledge, this is the first time that the elastic properties were measured for these acid systems. Measuring the elastic modulus (G’) and viscous modulus (G") were done using two techniques: Dynamic sweep and Creep tests.

Based on experimental results obtained, two regimes were identified: a pure viscous behavior and a semisolid elastic behavior. A viscous regime was noted below pH 2 where G" was dominant. Yield stress, Young's modulus, G’, and G" values were relatively small in the viscous regime. A semisolid elastic behavior was observed at pH greater than 2. Yield stress, Young's modulus, G’, and G" significantly increased at pH 2, and reached their maximum values at pH 3.2. G’ increased by 3 orders of magnitude while G" increased only by an order of magnitude during acid reaction with carbonate rock.

Maxwell and Kelvin-Voigt models were able to predict the experimental data for the viscous regime only. However, for the semisolid elastic regime, these models didn't accurately predict experimental results. Three-parameter model was able to predict the semisolid elastic regime only. Therefore, a new model was developed to predict the rheological properties of these acids over a wide range of pH values. Model predictions were in good agreement with experimental results.

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