Polymer-based acids, which have been used in the field for decades for acid diversion, were examined extensively through the literature. Recent lab work indicated that these acids could cause damage under certain conditions, while field data gave positive and negative results. The behavior of these acids is not fully understood. Permeability enhancement, radius of wormholes, diversion efficiency and damage mechanisms are determined in part by the rheological properties of these acids. Rheology of In-situ gelled acids exhibit strong viscoelastic properties at pH greater than 2. Therefore, the objectives of this study are to determine the elastic and viscous properties of these acids, how these properties change as the acid reacts with carbonate formations, and what their effect on acid diversions.
Experimental studies were conducted to measure the rheological properties for polymer-based acids using oscillatory rheometer. To the best of our knowledge, this is the first time that the viscoelastic properties were measured for these acids. Measuring the elastic modulus (G') and viscous modulus (G") were done using two tests: Dynamic sweep tests, which were used to define the shear rate dependent response; Creep tests, which were used to define the time depended response.
The behavior of gelled acids was independent of pH while the behavior of in-situ gelled acids varied with pH. Gelled and live in-situ gelled acids had weak viscoelastic properties while partially neutralized (pH 3.2) in-situ gelled acid had strong viscoelastic properties. Polymer filter-cake formed in case of gelled acid injection while in-situ gelled acids didn't form any polymer filter-cake. This is because of the in-situ gelled acid formed a thin layer of gel that minimize the leak-off which prevent the build of polymer filter-cake. The strong viscoelastic properties determine the effectiveness of diversion. As elastic properties increases, the formed gel will sustain around the wormhole and plug it. This will force the next acid stage to penetrate the weakest point and change its direction. For gelled acid, the inlet face washout and the polymer filter-cake reduced the core permeability to zero by plugged the core holder inlet during the water flow back. However after removing the plugging effect, a permeability enhancement was much higher than in-situ gelled acids. The wormhole obtained in case of gelled acid was nearly linear and thinner than the wormhole created by in-situ gelled acid. The results obtained can be used to better design matrix acid treatments when polymer-based acids are used in the field.