Experimental simulation, matrix acidification; geomechanical tests; computed microtomography.

The different phases of oil well construction and exploration can cause damage to the rock formation near the well, reducing its permeability and, consequently, oil production. Well stimulation techniques seek to circumvent these unwanted phenomena. Matrix acidizing in carbonates is a stimulation technique that consists of injecting a reactive fluid to dissolve carbonate minerals in the well rock matrix, creating channels for the flow of oil. Hydrochloric acid (HCl) is the most commonly used acid for this technique, but its high reaction rate with the carbonate can reduce its penetration into the formation. Therefore, in order to optimize acid consumption and increase the depth of the wormholes, it is necessary to develop stimulation fluids containing additives that reduce the rate of dissolution of the rock matrix by the acid. The aim of this work was to evaluate and understand the influence of stimulation fluids, under different injection flow rates, on the petrophysical and geomechanical properties of the rock. Computed microtomography (μTC), a 3D reconstruction technique, was used to analyze the wormhole patterns formed. To this end, characterizations of the reactive fluids and carbonate rock samples, porosity and permeability tests, geomechanical tests, flow tests in porous media and x-ray computed microtomography were carried out. The result was that the acid treatment reduced the porosity of the acidified samples, but significantly increased the permeability. The results also showed that the mechanical properties (Young's modulus and Poisson's ratio) showed a weakening of the rock after acid treatment. The samples acidified with 15% HCl showed Young's modulus values around 96% lower than those of the intact samples. In addition, the samples acidified with 15% HCl plus additives showed 22% lower values. On the other hand, the group acidified with 15% HCl had a Poisson's ratio 39% higher than the intact group. The group acidified with 15% HCl and additives had a 15% higher coefficient. In addition, computed microtomography visually confirmed that acid treatment effectively creates a path for fluid flow through the core.