Static acid dissolution of carbonate outcrops investigated by 1H NMR and X-ray tomography

The success of oil and gas exploration in carbonate reservoirs demands better strategies. Concerning the step of acid stimulation, the understanding of the nature of the carbonate matrices and their response to the acids can contribute for improvements of this process. We report a study about the reactivity of six different carbonate rocks with aqueous solutions of hydrochloric (HCl) and acetic (HAc) acids. The reactivity of the samples was determined by measuring the mass loss of the carbonate samples during the acid dissolution process. The results demonstrate that the dolomitization of the matrices is the main factor to decrease the kinetic of dissolution. Based on visual inspection, and micro-computed tomography, it was observed that the cylindrical outcrops with low dolomitization present different patterns of morphological dissolution upon HCl or HAc. For HCl, homogenous radial dissolution is observed, producing a smooth surface, while for HAc, the radial structure is practically preserved, but the surface become very rough. Concerning the inner effect of the acids, variations on the pore distributions of the outcrops were investigated by micro-computed tomography and pore distribution were observed during the dissolution cycles with the acids. This is an indication that the reaction occurs homogeneously from the surface, with negligible dissolution of the internal pore walls, since no pressure and flow procedures were employed in this study. •Static acid dissolution of carbonate outcrops investigated by time domain Nuclear Magnetic Resonance and X-ray tomography. • Acid stimulation in carbonate improve oil exploration in carbonate reservoirs. • Kinetics of acid dissolution and morphological changes were systematic investigated. • Chemical composition of the matrices is the more relevant parameter to the rate. • The mechanism of rock dissolution is different for HCl and acetic acid.