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The interaction of acidic fluids with carbonate rock formations is a key factor during reactive flow in carbonate acidizing operations. Although several studies have evaluated the reactivity of carbonate rocks, a critical gap remains concerning the influence of pre-experimental cleaning processes. Most studies involving outcrop rocks tend to overlook this aspect, yet the cleaning process can profoundly alter the surface characteristics of the rock, potentially leading to diverse and experimental results without reproducibility. Thus, understanding the impact of these cleaning protocols is essential for accurately interpreting the reactivity results and optimizing acid stimulation strategies.
This study investigates the effect of cleaning processes on the dissolution kinetics of Calcite and Dolomite through static dissolution using miniplugs. Two different permeability ranges were analyzed: low permeability (close to 2 mD) and high permeability (close to 100 mD). The cleaning methods used were based on hot Soxhlet extraction using nonpolar solvents according to API RP 40 standards: (i) toluene/acetone (TA); and (ii) chloroform (Ch). Therefore, three types of cleaning methods were considered: TA, Ch, and no-cleaning.
Miniplugs of 0.80 cm diameter and 1.0 cm length were prepared from larger cores, cleaned, and subsequently dissolved in HCl (1 M) under room conditions. The dissolution rate was quantified by the t50 parameter – the time required for 50% mass loss. The dissolution was verified in four time steps based on the t50: 1/4 of t50, 1/2 of t50, 3/4 of t50, and t50.
The results showed significant differences in the values of t50 and the concentrations of calcium ions −[Ca2+]− in the solution for both types of permeability and cleaning protocols. For Calcite samples, the non-cleaning and cleaned samples exhibited similar dissolution behaviors, suggesting a minimal impact of organic residues on acid-rock interactions. In contrast, Dolomite samples showed distinc dissolution trends, indicating that cleaning processes may influence the connectivity of the pore networks and reaction pathways. Moreover, the chloroform methodology revealed a greater increase in the acid dissolution for Dolomite. Micro-CT imaging provided detailed insights into the evolution of pore structures before and after dissolution. For both permeability groups, structural heterogeneities highlighted differences in reaction mechanisms and dissolution efficiency under varying cleaning conditions.
This study contributes to the broader understanding of cleaning-induced alterations in carbonate dissolution kinetics and their implications for subsurface applications. Future work will extend these findings to other carbonate lithologies and consider the influence of higher temperature and pressure conditions.
| Country | Brazil |
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