Effect of calcium content on the dissolution rate of dolomites in HCl acid

In previous dissolution studies, dolomite has been commonly assumed to have an ideal composition of CaMg (CO3)(2). The objectives of this work are to determine (1) if dolomites composing hydrocarbon reservoir rocks vary in chemical composition, specifically Ca content, and (2) the effect of Ca uptake on the dissolution rate of dolomite in HCl acid. Dolomitic reservoir rocks from different sources were analyzed using several techniques, including: X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and acid solubility. Most dolomites are Fe/Mn poor and nonstoichiometric with 1.02 < nCa <1.13, where nCa is the average number of Ca atoms per formula unit. To quantify the effect of Ca content on the dissolution rate of dolomite, the samples with similar mineral composition and microstructure, but different Ca uptake, were reacted with 21.5 wt% HCl in a rotating disk apparatus (RDA). At 24 degrees C dolomites with chemical composition Ca1.067Mg0.933(CO3)(2) dissolved 1.7 times faster than ones with composition Ca1.015Mg0.985(CO3)(2). At 50 degrees C the difference in the dissolution rates increased 2.2 times. Further increase in temperature decreased the difference in dissolution rates to 1.5 and 1.4 times at 75 and 100 degrees C, respectively. This non-linear dependence is explained by the shift of the dissolution process from surface reaction rate to predominantly diffusion (mass-transfer) limited. Results also indicate that Ca content in dolomites was not constant along the sampled intervals of the same wells. Since dolomite reactivity depends on calcium content, spatial variation in chemical composition of this mineral should be considered when designing well-stimulation operations. The present study innovatively determines the effect of chemical composition of dolomite on the dissolution rate of this mineral.

Automated summary

This study aims to determine if the chemical composition of dolomite in hydrocarbon reservoir rocks varies, particularly in terms of calcium content, and to investigate the effect of calcium uptake on the dissolution rate of dolomite in hydrochloric acid. The results show that dolomite reactivity depends on calcium content, and spatial variation in chemical composition should be considered when designing well-stimulation operations. The study innovatively demonstrates the impact of dolomite's chemical composition on its dissolution rate.