Hardness Enhancement of Carbonate Rocks by Formation of Smithsonite and Fluorite

Carbonate rock strengthening techniques are widely used in cultural heritage preservations. Recently, there has been a growing interest in their application in the oil and gas sector to improve the hardness and stability of fracture surfaces compared to the predominant host mineral. In this study, two rock lithologies were treated (chalk and limestone) at ambient temperature using 0.9 M NaF and 0.1 M ZnSO4 solutions with the aim of improving the hardness of the rock surfaces by transforming their mineralogy. The carbonate rocks were characterized before and after treatment using various techniques to establish changes in their mineralogical compositions (using x-ray diffraction (XRD), scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM–EDS), and X-ray photoelectron spectroscopy (XPS) techniques) and petrophysical/mechanical properties (using Brinell test, impulse hammer, scratch test, steady-state gas injection). The reaction rates were monitored using inductively coupled plasma optical emission spectrometry and ion chromatography (ICP-OES) over a period of 2–5 days at 4–6 h intervals. It was found that the NaF treatment yielded a 4% and 6% improvement in indentation resistance of limestone and chalk, respectively. The ZnSO4 treatment produced the most notable improvement in rock hardness (presented by Young’s modulus), with a 21% improvement for chalk and a 17% improvement for limestone, respectively. The novel findings in this study suggest that NaF and ZnSO4 are potential carbonate rock strengthening agents that can be utilized in various industrial applications, such as, for example, solving conductivity reduction problems arising in hydraulic fractures due to rock softening.