THE ROLE OF CARBONATE MINERALS IN HYDRAULICALLY FRACTURED WELL SCALING REACTIONS

Shale-Hydraulic Fracturing Fluid (HFF) interactions can cause many changes in hydraulically fractured wells, including compromised porosity and permeability. Mineral growth in microfractures and pore throats may diminish the ability of gas to flow through the formation and may account for lower late-stage production compared to the modeled values. Carbonates, dominantly calcite and dolomite minerals in shale have been shown to have a strong effect on the shale-HFF interface as they buffer the system’s pH via dissolution. The dissolution of carbonates can increase Ba concentration in the fluid phase by direct dissolution of Ba present in calcite and by Ca-Ba cation exchange with Ba-hosting clays. The increase in Ba in the fluid phase may further promote barite precipitation in subsurface environments. Carbonate buffering can also promote pyrite dissolution and precipitation of iron oxides, which are more favorable under more basic conditions. The impact of carbonate content on mineral precipitation reactions in hydraulically fractured wells is not fully understood because additional factors such as the clay content, the adsorption of ions in organic matter, and the formation of organo-metallic complexes can also contribute to mineral scaling at the shale-HFF interface. In this study, we investigated the role of carbonate minerals on mineral precipitation reactions by changing the carbonate content of shale while keeping all other factors such as its mineralogy, HFF composition, and all other experimental conditions identical. The Scanning Electron Microscopy with Electron Dispersive Spectrometry (SEM-EDS) results indicate differences in elemental distribution on shale’s surface, and the morphology of pyrite crystals between carbonate and carbonate free shale samples. The SEM-EDS results will be used in conjunction with fluid chemistry data to understand how carbonate minerals affect the dissolution of pyrite and scaling of barite and iron oxides in the two types of shale samples. The results from the study will help operators to design better HFF’s that minimize the impact of mineral precipitation on gas productivity.