Interactions between different acids and bauxitic-based ceramic proppants used in gravel-packed and fractured wells
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2017 Elsevier B.V. Proppants are widely used in hydraulic fracturing operations. They consist of solid particles with specific strengths and are used to keep the fractures open to enhance well production. They can be natural sand or artificial ceramic materials. Most ceramic proppants are alumina-based, using kaolin and bauxitic clays as the raw materials. The acid resistance of the fracturing proppant is an essential property. Acids are sometimes needed to clean the fracture by removing scale and clays that reduce the fracture conductivity. In this study, several experiments were conducted using a see-through cell, and an aging cell with 10 wt% HCl, mud acids (6.0 wt% HCl, 1.0 wt% HF and 12.0 wt% HCl, 3.0 wt% HF), i.e., (6:1 and 12:3), and formic (HCOOH), trisodium salt of N-(hydroxyethyl)-ethylenediaminetriacetic acid (Na3-HEDTA) and HF mixtures up to 350 F. The effects of varying acid system, temperature, soaking time, static and dynamic conditions were examined. The supernatant of solubility tests was analyzed with Fluorine Nuclear Magnetic Resonance (19F NMR) to identify the reaction products. Key elements concentrations were measured using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). A Zeiss Axiophot microscope was used to acquire images for the proppant particles to study particle shape and effect of acid solubility. Proppants were then analyzed by X-ray fluorescence (XRF) and X-ray diffraction (XRD). The proppant was sieved before and after the experiments. The residual solids were dried and analyzed using a scanning electron microscope (SEM) with Energy Dispersive X-ray Spectroscopy (EDS) capabilities. Effects of acid dissolution on mechanical performance of the proppants were also tested using an automated load frame. The results show that bauxite has a minimal solubility of 0.5 wt% in 10.0 wt% HCl. The proppant is readily soluble in different mud acids, 56.0 wt% dissolved in some cases. The amount dissolved increases with temperature, soaking time, acid concentration, and dynamic conditions. The fines generated and the changes in grain size distribution are detrimental to the proppant conductivity. 19F NMR showed aluminum fluoride complexes with different mud acids due to the high Al/F affinity. XRF showed that the solubility of the proppant occurs mainly in mullite and maghemite due to their high reactivity. Crush resistance test shows a direct relationship between solubility and mechanical strength of bauxite. Assessing solubility effects will help to better design acid treatments without jeopardizing the integrity of the proppant.
