Different Acid Systems Interactions with Sand and Ceramic Proppants Used in Gravel-Packed and Fractured Wells
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Proppants are solid particles, extensively used in hydraulic fracturing operations. These materials possess specific mechanical strength indispensable in keeping induced fractures open, resulting in up-surged well production. Proppants have different parameters including density, mechanical strength, internal porosity, shape, sieve distribution, and most importantly acid resistance. The acid resistance of the fracturing proppants, defined as the stability and suitability of proppants when they come into contact with different acids, is an important property. Numerous acids are used during the hydraulic fracturing process to remove the scale and clays affecting the fracture conductivity. Inopportunely, these acids adversely affect the proppants already existing in the fracture. The industry measures the acid solubility of proppants according to the API RP 19C/ISO 13503-2 standard. However, it fails to give any guidance on the anticipated final effect of acid dissolution on the mechanical performance of the tested proppants. This study investigates different factors affecting the interactions of different acid systems with sand and ceramic proppants under downhole conditions. Solubility experiments were conducted using translucent and aging cells at temperatures up to 350F. 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. Total key cations concentrations were measured using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). Proppants were then analyzed by X-ray fluorescence (XRF) and X-ray diffraction (XRD) to detect their mineral composition. After performing solubility tests, the residual solids were then dried and analyzed using Scanning Electron Microscopes (SEMs) with Energy Dispersive X-ray Spectroscopy (EDS) capabilities. Moreover, the effects of acid dissolution on the mechanical performance of proppants were also tested using an automated load frame. Experimental results show that monocrystalline sand proppants are soluble in regular mud acid (12.0 wt% HCl, 3.0 wt% HF), with a maximum recorded solubility of 10.0 wt%. Clay-based ceramic proppants are also soluble in mud acid, with much higher acid solubility compared to sand proppants. Proppant pack shows more compaction for clay-based proppants than that of sand proppants prior to and after acid exposure. Bauxitic ceramic proppants have a minimal solubility of 0.5 wt% in 10.0 wt% HCl. The proppant, however, is readily soluble in different mud acid solutions, reaching up to 56.0 wt% dissolution in some extreme cases. The higher solubility of ceramic proppants is attributed to HF attack at the grain boundaries. Understanding the effects of various acids on natural and synthetic proppants will improve production capabilities by promoting the design of acidizing regimens recommended during hydraulic fracturing operations.
