Mud-Acid Interactions With Sand and Clay-Based Ceramic Proppants Used in Gravel-Packed and Fractured Wells

Abstract

Summary Proppants are solid particles with specific mechanical strength that are widely used in hydraulic-fracturing operations. Their main purpose is to keep fractures open and increase well production. They can be naturally occurring sand grains or synthetic ceramic proppants. The acid resistance of fracturing proppants is an important property because acids are used during the hydraulic-fracturing process to remove the scale and clays that affect fracture conductivity. These acids affect proppants that are already present in the fracture, as well. Industry measures acid solubility of proppants according to the API RP 19C (2008)/ISO 13503-2 (2006) standard. This measurement produces a solubility number, but gives no guidance on the expected final effect of acid dissolution on the mechanical performance of tested proppants or on how acid-solubility values vary as a function of time, temperature, and dynamic conditions. This study investigates factors affecting the interactions of regular mud acid [hydrofluoric acid (HF)/hydrochloric acid (HCl) = 3:12] with sand and clay-based proppants under downhole conditions. Experiments were conducted by use of an aging cell at temperatures up to 300°F. The effects of varying temperatures, soaking times, and 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 aluminum, iron, silicon, titanium, and calcium concentrations were measured by 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). After the solubility tests, the proppants and the residual solids were dried and analyzed by use of scanning-electron microscopes (SEMs) with energy dispersive X-ray spectroscopy (EDS) capabilities. Effects of acid dissolution on mechanical performance of the proppants were also tested through use of an automated load frame. The results show that sand proppants are readily soluble in regular mud acid, with a maximum recorded solubility of 10 wt%. The amount dissolved increases with temperature, soaking time, and dynamic conditions. Clay-based proppants are also soluble in mud acid, with much higher acid solubility than that seen in sand proppants. The proppant packs show more compaction for clay-based proppants than for sand proppants before and after acid exposure. Understanding the effects of acid on natural and synthetic proppants will improve production by promoting the design of acidizing regimens used during hydraulic-fracturing operations.