During use on the hydraulic fracturing site, proppants experience repetitive mechanical handling starting at unloading from railcars and trucks to the point of final blending with the fracturing fluid. At each point of transfer, quantities of fine dust particles develop which can affect worker safety. Due to the increase of proppant consumption per site, the industry has a concern about a corresponding increase in the time-weighted average exposure of workers to high silica dust. Recently approved changes to OSHA’s crystalline silica rule require the permissible exposure limit (PEL) for respirable crystalline silica to be lower than 50 micrograms per cubic meter of air, averaged over an 8-hour shift.
This study first investigates dust generation associated with pneumatic transfer of northern white sand to sand storage units on location. A laboratory apparatus is designed to mimic location conditions and allow for continuous exposure of sand particles to 15 psi of air jet pressure. During this attrition process, generated dust is collected using a dust collection system. Weight percent and particle size of collected particles is measured. Average weight percent of collected sand is 1.6%, which, on a 10 000 000 lb job translates to a loss of 160,000 lbs of material. Particle size analysis of collected dust shows that 30% of it is below 10 microns and is by OSHA considered respirable. As attrition related changes in surface morphology affect strength performance, morphology of attrited sand grains is also evaluated via crush testing per ISO 13503-2. At 4000 psi, attrited samples measure 6 weight % higher crush than non-attrited samples.
The study also measures the effect of attrition on dust generation and mechanical stability of intermediate strength ceramic proppants. Following attrition testing, intermediate strength proppants show a 0.15% mass loss. XRD analysis of collected dust shows that none of the particles contain crystalline silica and are therefore not subjected to OSHA regulation. Mechanical performance of intermediate strength proppants, at 8,000 psi, is 1% higher on samples that were subjected to attrition than on non-attrited samples
This study further discusses advantages and limitations of designed attrition methods compared to field testing and evaluates sand dust mitigation costs.
Authors: Ian Kidd (Saint-Gobain Proppants), Tihana Fuss-Dezelic (Saint-Gobain Proppants), Jingyu Shi (Saint-Gobain Proppants), Craig Wilcox (Saint-Gobain Proppants), Raphael Herskovits (Saint-Gobain Proppants), Todd Kaul (Saint-Gobain Proppants)
Paper Number: SPE-184049-MS