3M™ Glass Bubbles HGS4K28 are hollow glass spheres designed for use in drilling, completion, and workover fluids, as well as cement slurries in the oil and gas industry. With a nominal density of 0.28 g/cc and an isostatic crush strength of 4,000 psi (pounds per square inch), HGS4K28 are well-suited for use in shallow to intermediate depth wells.