EXAMPLE 13
A protected particle was formed of a base particle having an average diameter of 200 μm. The base particle is a hollow sphere having a shell thickness of 48 μm. The base particle was formed of degradable sodium silicate. The interior pressure in the hollow cavity of the base particle is 14.7 psi. The density of the base particle is 2.06 g/cc. The outer surface of the base particle was coated with a polylactic acid (PLA) by fluid bed spray coating. The coating thickness was 0.5 μm. The protected particle had a crush strength of over 12,000 psi. The protected particles were added to a proppant slurry and constituted about 0.5 wt. % of the proppant slurry. The proppant slurry with the protected particles was pumped into the fracturing zones of a well. The outer coating of PLA degraded by hydrolysis in the well and the rate of degradation only increased to an appreciable rate once the PLA encounters the higher temperatures within the deep well's fractures (around 60-100° C.), and thereafter degraded within 24 hours which exposed the readily degradable sodium silicate hollow spheres to the well pressure, thus being hydrostatically crushed.
