In fact the change of density quoted requires 40% more by weight of the high density ceramic

In fact the change of density quoted requires 40% more by weight of the high density ceramic to fill the same volume than the low density proppant would require. Proppants are purchased on a weight basis and a sintered bauxite proppant may have a price 20 times that of silica sand or more. When the density factor is taken into account the cost of changing from a silica sand proppant to sintered bauxite proppant may result in a more than 30 fold increase in cost of this component. Many attempts have therefore been made to produce proppants of high strength relative to sand but with a lower density than that of sintered bauxite. In this context European patent application No. 0 087 852 by Dresser Industries, Inc is relevant. This is based on the use of Eufala aluminous ore which after calcination contains 40-60% Al2 O3 and 36.5-51% SiO2. A density of less than 3 is claimed for this product with a minimum permeability at 10,000 psi of 100,000 Millidarcies at ambient temperature.