The invention relates to a granular propping for use in petroleum extraction which has a high mechanical resistance and a density of between 3150 and 3250 kg/m.sup.3, the propping agent consisting of balls formed by melting, granulating and solidifying an initial composition comprising by weight, on the basis of the oxides, 47.1 to 48.7 parts of ZrO.sub.2, 52.9 to 51.3 parts of SiO.sub.2, the total of ZrO.sub.2 +SiO.sub.2 amounting to 100 parts, 8.2 to 16.9 parts of Al.sub.2 O.sub.3 and 4.1 to 8.5 parts of at least one oxide selected from the group formed by MgO, CaO, FeO and TiO.sub.2.

Hollow, fine-grained ceramic proppants are less expensive and improve fracture control when compared to conventional proppants (dense alumina, mullite, bauxite, zirconia, etc.). Hollow proppants of the present invention have been fabricated by spray drying, followed by sintering in order to obtain a dense case and a hollow core. These proppants generally have high sphericity and roundness (Krumbein sphericity and roundness greater than 0.8), have diameters on average between 2250 and 125 .mu.m, depending on proppant size required, and have strength equal to or greater than that of sand. The hollow core, the size of which can be controlled, permits better fracture control in hydraulic fracturing treatments since the proppant can be transported in lower viscosity fluids. Hollow proppants produced at the same cost/weight as conventional proppants also provide for lower costs, since less weight is required to fill the same volume. The fine-grained (preferably less than 5 .mu.m in diameter) ceramic case provides the strength necessary to withstand closure stresses and prevent crushing.

A composite, sintered, spherical pellet and a method for its manufacture are described. The pellet comprises clay and a member of the group of bauxite, alumina, or mixtures thereof; the pellet being prepared from at least one uncalcined or partially calcined ingredient. The pellet may have an alumina-to-silica ratio from about 9:1 to about 1:1. The pellet has a specific gravity of less than 3.40. Use of such pellets in propping hydraulically fractured subterranean formations is also described.

Sintered, spherical composite pellets or particles comprising one or more clays as a major component and bauxite, alumina, or mixtures thereof, are described, along with the process for their manufacture. The pellets may have an alumina-silica (Al.sub.2 O.sub.3 -SiO.sub.2) ratio from about 9:1 to about 1:1 by weight. The use of such pellets in hydraulic fracturing of subterranean formations is also described.

A two-phase ceramic particulate useful as a proppant in hydraulic fracturing operations is provided which comprises a vitreous matrix phase containing a crystalline alumina phase. The new ceramic particles are characterized by: (i) a concentration gradient of alumina which increases from very low in the center to relatively high near the surface of the particles; (ii) a fired density less than about 2.9 g/cc; and (iii) closed cell microporosity. Certain embodiments of the inventive ceramic particulate are also characterized by a Krumbein roundness of at least 0.8 and are chemically stable. The ceramic is made by a process comprising the steps of: a. mixing and pelletizing, with the aid of water, the dry raw materials including a mineral particulate such as nepheline syenite and a binder such as bentonite; b. drying the wet pelletizer product; c. mixing the dried pellets with a parting agent; and d. firing the mixture of pellets and parting agent at sufficient temperature and for sufficient time to cause vitrification to occur.