Stress-Corrosion Resistant Proppant for Oil and Gas Wells

The preferred proppants for gas or oil wells, at depths where silica sand is unsatisfactory due to the overburden pressure, are sintered bauxites containing from 3 to 40% of silica. The strongest proppants are made from relatively high purity bauxite. Higher silica content raw materials are used, however, to provide a lower density proppant with a sacrifice in strength and chemical stability as more silica is present in the product. Such proppants have been made from high silica bauxite with a silica content of 15 to 19% and from mixtures of bauxite or diaspore clay with higher silica content. This invention relates to proppants containing 10% or more silica with alumina.

Such proppants are made by pelletization of a mix in an intensive mixer and are taught in Australian Pat. No. 521930 issued Oct. 19, 1982, U.S. co-pending application Ser. No. 06/628,015, filed July 5, 1984, and U.S. Pat. No. 4,427,068, to Kennecott Corporation, or by a spray granulation method, as taught in U.S. Pat. No. 4,440,866.

Such proppants generally contain mullite (3Al.sub.2 O.sub.3 --2SiO.sub.2), alumina, and glass. Although the glass phase can be minimized by employing mixes of lower silica content, some glass phase is always present. Such glasses are undesirable in that they are more subject to corrosive attack by aqueous brines (present in the wells), under the influence of an applied stress, that is, in particular are subject to stress corrosion, leading to a decrease, with time, in the permeability of the proppant pack in the well. This decrease in permeability occurs both in the proppants containing 10 to 19% silica and even more so in the higher silica products and is a result of the stress activated corrosive strength degradation (i.e. failure) of ceramic proppants.

The present invention resides in the discovery that the resistance to corrosion of silica containing aluminous proppants can be significantly inproved by the addition of a small amount of zirconia to the raw mix used to produce the proppant. The zirconia can be in the form of ZrO.sub.2 or may be added as a salt or in the form of zircon to the mix. If added in solid form, it must be finely divided or may be added to the raw materials before milling so that it is reduced in size along with the other mix ingredients. Forming and firing may be conducted in the normal manner, as practiced in the art. For proppants containing 15 to 19% silica, between 1 and 2% of zirconium oxide, based on analysis of the fused product, should be added. More than 2% has been found ineffective, at such silica levels, and less than 0.7% has been found to be much less effective than 1.3% at average 17% silica levels. Increased silica content requires increased amounts of zironia. Thus at 40% silica, about 4% ZrO.sub.2 is required for optimum results. The level of zirconia required is also believed to be increased as the amount of alkali and alkaline earth oxides present in the mix increases. A ratio, by weight, of zirconia to silica, when the silica content is 10% or more, of from 1/20 to 1/6 is effective. These weight ratios refer to the silica content calculated as SiO.sub.2 and to the zirconia content calculated as ZrO.sub.2 even though these compounds are combined, in the case of silica, in the mullite and in the glass, and in the case of zirconia, presumably in the glass.

It has also been found that it is desirable to produce a smooth, flaw free, surface on the proppant spheres. Such surface can be controlled to some extent by control of the pelletizing methods, which may be the mixer pelletization or the spray granulation method.

Table I shows the analysis of five batches of bauxite used to make proppant by the mixer-pelletization technique. In batches 2 through 4, zirconia powder was added to give the indicated analysis for % ZrO.sub.2, except for batches 1 and 5 in which the zirconia was present in the bauxite as mined. The analysis is for the fired product.

Each batch was milled to reduce the particle size to an average of about 4.5 microns. The mixes were pelletized in an intensive mixer with 2% of a starch gum binder. Except for batch 5, extra care was taken in the pelletization to produce a smooth surface on the pellets.

After drying, the materials were fired at 1450.degree. to 1460.degree. C. in a rotary kiln to a density of 3.2 to 3.3 grams/cc.

Each material was then subject to conductivity testing in a test cell at 200.degree. F., in the presence of 2% KCl aqueous solution, and a closure pressure of 8,000 pounds per square inch. The following table shows the resulting conductivities in milli-darcy-feet, and the % retained conductivity at the start of the test and after 66 to 72 hours of exposure to the hot brine.