Drilling starch composition

According to the present invention the dispersant of drilling starch composition comprises dialdehyde starch and an inorganic chromate which is effective in dispersing clayey solids in an aqueous drilling fluid, and reducing the viscosity and gel strength of the drilling fluid containing same. The drilling starch composition contains from 60 to 90 percent of dialdehyde starch and from 40 to 10 percent of the inorganic chromate, on a weight basis. A particularly pre ferred composition contains from 70 to 85 percent of the dialdehyde starch and from 30 to 15 percent of the inor ganic chromate, on a weight basis, as this composition produces the most effective results in terms of viscosity and gel strength reduction

The drilling starch composition of the drilling fluid dispersant composition of the present invention is a per iodate oxidized dialdehyde starch preferably prepared by the electrolytic oxidation method.The drilling starches that are 50% or more oxidized, i.e. the number of original anhydroglucose units now in the dialdehyde form, per 100 units of the starch molecule, is 50% or more, have been found to be satis factory dialdehyde starches for use in the dispersant com positions of the present invention. It has been found that