Method for stabilizing boreholes

In a method of drilling a well through subterranean formations including at least one shale zone and at least one sandstone zone, the improvement comprising utilizing a water-based drilling fluid containing a stabilizing composition comprised of (1) one or more water-soluble high molecular weight polyfunctional polymers capable of forming several nucleophilic sites when treated with base and (2) one or more potassium salts whose anions are Bronsted-Lowry bases of sufficient strength to raise the pH of the stabilizing fluid to about 9.0 but which are relatively weak nucleophiles compared to the nucleophilic sites of the polymers, said polymers and said salts being present in an amount effective to stabilize said shale zone, said drilling fluid also containing particulate material having a size range of from 75 to 1,400 microns in a amount of from 30 to 50 pounds of particulate material per barrel of drilling fluid whereby said particulate material in combination with said stabilizing composition stabilize said sandstone zone.