Abstract: A water-based drilling fluid for use in shale formations having a given average pore size has solutes dissolved therein sized so as to allow the pores of the shale formations to at least partially restrict the solutes. For purposes of enhancing the restriction of the solutes by the shale pores, the dissolved solutes may have different radii. Additional solutes, preferably lower molecular weight salts, are dissolved in the drilling fluid to lower the activity of the water in the water-based drilling fluid.The same combination of additives, a first additive of dissolved molecular solutes sized to at least partially block a shale pore space and a second additive of dissolved molecular solutes selected to lower the water activity, are combined in a water-based carrier fluid and used in earth boreholes surrounded by shale formations as either a spotting fluid, or as a lubricant, or as a shale formation strengthener or as a completion fluid.