A water-base drilling fluid containing a high concentration of methylglucoside. The mud has been used to drill water-sensitive shales with less hole enlargement and fewer drilling problems. Methylglucoside is a polysaccharide containing methyl groups on the ring-like sugar structure. Being a rather large, nonionic molecule (resembling starch but highly water soluble), it ties up water molecules in concentrated solutions and it is thought to act as a low-efficiency osmotic membrane.