Abstract: A glycol ether block copolymer for use in low potassium content aqueous drilling fluids of the formula I R—O—[[—R1O]m[—R11O]n[—R111—O]p]H  I wherein the —R1O and —R11O groups may be in either order attached to the R—O group, and the R111O group, if any is spaced from the RO group by the R11O group and R1O group, and is different from the group R11 or R1O to which it is bonded directly. R is an organic group e.g. a hydrocarbyl group, R1is an ethylene group, each of R11 and R111, which are the same or different is a propylene, butylene ethylene group, m is 1-10 or an average of 1.0-10, while n is 1-10 or an average of 1.0-10, p is O or an average of 0.5-5 and the sum of m and n is at least 3.0.