The invention relates to a water-based drilling fluid composition
comprising a carboxymethyl cellulose (CMC), wherein the CMC is
characterized by forming a gel at 25.degree. C. after high-shear
dissolution in a 0.3 wt % aqueous sodium chloride solution, the final
content of the CMC in the aqueous sodium chloride solution being I wt %
for a CMC having a degree of polymerization of >4,000, 1.5. wt % for a
CMC having a DP of >3,000-4,000, 2 wt % for a CMC having a DD of
1,500-3,000, and 4 wt % for a CMC having a DP of <1,500, the gel being
a fluid having a storage modulus (G') which exceeds the loss modulus
(G'') over the entire frequency region of 0.01-10 Hz when measured on an
oscillatory rheometer operating at a strain of 0.2.