US Patent 7939469 Use of CMC in drilling fluids

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° 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.