Cationic polymer mud solves gumbo problems in North Sea

This paper reports on a recently developed cationic polymer mud, compatible with conventional polymer additives and designed to meet environmental regulations, which significantly minimized the gumbo problems common to the water-sensitive shales in the North Sea. The cationic polymer mud was used to drill highly reactive Tertiary shale formations which have caused severe gumbo problems on nearby wells drilled with other inhibitive water-based muds. Although many citonic polymers are toxic, aquatic toxicity tests performed by the Norwegian Statens Forurensningstilsyn (SFT) at the end of the test wells showed results far exceeding the SFT limits on the three species tested. The mud system on these wells was a seawater-based 15.0-ppg mud enhanced with 3% NaCl. A low molecular weight quaternary polyamine and a high molecular weight cationic polyacrylamide were used to suppress the swelling and dispersion of shales, respectively. Starch and polyanionic cellulose (PAC) polymers maintained fluid-loss control, and a lubricant reduced the torque and drag