Aq. polysaccharide compsn. used in well drilling - contains formate salt to improve thermal stability

摘要

An aq. polysaccharide compsn. (I) comprises (A) a water-soluble polysaccharide, 0.03-5% wt./vol. of (I), and (B) salt(s) of mono- or di-valent cations, 5-120% wt./vol. of (I); at least one salt (B) is formate, forming at least 0.05% wt./vol. of (I); the balance of (B), if any, is halide(s). Pref. (A) is a cellulose deriv., e.g. hydroxyethyl cellulose, or a microbial polysaccharide, esp. Xanthan; it is esp. 0.25-0.75% wt./vol. of (I). (B) is at least 9%; the formate is at least 2%, esp. at least 10% wt./vol. of (I). Halide if present is chloride or bromide. The cation is ammonium, Ba, Cs, Pb, Li, Mg, Mn, Rb, Ag, Sr, T1 or esp. Ca, K, Na or Zn. Incidental quantities of other salts, e.g. sulphates, nitrates and bicarbonates, may be present, e.g. when using brine to make up the compsn. pH is controlled to at least 9, e.g. with sodium hydroxide or carbonate. An antioxidant, e.g. 2-mercaptobenzothiazole, may be present.