The invention provides a method for preparing a fluid loss additive for
drilling fluid. The fluid loss additive is mainly prepared from the following raw materials in part by weight: 30 to 35 parts of
humic acid sodium, 20 to 30 parts of etherified
starch, 10 to 15 parts of
hydroxyethyl cellulose, 5 to 10 parts of
polyacrylamide and 5 to 10 parts of
water soluble silicone oil, preferably 30 parts of
humic acid sodium, 25 parts of etherified
starch, 10 parts of
hydroxyethyl cellulose, 7 parts of
polyacrylamide and 6 parts of
water soluble silicone oil. The preparation method comprises the following steps of: heating the raw materials to 120 DEG C under sealing and stirring conditions and under the pressure of 0.8 to 1.2MPa, keeping the temperature for 8 to 10 hours to obtain a semi-finished product; and
drying the semi-finished product at the temperature of between 85 and 100 DEG C, and adding 5 to 10 parts of sulfonated
asphalt into the semi-finished product to obtain the fluid loss additive, wherein at the step of synthesizing the etherified
starch, 50 to 80 parts of starch are soaked into 50 to 80 parts of 10-percent
sodium hydroxide solution for swelling for 24 to 48 hours, the
mixed solution is dried at the temperature of between 75 and 85 DEG C, the obtained product is ground to obtain the semi-finished product, 30 to 50 parts of chloroactic acid are added into 30 to 50 parts of the semi-finished product, the
mixed solution is sealed and stirred, the pressure is controlled to be 0.8 to 1.2MPa, the PH value is adjusted to 7 to 8, and the
mixed solution is heated to 135 to 145 DEG C and then kept at the constant temperature for 15 to 17 hours to obtain the etherified starch. The fluid loss additive possesses high anti-salt, anti-
calcium and anti-high temperature properties, and has the effects of carrying
drill cuttings and making mud cakes dense.