Abstract: The present invention provides a filtration control agent and a drilling fluid containing the same. The filtration control agent comprises the product obtained from the reaction of water, lignite, aldehyde and sulfonate polymer at 180-220° C. and the units containing sulfonate groups in the molecular chains of sulfonate polymer are at least 30 wt %. The filtration control agent provided by the present invention features a good filtration loss reduction effect and a low viscosity effect and can improve the rheological behavior of the drilling fluid, thereby meeting the requirements of the drilling of deep wells, ultra-deep wells and ultra high pressure formations.

Abstract: The present invention provides a modified starch, preparation method and use of the same, also provides a drilling fluid comprising the modified starch which contains bi-substituted starch structural units and tri-substituted starch structural units, wherein, the tri-substituted starch structural units are represented by the following formula (1), the bi-substituted starch structural units are the structural units represented by the following formula (2) and/or the structural units represented by the following formula (3), and the total content of the bi-substituted starch structural units and tri-substituted starch structural units accounts for 20 wt % or more of the modified starch, preferably 20-30 wt %, the weight-average molecular weight of the etherified starch is 50,000-600,000, preferably 80,000-580,000, wherein, R1, R2, and R3 are C1-C5 alkylene respectively, and M1, M2, and M3 are H, alkali metal element, or alkaline earth metal element respectively.