to provide an additive for drilling mud , used in drilling for petroleum , natural gas , civil engineering , mines , etc ., which has high heat resistance and high mechanical shear resistance , of which the viscous property does not degrade even under high - temperature and high - shear conditions ; has high pseudoplastic flowability and therefore secures high flowability during discharging ; and is a naturally - derived one and is therefore highly biodegradable and has least adverse impact on environment . an additive for drilling mud , which contains cellulose fibers having a number - average fiber diameter of from 2 to 500 nm and a fiber aspect ratio of 50 or more and having a cellulose i - type crystal structure .