A well stimulation and or treatment fluid that includes water, other additives, and a biocide consisting of 2,5-dimethyl-1,3,5-thiadiazinane-2-thione in an amount effective to inhibit bacterial growth and minimize antagonistic reactions between the biocide and other additives. Also disclosed are well injection compositions, stimulations, squeezing, waterflood, packing, cement compositions, and methods for cementing.