| With the increasing of complex situations in the process of drilling, Filtration reducing agents have to face the challenges of high temperature, high salinity and environmental protection. The graft copolymer intermedium which used potassium peroxodisulfate and sodium sulfite as the initiator system was prepared by carboxymethyl starch(CMS) and 2-acrylamide-2-methyl propanesulfonic acid (AMPS) and acrylamide (AM). Operation conditions were optimized with the Design-Expert 8.0 software, The optimum reaction conditions were that the degree of substitution of CMS was 0.29, the molar ration of AM:AMPS was 3:1, the mass ratio of monomer and CMS was 0.38:1, reaction temperature 65℃, reaction time 2h, pH value 7.0. The modified carboxymethyl starch as filtration control agent was formulated with the graft copolymer and film forming agent at the mass ratio of 1:2.Laboratory experiments were done on the evaluated of the new type of filtration control agent, which showed that it has very good abilities of depressing the filter loss at both fresh mud and salty mud. With the additive of 1% filtration control agent, the API filter loss rate of pre and post aging could be controlled at 81.3% and 82.1% at the fresh mud. At salty mud, saturated salty mud and sea water mud, the API filter loss rate of pre and post aging could be controlled at 90.5%, 91.9%, 90.8%, 90%, 93.3%, 93%. The agent also has very good abilities of saturated brine, 2.5% calcium and 150℃temperature-resistant. Compared with filtration control agent OCL and CMS, and the ratio of performance and cost is relatively high. The environmental protection of experiment results showed that the new agent was degradable and will not pollute the environment.
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