The Development of a Modified Starch through Inverse Phase Emulsion Copolymerization and its Performance as a Filter Loss Reducer

To overcome the gelling problem encountered in synthesizing modified starch filter loss reducer through aqueous free-radical polymerization, inverse emulsion polymerization was used to graph copolymerize starch with AM, AMPS and NVP whose molecules contain amide group, sulfonic acid group and benzene ring, respectively. The product of the copolymerization is a new filter loss reducer ESt-g-NAA with high temperature and salt and calcium resistance. The optimum synthetic conditions of the ESt-g-NAA were determined through orthogonal test: reaction temperature 60 ℃, molar ratio of the monomers n_NVP∶n_AM∶n_AMPS= 1∶2 ∶3, and the HLB value = 5. A 3% bentonite base mud treated with 3% ESt-g-NAA had filter loss between 7.6-15.2 mL after being aged at 100-180 ℃ for 16 h, the high temperature filter loss of the ESt-g-NAA treated mud was always less than the high temperature filter loss of the WSt-g-NAA (an aqueous polymerization product) treated mud with the concentrations of ESt-g-NAA and WSt-g-NAA in the base mud being the same. The ESt-g-NAA in the experiments showed excellent salt and calcium resistant capacities; a saturated salt water drilling fluid treated with ESt-g-NAA had filter loss of only 5 mL after being aged at 150 ℃ for 16 h. A drilling fluid containing 0.5%-20% CaCl₂ had filter loss between 5.0-15.5 mL after being aged at 150 ℃. If aged at 170 ℃, the drilling fluid was able to resist the contamination by 10% CaCl₂; aging at 180 ℃ greatly impaired the calcium resistance of the drilling fluid. The filtration control mechanisms of ESt-g-NAA were revealed by analyzing the effects of ESt-g-NAA on the size distribution of the clay particles in the base mud and by observing the mud cakes under microscope.