soda 20 g of acrylic acid 15 g of methacrylic acid and 35 g of acrylamide 25 g of polyethylene glycol monomethyl ether maleate the molecular weight of which is about 500 was weighed placed into a reactor and completely liquefied at 90 u00b0 c followed by addition of 0 15 g of alkyl benzene sulfonic acid and dropwise addition of 0 5 ml of acrylic acid 120 ml of 7 white oil 10 ml of op 10 8 ml of tween80 and the aqueous solution i were successively added into the reactor and 9 ml of the initiator solution ii was dropwise added into the reactor a comb polymer salt iii was finally obtained the salts of the polymers in examples 1 to 3 were tested by the infrared spectrometer and the nuclear magnetic resonance spectrometer and the data thereof showed that the molar ratio among the three structure units was substantially the same as the molar ratio among the corresponding reactants added in the reactions 25 g of bentonite for slurry formulation was weighed and dissolved into 500 ml of water the resulting mixture was stirred for 20 min and stood for 24 h to obtain a bentonite slurry the apparent viscosity and the api filter loss of the bentonite slurry were tested to be 18 mpa u00b7 s and 24 ml respectively 2 5 g of the comb polymer salt i was weighed and added into the bentonite slurry obtained in comparative example 1 the resulting mixture was stirred for 20 min to obtain a mud slurry the apparent viscosity of the mud slurry was tested to be 37 mpa u00b7 s.