Polysulfonate drilling fluid is a kind of high temperature resistant drilling fluid system formed by combining polymer drilling fluid and sulfonated drilling fluid in drilling practice. Although polymer drilling fluid has outstanding advantages in improving ROP, inhibiting formation mud making and improving wellbore stability, on the whole, its thermal stability and the quality of mud cake are not suitable for use in deep wells with high well temperature. Especially for hard and brittle shale formation, it is often necessary to add some sulfonated treatment agents to improve the quality of mud cake in order to reduce the HTHP filtration of drilling fluid. Therefore, it is natural to gradually combine the two systems. Polysulfonate drilling fluid not only retains the advantages of polymer drilling fluid, but also improves its mud cake quality and rheology under high temperature and high pressure, which is conducive to the improvement of deep well drilling rate and the stability of borehole wall. The temperature resistance of this kind of drilling fluid can reach 200-250 ℃, and the salt resistance can reach saturation. Since the 1980s, this system has been widely used in deep well drilling in various oilfields.

Proper bentonite content is the key to maintain good performance of polysulfonate drilling fluid, which must be strictly controlled. If the quality of mud cake becomes worse and HTHP filtration increases, the dosage of smp-1, SMC and sulfonated asphalt should be increased in time; If the rheological properties do not meet the requirements, the proportion of polymers with different relative molecular weight and the content of bentonite can be adjusted; If the inhibition is poor, the dosage of polymer coating agent can be appropriately increased or an appropriate amount of KCl can be added.

The main treatment agents used in polysulfonate drilling fluid can be roughly divided into two categories: one is inhibitors, including various polymer treatment agents and inorganic salts such as KCl, which are mainly used to inhibit formation slurry making, which is conducive to formation stability; The other is dispersant, including various sulfonated, lignite treatment agents, cellulose and starch treatment agents, which is mainly to reduce filtration loss and improve rheology, which is conducive to the stability of drilling fluid performance. In different well sections of deep wells, due to different well temperature and formation characteristics, the use of the two types of treatment agents should be different. The upper formation is mainly to enhance the inhibition and improve the drilling rate, while the lower formation should be mainly to resist high temperature and reduce filtration.

At present, Chinese drilling fluid scientists and technicians have accumulated rich experience in the field application of polysulfonate drilling fluid. They usually referred to the above two types of treatment agents as “poly” and “sulfonic” respectively, and proposed “more poly and less sulfonic” or “only poly but no sulfonic” in the upper formation of deep wells; For the implementation principle of “less sulfur and more accumulation” or “only sulfur and no accumulation” in the lower formation, the dividing point is roughly 3000-4000m deep. According to this principle, polysulfonate drilling fluid has been widely used in many oilfields in China.