Research On Viscosity Reducer Of Ultra-high Temperature And Ultra-high Density Water-based Drilling Fluid

With the exploration and development of oil and gas resources towards deeper and deeper strata,downhole high temperature and high pressure are becoming more and more common.This requires the drilling fluid to deal with the formation pressure with a higher density.The higher the density of the drilling fluid,the higher its viscosity is generally.In addition,because the downhole high temperature may lead to the thickening of the drilling fluid,it is necessary to add viscosity reducer to the drilling fluid to improve the rheological properties of the drilling fluid.This paper aims to develop a polymer viscosity reducer with good viscosity reduction effect in ultra-high temperature and ultra-high-density water-based drilling fluid system.Through molecular structure design,PAAT is synthesized by aqueous solution polymerization with acrylic acid（AA）,2-acrylamide-2-methylpropanesulfonic acid（AMPS）and tannic acid（TA）as monomers and ceric ammonium nitrate as initiator.PAAT has good water solubility and can be used directly in solution.The optimum synthesis conditions are determined by orthogonal design of monomer ratio,initiator and reaction temperature.Finally,the optimum reaction conditions are as follows:molar ratio n_AA:n_AMPS=19:18,mass ratio m_AA/AMPS:m_TA=9:1,reaction temperature 80℃,initiator dosage 1%and reaction time 8h.The structure of PAAT is characterized by FT-IR and ¹H-NMR,and the synthesized polymer is determined as the target product.The molecular weight of PAAT is determined to be 11396 by molecular weight test.The pyrolysis temperature of the target product is determined to be 496.9℃by thermogravimetric analysis,which proves that it has good temperature resistance.Through the evaluation of PAAT in base mud and water-based drilling fluid system,the results show that the viscosity reduction rate of PAAT in high concentration base mud can reach 66%and the temperature resistance can reach 240℃;In the water-based drilling fluid system with density of 2.2 g/m L and aging temperature of240℃,the viscosity reduction rate can reach 44%,which is superior to similar foreign products and domestic viscosity reducers XY-27 and SMS-19 in viscosity reduction and temperature resistance.Zeta potential results show that PAAT can make the dispersion system more stable and effective;The results of particle size distribution show that PAAT can not only make the particle size distribution of clay more uniform and fine,but also make barite more dispersed to a certain extent;The results of transmission electron microscope show that PAAT has a very significant dispersion effect on clay,which is significantly better than similar foreign viscosity reducing agents;In addition,it is also analyzed that tannins can form chelates with Al³⁺on the clay surface through coordination bonds,resulting in more stable adsorption.Finally,based on the viscosity reducer PAAT,a set of ultra-high temperature and ultra-high-density water-based drilling fluid system is formed.When the system is weighted to 2.45 g/m L,it still has good rheology,high-temperature and high-pressure filtration（<15m L）,and the temperature resistance can reach 240℃.