Synthesis And Solution Rheology Of Crosslinked Xanthan Gum

The onshore oilfields of our country are all at middle and late stages of water flooding, and enhanced oil recovery (EOR) technologies are used to increase oil-displacing efficiency. Polymer flooding is one of the extensively used EOR technologies. The partially hydrolyzed polyacrylamide (HPAM) is currently a widely used oil-displacing reagent. Its solution possesses a high viscosity and good oil-displacing ability but poor salt and shear resistances, not suitable for application in oil reservoirs with high temperature and salinity. Therefore, developing polymer flooding systems with strong salt and temperature resistances to solve the puzzles met in polymer flooding of oil reservoirs with high temperature and salinity has become one of the key research points in the oilfield chemistry. The double helix and branched structure of xanthan gum (XG) molecule endow itself with good salt and shear resistances, but its poor temperature tolerance limits its large-scale application. In this dissertation, crosslinking modification of the XG is studied to enhance its temperature resistance and provide evidences for its application in EOR of oil reservoirs with high temperature and salinity.Main contents and conclusions of this dissertation are listed as follows:(1) A water-soluble weakly crosslinked XG (SP-c-XG) was synthesized using sodium trimetaphosphate (STMP) as a crosslinker. The synthesis conditions were optimized based on the viscosity of the SP-c-XG solutions. We investigated the effects of electrolyte (NaCl and CaCl2) concentration (0-5 wt%), pH (2-11), and temperature (20～70℃) on the rheological properties of the SP-c-XG solutions which were compared with those of XG solutions. Results show that, under the studied conditions, the rheological curves of both SP-c-XG and XG solutions are pseudoplastic and can be described by a Herschel-Bulkley model. The yield stress, apparent viscosity and dynamic moduli of the SP-c-XG and XG solutions decrease then increased with increasing electrolyte concentration, but increase then decrease with increasing pH. Increasing temperature resultes in a decrease of these rheological parameter values for both SP-c-XG and XG solutions. The SP-c-XG and XG solutions exhibite similar rheological properties, but the former exhibites higher yield stress and apparent viscosity, and especially stronger elasticity and temperature resistance, indicating a good promise for application in EOR area.(2) Water-soluble weakly crosslinked XG and purified XG (XGm) were prepared using glutaraldehyde (GA), phenol-formaldehyde resin (PR) and HX as crosslinkers, and simply marked as GA-c-XG (XGm), PR-c-XG (XGm) and HX-c-XG (XGm). Their solutions have enhanced temperature resistance, and especially the viscosity of the HX-c-XG solution kept constant for 140 days at 90℃, indicating a very strong temperature resistance. Oil displacement results show that in brine with salinity of 100000 ppm, the oil-displacing capability of the HX-c-XGm system is obviously better than that of the HPAM solution, revealing a good application prospect in EOR area.