Rheology And Oil Displacement Capacity Of Anionic Branched Polysaccharides And Their Complex Systems

At present, most of oilfields in our country have come into the middle or later stage and the crude oil yield decreases substantially, even though two third of crude oil reserves are still left in the formation. Thus, lots of technologies have been adopted to enhance the oil recovery and keep the oil yield constant in all the oil fields. Among them, polymer flooding with partially hydrolyzed polyacrylamide (PHPA) being the main component is one of the most widely applied technologies. However, some problems such as the abrupt increase of water cut and decrease of oil content of produced liquid arise recently after the PHPA flooding has been utilized. How to enhance the oil recovery is what needs to solve as soon as possible. In addition, all the oilfields of our country possess some oil formation with high oil content and salinity (greater than20000ppm or20g/L) which are still hardly exploited due to the weak salt resistance (less than10000ppm in general) of widely used PHPA system and lack of efficient methods or technologies for the enhanced oil recovery. How to exploit this kind of oil formations is also becoming to be a significant research issue in the field of oil exploitation.The development of new oil displacement systems is an important approach to further enhance the oil recovery after PHPA flooding and to exploit oil formation with high salinity. PHPA is an anionic flexible linear macromolecule with weak salt and shear resistance while some polysaccharides with branched structures possess rigid backbones and therefore are expected to show strong salt and shear resistance, having potential application value in the forced flooding field. In view of these thoughts, the rheological properties of three types of polysaccharides and/or their complex solutions were studied in this dissertation. The referred polysaccharides include an exopolysaccharide (SM-A87EPS) secreted by a deep-sea mesophilic bacterium, xanthan gum (XG) and a cationic cellulose, JR400. The influences of experimental conditions such as pH, electrolytes and temperature on the rheological properties of each system were investigated, after which their oil displacement capacity were evaluated via the lab stimulation experiments. These would enhance our scientific knowledge about rheology of polymer solutions and provide the scientific basis for the development of new polymer flooding systems.The main research contents and conclusions are listed as follows:(1) The rheology of SM-A87EPS solution at various concentrations, pH, electrolyte concentrations, temperatures and shear rate were investigated and their corresponding mechanisms were discussed. It is found that the SM-A87EPS solution, as a new potential polymer flooding system, shows stronger salt and shear resistance, better pH stability and more obvious viscoelasticity than the PHPA solution, which meets the demand of forced flooding systems. New methods were proposed to determine the critical concentrations (overlapping and crossover concentrations) of polymer solutions. Besides, the power law relationship between critical Reynolds number and concentration of SM-A87EPS solution was confirmed and the truth that Reynolds number and critical Reynolds number of polymer fluid only depend on the fluid viscosity was proved.(2) Rheological properties of SM-A87EPS/PHPA complex solution were studied and emphasis was placed on the influences of composition ratio, total concentration, pH, electrolyte and temperature. Further, the interacting mechanism between SM-A87EPS and PHPA was discussed. It is found that the mixing of SM-A87EPS and PHPA causes the decrease of viscosity of solution. The higher the total concentration and temperature get, the more obvious the viscosity decrease is. With the variation of electrolyte concentration and pH of solutions, the viscosity could be enhanced after the mixing but the viscosity increase is not significant.(3) Stability of XG/JR400complex solution was studied. Two stable regions and one flocculation region are shown with the variation of total concentration and composition ratio. In the stable regions, rheological properties of XG/JR400mixture were investigated at different composition ratio, pH, shear rate and electrolyte concentrations. After the mixing of XG and JR400solutions, the viscosity of solution is increased obviously, but with increasing shear rate or electrolyte concentration, the viscosity increase turns weak. The salt, shear and temperature resistance of XG/JR400mixture show no enhancement comparing with that of XG solution at the same concentration, which indicates no potential application value of XG/JR400mixture in the forced flooding field.(4) The oil displacement capacity of SM-A87EPS solution and hydrogen-bonded SM-A87EPS/PHPA mixture were evaluated via the lab stimulation flooding experiment. It is found that the oil displacement capacity of SM-A87EPS solution is obviously stronger than that of PHPA solution in the high salinity water. The SM-A87EPS solution is expected to be applied as a new polymer flooding system in the oil formations with high salinity or after the flooding of PHPA solutions.