| According to the investigation actuality of the basic theory study on polymer flooding, some problems with rheology in the process of polymer flooding, percolation in porous media and mechanism of oil displacement are studied systematically. High-concentration polymer flooding was studied. The results of the study are significant to enrich the basic theory study of relative fields and accelerate development of these subjects. The research results of the paper can be divided into four special topic, which will be briefly introduced separately as follows: 1. Viscous-elastic behavior of polymer solutions The viscous and viscous-elastic behaviors of partially hydrolyzed polyacrylamide (HPAM) solutions and xanthan gum(XA) solutions were measured by rheometer. The effects of some factors on non-Newtonian property of polymer solutions were analyzed. It is discussed that Maxwell model can be used to describe stress relaxation tests and dynamic mechanical tests, and the molecular theory may also be used to calculate the relaxation time of polymer solutions. Based on steady shear flow experiments it is shown that first normal force difference and apparent viscosity of HPAM solution increase with the increase of shear rate or molecular weight or concentration or the decrease of salinities. Based on dynamic mechanical tests, it is shown that the viscoelasticities of XA solutions also increase with the increase of concentrations as HPAM solutions. No obvious difference exists between the energy-storage moduli and the loss moduli of both types of polymer solutions. Through analyses on the test results, it is concluded that HPAM solutions is a viscous-elastic fluid with relative higher first normal force difference and XA solutions is also a viscoelastic fluid with no first normal force difference or low first normal force difference. 2. Rheological behavior of polymer solutions in porous media The curves that pressure differences vary with flow rates in various conditions are measured when HPAM solutions and XA solutions flow through porous media. The results show that the pressure difference of HPAM solutions increase quickly with the increase of flow rates, which shows the existence of viscoelasticity. However, no such a phenomenon exists for XA solutions. The two kind of fluids behaved different rheological properties. The factors , such as concentrations, molecular weights and salinities, effect on the rheological properties of HPAM solutions. There was a good linear relation between elastic viscosity/shear viscosity and shear rate. Under seepage flow of oil reservoirs, It can occur a elongation flow of HPAM solutions. 3. Machanism of polymer flooding In the paper, the mechanism of viscous-elastic polymer solution increasing oil displacement efficiency is investigated by studying the forms of residual oil in porous media and the flowing behavior of the polymer solutions. Research results of flooding visualization cores show that viscous-elastic fluid flooding can decrease the residual oil saturations of cores with all types of wettabilities. Core results show that the recovery factors of both HAPM and XA flooding increase with higher concentration and higher viscosity. However, when the incremental oil recovery of XA flooding in artificial cores reaches about 8.6% OOIP (original oil in place), the displacement efficiency of XA flooding levels of, but an incremental oil recovery of more than 20% OOIP can be obtained from HPAM flooding with a high concentration, the higher the concentration the higher the recovery, the recovery does not level of. Under the same capillarity number, the higher the elasticity of displacing phase is, the higher the oil displacement efficiency is, and the lower the residual oil saturations are. 4. The pilot test and laboratory test of high-concentration polymer flooding The influence factors such as concentrations, molecular weights and injection rate on the injectivity of high concentration polymer flooding are investigated and some stimulation measures are suggested. The viscous-elastic property of HPAM fluid not only increases the volumetric sweep efficiency but also the displacement efficiency of the pay zone on a micro scale. A novel method has been developed to enhance the oil recovery in highly heterogeneous and high permeable reservoirs. The results on artificial cores with a Dykstra-Parson Coefficient of 0.72 show that an incremental recovery over water flooding of more than 20% OOIP can be obtained by early time injection of the high molecular weight, high concentration polymer solution, the earlier the better. The incremental recovery is comparable to alkali/surfactant/polymer flooding but at a lower cost. In Daqing Oil Field, a pilot test of high concentration polymer injectivity was conducted on 6 injection wells in August 2002. Three wells are injecting in high permeable pay zones and the other 3 wells in low permeable pay zones. The range of effective permeability (Keff) of the pay zones is 0.354 ~1.06 um2. The results show that high permeable formations (Keff>0.890　um2) are suitable for continuous injection of high concentration polymer solutions (concentration 2,000 ppm ~ 2,500 ppm, molecular weight 17 million Daltons, viscosity 200 ~ 250 mPa.S). In September 2003, the average fluid production of each well was 94.2 tons per day, the average daily oil production of each well was 23.7 tans and average water cut was 74.9%. Compared to low concentration (1,000 ppm) solution flooding, the average fluid production per day of each well remained steady, the daily oil production of each well increased 6.2 tons (an increase of 35.4%) and the average water cut decreased by 6.6%. The results show that the injection of high concentration polymer fluid is very effective and the pilot area will be further expanded.
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