| The main developed oil reservoirs in Dagang Oil-field belong to medium-low temperature,medium-high permeability and high-heterogeneity reservoirs.After more than50 years of development,it has entered the development period of extra-high water cut.It is urgent to adopt greatly enhanced oil recovery technology.After years of scientific and technical personnel scientific and technical personnel collaborative research,the chemical flooding technology（polymer flooding）in Dagang Oil-field has achieved obvious EOR effect,and has become one of the main technologies for the continuous and stable production of crude oil in the oilfield.With the expansion of the industrial application scale of polymer flooding,the increase of the types of applied oil fields and the new requirements of green and environmental objectives,it is urgent to clarify and solve the national and social concerns about the ecology and development environment of polymer flooding.（1）There is a risk of accidental leakage of polymer solution during surface preparation and injection.It is necessary to clarify whether the polymer will pollute the water source and soil in the mining area,and whether it will affect biological life activities;（2）It is necessary to solve the problems of wellbore blockage and reservoir damage caused by polymer derivatives formed by the interaction of polymer with microorganisms in water,metal ions or rock mineral components;（3）It is necessary to relieve the problems of seepage resistance increasing,liquid absorption index decreasing and injection profile reversal caused by the retention of polymer in the low-permeability layer.Therefore,theoretically guided by polymer materials,microbiology,physical chemistry and reservoir engineering and technically by the instrument detection,chemical analysis and physical simulation in this paper.The impact and mechanism of polymer in polymer flooding on the ecological and development environment taking the surface water source,soil and reservoir of Dagang Oil-field as the research object were carried out.It had important application and academic values for responding to social concerns and expanding the application scale of polymer flooding technology in Dagang Oil-field and improving the development effect of polymer flooding.The main research works,conclusions and understandings of this article were as follows:（1）The polymer used in Dagang Oil-field polymer flooding test area was partially hydrolyzed polyacrylamide（BHHP-112）,its physical and chemical property indicators met the index requirements of Type II products in the petroleum and natural gas industry standard of petroleum natural gas profession standard（SY/T5862-2020）.In case of accidental polymer leakage in polymer flooding caused by human,machines,materials,laws,environments and other factors,the biological damage pollution characteristic was the acrylamide monomer,and the characteristic pollutant affecting the ecological and development environment was the polymer solution containing acrylamide monomer.（2）Based on the toxicological data literature investigation and the derivation of supplementary experimental data,it was confirmed that the acute environmental reference value of the polymer used in the polymer flooding test in Dagang Oil-field was 672.93mg/L,the chronic environmental reference value was 67.43mg/L,and the boundary polymer concentration of ecological environment safety in polymer flooding was from 3150mg/L to4000mg/L.At present,the concentration of polymer used in Dagang Oil-field was lower than the ecological environment safety boundary concentration,and the environmental safety risk in the mining area was low.（3）There are many kinds of bacteria in the soil of polymer flooding and water flooding areas of Dagang Oilfield,and the coastal flora has distinctive characteristics.The content of Pseudomonas aeruginosa in the soil of the two areas with polymer and oil pollution remediation functions exceeds 70×109 per gram.When the polymer solution moves from the surface to the depth of 1400mm,most of it is adsorbed and trapped by the soil,and the residual amount is only 3.2%～16.8%.After contacting with polymer solution,the microbial community in soil changed a little,but the main groups changed little,and Pseudomonas aeruginosa developed and grew preferentially.The ecological environment of polymer flooding area is not polluted by polymer.（4）The starting pressure gradient after polymer flooding in the low permeability(Kg=293.2×10-3μm2)of the reservoir was 390 times and 10 times higher than that in the high permeability(Kg=3610.0×10-3μm2)and medium permeability(Kg=1243.1×10-3μm2).The great difference of starting pressure gradient at each layer was the internal reason for liquid injection profile reversal in the subsequent polymer flooding process.The parameters of the rock such as average pore radius,median pore radius,sorting coefficient,skewness,kurtosis,mean radius,characteristic structure coefficient and homogenization coefficient generally decreased after polymer flooding,and the permeability loss rate of several cores exceeded30%.（5）At the initial stage of polymer flooding after water flooding,polymer solution mainly entered the high permeability part of the reservoir and was detained.The retention resulted in an increase in the seepage resistance and a decrease in the liquid absorption index of this part,and an increase in the injection pressure of the whole well（the liquid injection rate remains constant）,which leaded to an increase in the differential pressure of liquid absorption and an increase in the liquid absorption volume at the middle and low permeability parts.With the polymer flooding entered the middle and late stage,on the one hand,the percolation resistance in the middle and low permeability parts of the reservoir increased and the liquid absorption index decreased due to polymer retention.On the other hand,the liquid suction pressure difference tended to the maximum(Pfa-Pe)because the injection pressure rise was limited by the reservoir fracture pressure.The combined effect of the liquid suction index and the liquid suction pressure difference caused the liquid suction volume at the low permeability part of the reservoir to decrease significantly,and finally caused the liquid injection profile to reverse.In addition,the interaction of polymer with Fe3+and bacteria in water and minerals in reservoir would generate polymer derivatives,which would cause reservoir damage and accelerate the reversal of liquid injection profile.（6）The formation of microfractures in the reservoir damage area of polymer flooding injection wells not only reduced the regional seepage resistance,but also increased the contact area between carbonic acid and polymer retention area.Based on the differential acidification characteristics between carbonic acid and polymer solution,polymer gel and reservoir minerals,and the characteristics of increasing contact area of microfractures,the integrated treatment technology of“profile control/micro-fracture/plug removal/oil displacement”in polymer flooding blocks was created. |
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