Research On Scaling Mechanism And Prevention Of Injecting Water In Ultra-low Permeability Reservoirs

Water injection development is one of the effective means to improve oil and gas recovery in current oilfields.Water injection not only extracts crude oil to the surface,but also maintains formation pressure.However,for high-temperature and ultra-low permeability reservoirs,water injection can also cause scaling problems,resulting in reduced oil and gas formation porosity,blockage of water injection pipelines,and increased pump pressure,which seriously affects oilfield production operations.Therefore,exploring the reasons for the scaling of injected water in the target block and prevention techniques have important practical value for the development of ultra-low permeability oil and gas fields.This paper analyzes the ion composition of the water samples,the scale samples taken and the reservoir cores of the two blocks,and carries out theoretical predictions and experiments on the scaling trend of injected water under different influencing factors to study the scaling mechanism of injected water in ultra-low permeability reservoirs.Static experiments on the compatibility of formation water and injected water are used to obtain the scaling trend of mixed liquids under different conditions.At the same time,the dissolution of formation cores by highly alkaline injection water is analyzed to form the compatibility of injected water and formation and countermeasures.Combining the existing oilfield water scaling chemical control technology and the characteristics of high temperature and ultra-low permeability of the reservoir,the‘softening-filtering-scaling’joint control method is determined,and the best softener,flocculant,scale inhibitor,and filter are selected through experiments material.Finally,through the core displacement experiment,dynamic simulation study the actual effect of the injection water scaling prevention technology.The research results show that the injected water in the two target blocks is severely scaled under high temperature conditions in the formation,and the type of scale is mainly carbonate scale.Therefore,it is easy to block the micro pores of the ultra-low permeability reservoir during the water injection flow process.Theoretical predictions and experimental results have confirmed that temperature,p H value and HCO₃^-mass concentration have a great influence on the scaling tendency of injected water.At low temperature,the compatibility of formation water and injected water is good,but at 95°C or higher and high p H value,the scale of the mixed solution is serious,and it increases with the increase of the proportion of injected water;4 wt%Al（NO₃）₃ has a good inhibitory performance in the experiment of controlling the formation of softened water erosion.After adding the softener Na OH,the scaled grains of the injected water will grow rapidly in a short period of time,and the D₁₀ of the scaled grains will reach 8-10μm at 1 h.Combining the physical characteristics of the reservoirs in the two blocks,an ultra-microfiltration membrane with a pore size of 0.45μm is selected.It is also used with CPAM,a polymer flocculant.In order to further solve the problem of remaining calcium,magnesium ions and smaller scale particles entering the target reservoir,HPMA was selected as a high-temperature scale inhibitor;under the conditions of the reservoir,HPMA not only effectively solves the problem of scaling in the mixed solution,but also performs at high temperatures.It is stable and has the characteristics of strong anti-scaling ability in neutral solution.Based on the above research,the combined prevention and control method of‘softening-filtration-scaling’was finally determined,which completely solved the problem of scaling of injected water in high-temperature and ultra-low permeability reservoirs.