Research On Corrosion Performance Of Reinjection Water And Scaling Tendency Of Mixed Injection In Low-permeability Oilfields

This paper is aimed at the fact that low permeability oil fields in southern Shanbei need water injection to maintain the formation energy and the utilization of produced water,The corrosion and scaling problems that may occur in the mixed reinjection of oilfield sewage,clean water in this area and formation water are studied,The effects of water quality,corrosion performance and mixing conditions of different water sources on the mixing process and scaling tendency were discussed,The basic parameters and basis are provided for the pretreatment process to solve the corrosion and scaling of pipeline and equipment and formation plugging in the process of oilfield reinjection.Firstly,the water quality of fresh water,sewage and formation water（-31,-6,-12）in three blocks was analyzed.The Ph,the concentration of anions and Cations,the content of suspended solids and the mineralization degree of the five kinds of water are quite different.Clear water has the highest Ph,next is formation water-31,the other three have smaller differences in Ph.The most mineralized is formation water-31,the other mineralization degrees from big to small are sewage,Formation Water-6,formation water-12 and clear water.Clear water belongs to Na₂SO₄water type,wastewater and formation water belong to Ca Cl₂type.The suspended solid content of the five kinds of water all exceeded the standard.Sewage,formation water-31 is the oil content exceeding the standard,The saprophytic bacteria and sulfate-reducing bacteria in sewage and formation water-6 exceeded the standard.The corrosion behavior of five kinds of water was studied experimentally.The corrosion rate of clear water,formation water-31 and formation water-12 exceeded the standard.The corrosion of clear water belongs to the electrochemical corrosion of total oxygen evolution.The corrosion of sewage and formation water-6 is localized pitting.The main type of intergranular corrosion,the auxiliary type of Pitting Corrosion,Formation Water-31 and formation water-12 belong to the local corrosion type,the intergranular corrosion of main,the crevice corrosion of auxiliary.Chloride Ions,Sulfate-reducing bacteria and saprophytes in water,and suspended solids are the main factors affecting corrosion.The experimental study was carried out under the experimental condition that clean water and sewage were mixed with formation water respectively.The Ph of mixed water,the concentration of scaling Cation,the rate of change of total concentration and the amount of scaling are affected by Formation Water,temperature and mixing ratio in different blocks.Under the condition of the same mixing ratio,the scaling amount of formation temperature（40℃）is larger than that of normal temperature.Clear water is mixed with three types of formation water,the biggest scale is the mixture of clear water and formation water-31,the less scale is the mixture of clear water and formation water-6,the least scale is the mixture of clear water and formation water-12.When clear water is mixed with different formation water,When the change rate of the total concentration and the amount of scale formation reach the maximum,the mixed ratio is similar.The optimal mixing ratio is:the ratio of clear water to formation water-31 and formation water-6 is 8:2,the mixture ratio of clear water and formation water-12 is 5:5.Ca CO₃is the main component by mixing clear water with formation water-31 and formation water-12,the scale particles are large and loose.The scale formed by the mixture of clear water and formation water-6 is mainly Ca CO₃,in addition,there is a small amount of silicon crystal.The crystalline morphology is smaller and denser.When sewage is mixed with three types of formation water,the maximum amount of scale formation occurs when sewage is mixed with formation water-6,the amount of scaling produced by the mixture of sewage and formation water-31 is larger,the minimum amount of scaling is produced by mixing sewage with formation water-12.When sewage is mixed with three types of formation water,the maximum change rate of the total concentration of scale-forming cations is not consistent with the mixed ratio corresponding to the maximum scale-forming quantity.It is caused by the high content of corrosive bacteria and suspended solid in sewage.The optimum mixing ratio（corresponding to the maximum change rate of the total concentration of scale-forming cations）:the mixture ratio of sewage with formation water-31 and formation water-6 is 6:4,the mixture ratio of clear water and formation water-12 is4:6.When sewage is mixed with formation water,the main scaling component is Ca CO₃.When sewage is mixed with formation water-31 and formation water-6,the scale crystals are larger and looser.When sewage and formation water-12 are mixed,the scale crystals formed are smaller and denser.In most mixed conditions,compared with the scale amount produced by the mixing of Clean Water and formation water,the scale amount produced by the mixing of Sewage and formation water is larger.