Study On Casing Damage Mechanism And Prevention Measures In Ansai

The damaged oil well or water well casings in oilfields are called the casing damage Wells.In many domestic and oversea oilfields,with the increase of the developing time and with the frequent adjustment and implementation of the developing plan,especially for the oil reservoir employing waterflooding,the technical situation of the oil/gas/water well casing will gradually get worse and even damage,due to the different geological,engineering and management conditions.The damaged casings cause that the well cannot operate normally.Ansai oilfield is mainly located in the"Four Rivers/Three Reserviors"environmentally sensitive area?"Four Rivers":Yan river,Xingzi river,Changwei river,and Xiaochuan river;"Three Reservoirs":Wangyao reservoir,Hongzhuang reservoir,and Zhongshan Chuan reservoir?,in which the water resources are scarce,the drinking water is very precious and the ecology is fragile.Therefore,the environmental protection of the area is a heavy burden.When the casing of oil and water well is damaged,a passage is formed between the wellbore and the underground water body,which favors the flow of crude oil and deep produced water to the superficial Luo river and the surface,causing the risk of environmental pollution and even the pollution of drinking water.The damaged casing also affect the stable oil field production.Since the discovery of the first damaged casing in Ansai oilfield in 1990,the average increased number of casing damage well is about 70?80 per year,and the existed casing damage well at present is 1612,indicating that the hidden trouble wells increase frequently.Because parts of the hidden trouble wells contains many damaged sections,the hidden trouble wells have become a main threat to water supply,environmental safety,and the normal oil/water production of oilfieds.In this paper,an in-depth study on the causes of casing inner/outer corrosions was carried out by means of casing inspection,casing damage water analysis,hanging plate corrosion test and liquid production corrosion test.The results showed that the main causes of casing damage in Ansai oilfield are casing outer corrosion,cement refluxing,high salinity of water,CO₂ and oxygen content in superficial water.Based on the present state of the casing damage wells,combined methods are used to treat or control the casing damage wells,such as the use of casing cathodic protection,oxygen isolation,adding corrosion inhibitor methods to reduce the corrosion rate of casing.Meanwhile,different treating ways are selected in view of the casing corrosion degree.One is for the casing damage Wells that the wellbores could still be used:long-term packer+mechanical seal water oil production technology,at the same time supporting delay internal corrosion and enhance the governance of casing damage Wells setting ratio.Second is for the oil and water wells with severe casing damage and cement return to high or low level:implement hidden trouble control well secondary cementing technology,and select secondary cementing technology through comparative analysis of different cement slurry systems.Third,the experimental development of casing chemical plugging,expansion pipe subsidies technology,and constantly explore new technologies and new experiments to improve casing damage well management in Ansai oilfield.Through the comparison of several technologies and the application of field tests,it is shown that the secondary cementing technology is suitable for casing damage control in Ansai oilfield,which improves the economic benefits of the oilfield.