Study on risk assessment and prevention measures of mining face affected by abandoned oil well

With the gradual shift of China′s coal resource development to the northwest, there were many examples of projects in Ordos Basin where shallow coal and deep oil and gas resources interfere with each other. In the area where coal and oil and gas resources overlap, abandoned oil and gas wells left by oil and gas development are important disaster-causing factors that affect the safe mining of coal face. Based on the geological conditions of the working face, the mining technical conditions and the past operation of the abandoned oil wells, the hazard risk level of abandoned oil wells in Hecaogou coal mine was evaluated by hazard identification and risk assessment theory. It is considered that the abandoned wells in the mining process of working face may cause three types of disasters including gas explosion, massive influx of oil and water as well as toxic and harmful gas influx, and their risk levels are relatively dangerous, slightly dangerous and highly dangerous. By means of theoretical analysis, technical optimization and field test,the safety and environmental protection treatment technology of abandoned oil wells in the field was studied.After comparative analysis of the schemes, the "three-stage" plugging construction methods including the deep plugging of the oil layer, the casing plugging from the bottom of the well to the surface, and the surface casing, as well as the five process routes including preliminary analysis and diagnosis, professional well access, wellbore detection, segmented plugging and pressure test sealing were finally proposed, and the new technology practice of plugging the abandoned oil wells were carried out on site.The results show that the new plugging technology of the abandoned oil wells can cut off the connection between the production coal seam and the deep high pressure oil, gas and water layers, and block the passage of leakage to the production coal seam,effectively preventing the major accidents such as fire, flood, poison gas and explosion which may be caused. This technology liberated abandoned oil wells for a total of 233,700 tons of coal in No. 5 coal seam of Hecaogou coal mine, and produced good technical and economic benefits.