Main environmental geological problems in oil and gas mineral development

The exploitation of oil and gas in northwest China is mainly distributed in Junggar basin, Tarim basin and Turpan-Hami basin in Xinjiang, Yumen Jiuquan basin in Gansu, Qaidam basin in Qinghai and Ordos basin. They all belong to the Gobi desert and the Loess Plateau, where the ecological environment is extremely fragile. The exploitation of oil and natural gas leads to loose sand layer, destroys sparse vegetation and intensifies the degree of land desertification. In the process of oil and gas exploration and development, pollutants such as drilling mud, crude oil and wastewater, and natural gas venting pollute water, soil, vegetation and atmospheric environment. For example, before 1998, the crude oil wastewater discharged from Karamay Oilfield in Xinjiang caused a large area of land pollution. The oil and gas development zone in the Loess Plateau is a fragile ecological environment with natural collapse, landslide and soil erosion, and oil and gas development has intensified the occurrence of soil erosion, collapse and landslide geological disasters.

The most serious environmental geological problem of petroleum development in northwest China is northern Shaanxi. This area is also the birthplace of China's petroleum industry, with the first onshore oil well in China-Yan 1 well. Since 1993, due to the chaotic management, some county-level governments have issued oil exploration licenses illegally and ultra vires. In addition to the state-owned Changqing Petroleum Exploration and Development Bureau and Yanchang Oil Mine, there are various county drilling and production companies and individual oil producers affiliated to Yanchang Oil Mine. Northern Shaanxi has become the only area in China with private oil exploitation.

From the recovery ratio of different enterprises (Table 3-5), it can be seen that Changqing Oilfield Company, Yanchang Oilfield and local and individual oil mines that exploit low permeability oil in the same area have an average recovery ratio of 8% due to small private exploitation scale and low mining technology level; The primary recovery rate of Changqing Oilfield Company is 20.5%, and the cumulative recovery rate of secondary oil recovery can reach 30% after water flooding technology is adopted. In other words, local and individual recovery rates only account for 26.67% of the total recovery rate of Changqing Oilfield Company. In other words, local and individual oil mines make 73.33% of the resources unusable, resulting in the waste and destruction of valuable oil resources.

Table 3-5 Oil Recovery Ratio List of Different Petroleum Enterprises in Northern Shaanxi

Road construction, well site development, oil pipeline laying, etc. In the Loess Plateau, vegetation is destroyed, and abandoned soil accumulates into gullies and slopes, which aggravates soil erosion and becomes the main environmental problem of oilfield development zones. On July 9, 2004, People's Daily reported that "oil and gas development in northern Shaanxi caused a lot of soil erosion-the new soil erosion reached 690× 104t". It has aroused great concern of the central leadership and made important instructions. At the same time, landing crude oil, oil production wastewater, abandoned well pad mud pit, etc. It will also cause water and soil pollution. Shaanxi Jingbian Ansai Oilfield is located in the upper reaches of Dali River. From 1990 to 200 1, nearly1,000 oil wells were drilled in Qingyang, Jingbian, with a range of 2 15km2, which caused shallow groundwater leakage, deep salt water overflow, exhausted available groundwater resources, chaotic civilian mining, indiscriminate honeycomb mining, and mutual infiltration and pollution between oil layers and water layers. In April and June, 2000, 5438+ 10, CCTV focus interviews exposed the area twice. See Table 3-6 for monitoring results of harmful substances in surface water of Ansai Oilfield Development Zone 199 1 and 1992 of Changqing Oilfield Company. Among them, the most serious environmental pollution is petroleum, which exceeds the standard by 32 times, sulfide by 1.20 times, volatile phenol by 4.2 times, chemical oxygen demand by1.7/kloc-0 times and biological oxygen demand by 5.23 times. Among them, the serious places exceeding the standard are mainly in Wang Yao Reservoir and the upper reaches of Xing Zi River and fengcun (Table 3-6).

Table 3-6 Monitoring Results of Harmful Substances in River Water of Ansai Oilfield Unit: mg/L

Since 1990, there have been nearly 700 oil wells in the 820km2 basin of Yao Yao Reservoir, the drinking water source in Yan 'an, with an average of1.2km2. At the initial stage of oilfield development, Yan 'an Wang Yao Reservoir was once seriously polluted due to improper measures. In the process of development, the mud pit stays at the well site, oil pipeline accidents, vehicle transportation accidents, etc. , resulting in failure to clean up the landed crude oil in time, or the landed crude oil is piled up around the well site or directly discharged outside the well site, resulting in land pollution in the well site. Since 1992, there have been many serious pollution accidents caused by pipeline rupture in Ansai oil region. In addition, criminals in the development zone steal crude oil, drill holes in pipelines and steal oil by wooden boats on the water surface, which leads to serious water pollution in reservoirs. From July 1993 to July 10, continuous rainfall occurred in the reservoir, and crude oil, sewage and crude oil not buried in the well site were brought into the reservoir by floods, resulting in the water surface in the reservoir area being covered with dirty oil. According to the monitoring of the provincial environmental monitoring center, the average value of petroleum in the water is as high as 1.74mg/L,1August 3, 19941day, and the reservoir basin is flooded with heavy rain again, causing the whole water body to be seriously polluted again. The highest oil content is 5.9mg/L, which exceeds the standard 1 17 times. According to the monitoring of water quality of Yao Yao Reservoir by Yan 'an Water Supply Company 1999, the oil content is as high as 0.4mg/L, which is 7 times higher than the standard. The monitoring in June 2000 showed that the oil content at the outlet of the reservoir reached 0.2mg/L, which was 3 times higher than the standard. The oil in the water inlet exceeds the standard by 3 times. Judging from the above monitoring results, the local water source has been seriously polluted by oil.

During the investigation of Wang Yao Reservoir in 200 1, the project team found that the water pollution situation was obviously reduced. This is because Changqing Oilfield Company has taken a series of active and effective environmental protection measures, such as crude oil pipeline transportation, building a fence at the well site and landing a crude oil collection pool, which has minimized the pollution of oil development to the land, roads and rivers in the mining area. In addition, the Shaanxi provincial government has intensified its efforts to rectify the oil development market in northern Shaanxi and severely cracked down on criminals who steal crude oil. However, local drilling and mining companies and private mining enterprises in northern Shaanxi have not paid enough attention to environmental protection, and environmental protection facilities have not kept up. The situation of low oil recovery caused by small-scale decentralized development and serious pollution of water, soil and vegetation caused by exploration and development of crude oil wastewater has not been effectively curbed.