Oil is not only one of the main sources of energy for human beings, but also one of the sources of human environmental pollution. According to statistics, more than 8 million tons of oil enter the world environment every year, polluting soil, groundwater, rivers and oceans. With the massive exploitation and utilization of oil in the Loess Plateau region, the region is characterized by a large oil extraction area, many oil wells, low production and backward development technology. The pollution it brings to the natural environment is becoming more and more serious, directly affecting the ecological and living conditions of the region. The situation in localized areas has been extremely serious, threatening local agricultural production and farmers' living environment. Petroleum-based substances have become one of the region's key pollutants, soil, rivers and other areas have been different degrees of petroleum-based pollution.
Ordos Basin main oil and gas system
Ordos Basin is a multi-rotation superposition of oil and gas basins, across the Shaanxi, Gansu, Ningxia, Jin, Inner Mongolia 5 provinces (regions), an area of 320,000km2, the sedimentary macro-thickness of the Phanerozoic. Basin base for the Paleoproterozoic - Paleoproterozoic metamorphic system, in the Middle and New Paleoproterozoic for the cleavage trough basin, sediments for the shallow sea clastic rock - carbonate rift filling type; Early Paleoproterozoic for the Klatun Basin, sediments for the land surface of the sea carbonate plateau type; Late Paleoproterozoic - the Middle Triassic for the Klatun Pass Basin Late Paleozoic - Middle Triassic is the Craton depression basin, sediments from the coastal carbonate rock type transition for terrestrial clastic rock terrace type; Late Triassic - Cretaceous is a large inland depression basin, sediments for the inland lakes, fluvial deposition; Cenozoic rise as a whole, the main body of the basin for the gentle westward-dipping slopes, sediments for the three toes of Ma red clay and a huge thickness of the wind-formed loess; the perimeter of the fracture basin occurs and develops. The four sets of oil and gas-bearing systems that have been explored and developed in the basin are all stratigraphic-rocky oil and gas reservoirs.
1.Upper Triassic Yanchang Formation Rock Reservoir Oil-bearing System
Earliest exploration and development of Yanchang Formation oil-bearing system hydrocarbon source rock to Yanchang Formation deep lake and shallow lake phase black mudstone, shale and oil shale, hydrocarbon centers are distributed in the southern part of the basin, Majiatan - Dingbian - Huachi - Zhiluo - Binxian range, the hydrocarbon centers are distributed in the southern part of the basin. Zhiluo - Binxian range, the thickest oil source rocks up to 300 ~ 400m, favorable oil-generating area covers an area of 60,000km2 (Figure 3-3), reservoir rocks around the distribution of oil-generating depressions, the northern flank of the gentle slopes of the belt with Dingbian, Wuqi, Zhidan, Ansai and Yan'an and other five large delta and delta leading edge of the sand, and the southern flank of the steeper slopes of the belt is the development of the Huanxian and Xifeng and other accumulation rate of the river is faster! The south flank of the steeper slopes of the belt, the development of Huanxian and Xifeng and other fast accumulation rate of river sand body and underwater sedimentary sand body. The storage and seepage conditions are improved by cracks and secondary porosity of turbidite zeolites, the enclosure is improved by compaction structure, and the shading is improved by the lateral change of lithology in the upward-dipping direction.
2.lower jurassic yanan group sandstone oil reservoir oil-bearing system
Yanan group sandstone oil reservoir to freshwater - brackish water lake phase deposition of the upper triassic prolongation group of hydrocarbon source rock as the main oil source rock, is a mixed type of cheesecake; to the swamp phase coal system deposition of the jurassic yanan group as the auxiliary hydrocarbon source rock, is a humus type of cheesecake, the southern north of shaanxi, the clothing of the coal system is more characterized by high oil content rate. The Yixicun coal system in southern Shaanxi is characterized by high oil content. At the end of Triassic, the Indo-Chinese movement uplifted the whole Ordos Basin. At the top of the Triassic, erosional landforms were formed, cutting the Extension Formation in the form of paleochannels. The largest Gan-Shaan paleo river converged the Qingshi paleo river, the Ningshaan paleo river and the Zhiluo paleo river from southwest to northeast, and the opening extended to the south (Figure 3-4). Indo-Chinese erosion surface of the occupation of the river channel cut the Yanchang Formation, become the oil and gas underflow channel, overflow of the erosion surface of the oil and gas first to the ancient riverbed within the Fuxian Formation and Yan'an Formation bottom sandstones transport and gathering, but also to the Yan'an Formation of the upper part of the various sandstone bodies and the ancient riverbed on both sides of the side of the sand body transport, gathering, to the compacted structure and a large number of lithological enclosure as the main form of its enclosure.
Figure 3-3 Sedimentary phases during the sedimentary period of the Late Triassic Yanchang Formation in the Ordos Basin
3.Ordovician Majiagou Formation carbonate gas-bearing system
The hydrocarbon source rocks of the Ordovician shallow marine carbonate rocks in the Ordos Basin mainly consist of microcrystalline and mud-crystalline cherts, muddy cherts, muddy dolomites and anastomosing dolomites, which are as thick as 600~700 m. Hydrocarbon generation centers are located in the east of the area of Yulin-Yanan, in the west of the area of Yuhuan-Yanan, and in the west of the area of Yuhuan-Yanan. The hydrocarbon production centers: the eastern part is in the Yulin-Yanan area, and the western part is in the Huanxian-Qingyang area, which produces humus-type cracked gas. Garidong movement makes the whole Ordos Basin uplifted, after 130 Ma weathering and stripping, resulting in the formation of quasi-plaining paleokarst landforms on the top of the Ordovician system, and there are north-south oriented broad submarine platforms distributed in the central part of the basin along the Jingbian area, with submerged grooves and depressions around the periphery. Under the dual role of the cover of the overlying Carboniferous Coal System Fe-Alumina rocks and the lateral shielding of the Ordovician Saltpaste layer to the east, the paleo submarine platforms have become large hidden enclosures (large-scale hidden enclosures) for natural gas transportation and aggregation. large hidden enclosure (Figs. 3-5).
4.Carboniferous-Permian coal gas-bearing system
The Carboniferous System of the Ordos Basin was deposited in the river-lake and tidal flat phases, and the Permian System was deposited in the transitional phase between land and sea and inland river-lake phases, with clastic rocks as the main source, and only a small amount of carbonate rocks in the Carboniferous System. The hydrocarbon source rock is mainly the coal system of the Taiyuan Group of the Carboniferous System and the Shanxi Group of the Lower Permian System, and the microstructure consists of lenticels and filaments, and the casein is of the humus type, and the components of the coalbed methane are mainly methane. The coal beds in the north of Dongsheng and Yulin areas are 20m thick, and the dark mudstone is 50-90m thick, with a range of about 70,000km2; the coal beds in the south of Fuxian and Huanxian areas are 5-10m thick, and the dark mudstone is 10-100m thick, with a range of about 60,000km2.The reservoirs are mainly composed of sandstones, and the main source areas are in the north of the area of Dazingshan and Tora Mountain, with the superposition of the sand bodies of various layers, which is spectacular. The sedimentary center of Shanxi group is located in the southern part of the basin around Luochuan-Qingyang, with the most developed sand body in the northern part of the basin, **** there are 6 large sand bodies extending into the basin, and the internal control of each large sand body is controlled by the ancient river network, which is in the form of complex strips. The storage and seepage conditions are improved by fracturing and post-generation diagenesis, and the trap is sheltered by compaction structures and lithology in the up-dip direction.
Figure 3-4 Schematic diagram of the Early Jurassic Ganshaigu River in the Ordos Basin
The main geo-environmental problems caused by petroleum development
(I) Generation of petroleum pollutants
During the process of exploration and development of petroleum, in all the links from geologic exploration to drilling and petroleum transportation, due to the many contents of the work, the difference of work process, the complexity of construction, the different management level, and the equipment and equipment, there are many different ways of construction, and there are different levels of management. Petroleum-based pollutants may be generated at every stage of oil extraction (Figure 3-6).
The petroleum-based pollutants generated during the different operational periods of oil extraction are specifically described as follows:
1. Drilling Period
Drilling wastewater and oil-containing mud containing petroleum-based pollutants are generated during drilling operations in an oil field. This is generated during the drilling process by the oil pollution from flushing the ground and equipment, mud loss during drilling operations, and leakage from the mud circulation system. The wastewater contains pumping concentration between 50 and 1200mg/L, and the volume of water ranges from a few tons to tens of tons. In addition, in some cases, before reaching the high oil-bearing layer, a certain amount of low oil-bearing strata has to be passed through, thus causing the oil to be brought to the surface along with the drilling mud. At the same time, once the high oil-bearing formation is reached, a small amount of highly concentrated oil may be spewed out when the ground pressure is high.
Figure 3-5 Paleomorphological map of Ordos Basin Ordovician top surface (according to Fan Zhengping and others, 2000)
Figure 3-6 Schematic diagram of the sources of petroleum pollutants and the pollution pathway in the oil extraction process
2. Oil extraction period
During the oil extraction period (including the normal operation and well washing), the wastewater discharge includes the oil extraction wastewater and the wastewater from well washing. In the underground oil-bearing strata, oil and water are present at the same time, in the process of oil extraction, oil and water are pumped to the ground at the same time, the oil-water mixture is sent to the crude oil gathering and transportation system of the oil station for dewatering, desalination treatment. The wastewater that is desalted is oil recovery wastewater, also known as "extraction water". As the oil recovery wastewater is extracted from the oil reservoir together with the original pumping, and produced by the crude oil dewatering treatment, this part of the wastewater not only contains a variety of salts and gases dissolved into the stratum in the high temperature and high pressure oil reservoir, but also contains some other impurities. More importantly, due to the effect of dewatering at the oil processing station, this part of the wastewater carries crude oil - petroleum pollutants; in addition, within the study basin, there are also simple dewatering methods, such as gravity separation, and is mostly found in single well dewatering of oil wells. Generally, the oil extraction wastewater from oil wells contains pumping concentration in thousands of mg/L, and the average discharge from a single well is tens of m3/d. Well washing wastewater is the effluent produced by periodic flushing of injection wells or the oil-containing wastewater produced by wells that need further overhauling or washing operations due to the damage of equipment, clogging of the oil layer, corrosion of pipelines, and other reasons after a period of extraction.
3. Leakage in the process of crude oil storage and transportation
Crude oil in the storage and shipment process due to leakage of crude oil on the ground, as well as crude oil in the pipeline centralized transportation process of some of the intermediate links may result in a certain amount of crude oil leakage or generation of oily wastewater.
4. Accidental pollution
Accidental pollution includes natural and man-made factors: natural accidents, including blowouts, equipment failure and the use of vehicles to transport landslides triggered by traffic accidents caused by crude oil leakage. Yan'an area surface loess structure loose, hydraulic scouring intense, due to landslides and pollution caused by more frequent accidents. Man-made accidents refer to pollution accidents caused by various man-made factors, such as the destruction of oil extraction equipment, oil pipelines and the overturning of vehicles caused by man-made traffic accidents during the transportation of crude oil vehicles. Accidental pollution has a large amount of pollution, serious harm, difficult to predict the characteristics.
(II) The impact of oil extraction process on soil and water environment
Pollution can be produced in all aspects of the oil, the pollution object is mainly soil, followed by surface water bodies, groundwater pollution is mainly indirect pollution, there is no obvious indicators in the Ordos Basin to show that the oil leakage or infiltration of pollution of the groundwater, i.e., no petroleum pollutants have been detected in the groundwater. However, in the process of oil development, the water quality of groundwater changed significantly, the mineralization increased significantly, and other indicators also changed greatly.
1. Impact on Soil
(1) Impact of Landed Crude Oil on Soil Environment
A large amount of spilled crude oil enters into the soil, which affects the survival of microorganisms in the soil, causes salinization of the soil, destroys the structure of the soil, and increases the content of petroleum-based pollutants. After a crude oil spill, crude oil contaminates (expands) a larger area in non-permeable bedrock and clay-heavy soils, while the expansion of impacts in loose soils is smaller. It is particularly emphasized that clay-heavy soils are mostly cultivated soils, and crude oil overlying the surface will reduce soil permeability and soil fertility. In the initial leakage accident, crude oil in the soil seepage to a certain depth, with the prolongation of the leakage time, the seepage depth increases little, according to the field investigation in the Longdong oilfield and Shaanxi oilfield, etc. shows that the crude oil generally in the soil inside the depth of more than 50cm accumulates, so the crude oil leakage is mainly contaminated with the soil tillage layer.
(2) Vertical penetration law of petroleum-based pollutants in soil
Ordos Basin has a dry climate, low rainfall, and the surface of the ground is mostly covered by Gobi gravel, with poorly developed soil and high sand content, so the petroleum-based pollutants produced by oilfield development in this basin are more likely to migrate along the downward seepage of the soil inclusion zone and jeopardize the ecological environment. The migration rate is determined by the adsorption capacity of the soil for the pollutants. Generally the specific gravity of crude oil is less than 1, and it is neither static in the soil for a long time, nor similar to the rapid migration of soluble substances up and down. In order to figure out the migration status of oil substances in the soil, the migration law of oil pollutants in the soil of the oil field area was studied by using the method of field sampling and analysis.
The content of petroleum substances in the soil profiles near the well sites of Xifeng Oilfield and Qincheng Oilfield in Longdong were measured respectively, and the results are shown in Table 3-5 to Table 3-7.
Table 3-5 Vertical distribution of petroleum pollutants in the soil layer of Qincheng Oilfield
Table 3-6 Vertical distribution of petroleum pollutants in the soil layer of Xifeng Oilfield
Table 3-7 Longitudinal distribution of petroleum pollutants in the soil layer near the venting pool of the Xing2 well in Ansei, Shaanxi Province
From Table 3-5 to Table 3-7, it can be seen that, due to the adsorption of the soil, the petroleum pollutants gradually decrease with the increase of the distance of the longitudinal profile of the soil layer, and the pollutants in the area within 50 cm decrease very quickly. Petroleum pollutants mainly accumulate in the soil surface layer within 80cm, and it is generally difficult to infiltrate to below 2m. Changqing oilfield is located in the area of wind sand soil and gray-brown desert soil soil, coarser particles, looser structure, higher porosity, vertical permeability coefficient is larger than the general soil. However, due to the location of the oilfields in the northwest of the arid climate, rainfall is low, the water content in the soil is very low, so that the migration of pollutants infiltration is greatly weakened, and there is very little large amount of precipitation leaching effect, so the oilfield development process of these landfalls of crude oil is only accumulated in the surface of the soil, the penetration degree is shallow, the deeper soils have less impact.
2. Impact on surface water bodies
Ordos oilfield spans 3 provinces (areas) of Shaanxi, Gansu and Ningxia, and there are 3 main water systems in the territory, i.e., the water system of Malian River in Longdong, Gansu, the water system of Yanhe River in Yan'an, Shaanxi, and the water system of Wuding River in Jingbian, Shaanxi. These three major water systems have been polluted to varying degrees during the oil development process.
The most important pollutants in the surface water of Longdong Petroleum Development Zone are COD and chloride, of which COD is the most serious pollution, all of the 14 samples exceeded the standard, and the exceedance of Huanjiang is particularly serious; chloride pollution index in addition to Hulu River, Guchengchuan and Puhe River in the various samples of the point of the non-exceedance of the rest are exceeded, and also to the Huanjiang for the most. pH value are not exceeded; petroleum class in addition to the serious exceedance of Huanjiang Hanjiawan cross-section The rest of the samples were between 0.04 and 0.3mg/L; volatile phenols in addition to the Ruiyuan River Huachi Yuele section exceeded the standard by a factor of 1, the rest did not exceed the standard; Huanjiang Hongde Bridge, due to geological reasons, the TDS content is very high, and this part of the bitter water downstream affects the quality of the downstream water, but with the downstream water volume increases, the mineralization is gradually reduced.
Overall, in the Longdong region, the Huanjiang and Malian River main stream of the most serious pollution, followed by the Zuiyuan River, Pu River pollution is the least. The main streams of Huanjiang and Malian River can no longer meet the requirements for the functional use of Class III water bodies, and Ruoyuan River and Pu River can no longer meet the requirements for the functional use of Class II water bodies.
Based on the hydrological data (Table 3-8) from 1987 to 1992 at the hydrological station in Wuqi County, it can be seen that before the large-scale development of petroleum resources, the annual average values of sulfate, chloride, and hexavalent chromium content in the river water of the upstream of the Beiluo River exceeded the national standard of Class III, especially the chloride and sulfate content exceeded the standard by 2-3 times, and the degree of mineralization was greater than 1,000, and most of the water was high in TDS, and the total hardness was higher than the standard. TDS water, and the total hardness is between 500 and 600mg/L, exceeding the standard seriously.
Table 3-8 Wuji County hydrological station water quality monitoring value statistics unit: mg-L-1
Luohe upstream area of the water mineralization and a variety of salts exceeded the standard with the Luohe upstream groundwater recharge area of the Cretaceous, Tertiary (Paleo, Neoproterozoic) stratum salt, groundwater mineralization or salt content itself is high. In Wuji area, there exists Wuji Ancient Lake at the southern edge of Baiyu Mountain, which forms saline strata after drying up and inputs a large amount of salts into Luohe River during groundwater recharge. There are a large number of salt ponds and salt-bearing strata in Dingbian area in the northwest of Wuji, and the recharge of salt into the groundwater to the southeast should not be ignored. since the 1990s, after the large-scale development of petroleum resources, the TDS, hexavalent chromium, ammonia nitrogen, chloride, permanganate index, sulphate, and total hardness have shown a significant upward trend, which indicates that the current upstream of the Luhe River has "high salinity and mineralization (TDS)". (TDS), high hardness" is in the local higher on the basis of further water pollution caused.
Northern Shaanxi, Petroleum Development Zone, the surface water body of hexavalent chromium are exceeded, other heavy metals are not exceeded, most of the volatile phenols do not exceed the standard, only two samples exceeded the standard, exceeded the standard were 1.8, 0.6 times, relatively speaking, chemical oxygen demand and ammonia nitrogen exceeded the rate of a little bit larger. Chloride exceeded the most serious, exceeding the rate of 63%, followed by sulfate, sulfate more than half of the cross-section exceeded the standard, followed by nitrate and total phosphorus, fluoride all do not exceed the standard.
Table 3-9 shows the monitoring results of harmful substances in ground water of Changqing Oilfield Company's Ansai Oilfield Development Zone in 2006 and 2007. The most serious environmental pollution is petroleum, the maximum exceeded 32 times, sulfide maximum exceeded 120 times, volatile phenol maximum exceeded 4.2 times, COD maximum exceeded 1.71 times, BOD5 maximum exceeded 5.23 times. The serious exceedance locations are mainly in Wangyao Reservoir and Fengzhuang upstream of Xingzi River. From Table 3-9, it can be seen that the monitoring data in August 2007 exceeded the standard higher than the value of April 2006 monitoring data.
Table 3-9 Monitoring Results of Harmful Substances in Surface Water in Ansai Oilfield Area of Changqing Oilfield Company Unit:mg-L-1
3. Impacts on Groundwater
The groundwater in Ordos Basin is buried deeper, and in combination with the above characteristics of pollution of the soil and surface water bodies, the fallout of crude oil and petroleum wastewater have no impacts on groundwater, and the impacts of oil The impact of development on groundwater is mainly the impact of injection wells on groundwater, which is mainly in the process of oil development, a large number of skimming groundwater, changing the groundwater environment.
(1) Groundwater pollution status
In Longdong oil area, the groundwater of each major oilfield block makes the indicators in groundwater exceed the standard seriously due to oil extraction activities (Table 3-10). The most serious exceedance of ammonia nitrogen in the groundwater of Maling oilfield, the monitoring results of all exceeded the standard, hexavalent chromium 6 monitoring sites in 5 exceeded the standard or close to the standard value; chloride has also exceeded the standard phenomenon. There is one serious exceedance of coliform indicator in the groundwater of Huachi Oilfield, and COD exceeds or is close to the standard value in each point. Ammonia nitrogen, hexavalent chromium, chloride, total bacterial count, coliform bacteria in the groundwater of Fanjiachuan Oilfield all exceeded the standard, among which, coliform bacteria pollution is the most serious; in addition, fluoride also exceeded the standard. Overall, the water quality is poor and not suitable for human consumption. These contaminations have a great deal to do with oil development, but there are also other contaminating factors.
Table 3-10 Groundwater Quality Indicators in Longdong Oilfield Unit:mg-L-1
Overall, the main pollutants in the groundwater of Longdong Oilfield are COD, 56.25% exceeding the national standard of Class III, followed by chloride, 31.43mg/L; pH does not exceed the national standard of Class III; petroleum is undetected in all; and mineralization ranges from 452.5% to 440.0%, with fluoride exceeding the national standard. Mineralization varies from 452.67 to 15736.00mg/L.
Northern Shaanxi, petroleum, hexavalent chromium, chloride, nitrate, sulfate partially exceeded the standard, the rest of the test items are not exceeded; individual areas of petroleum exceeded the standard more than ten times, some of the well water and spring water hexavalent chromium exceeded the standard, is not very serious; some of the samples exceeded the standard of chloride is more serious, the highest exceeded the standard 500 times. Nitrate has 1 well water sample exceeded the standard. Spring water has a larger pH value, well water is second, the oil layer water is the smallest (Table 3-11).
Table 3-11 Comparison of TDS, hardness, and chloride ion content of formation water and river water in northern Shaanxi
Continuation of the table
The mineralization, hardness, and chloride ions of the groundwater in each place were analyzed in comparison with those of its surface water in order to reveal the interrelation of the surface water with the groundwater. The river water samples selected in the table were chosen according to the location of the sample point of the stratum water, in the vicinity of the stratum water. Comparison of the selected well water and spring water with the corresponding river water shows that the TDS, hardness, and chlorine ions of the well water are lower than that of the river water, and it seems that the quality of the groundwater is better than that of the surface water in the same area from the other indexes, which is consistent with the fact that the local residents basically drink the groundwater as found out in the investigation.
Shanxi Jingbian Ansai oil field is located in the upper reaches of the Dali River. From 1990 to 2006, nearly 1,000 oil wells have been drilled within the 215km2 of Jingbian Qingyang Fork, resulting in leakage of shallow groundwater, overflow of deep, highly saline water, and the depletion of groundwater resources, coupled with chaotic civilian mining and indiscriminate honeycomb mining, which has resulted in the pollution of the oil layer and the water layer by mutual infiltration, with 80% of the wells drying up, and some of them producing bitter water quality. The water quality of the wells that produce water is bitter and undrinkable.
(2)Analysis of the impact of injection wells on groundwater
Taking the Longdong area as an example, at present, the Longdong Oilfield*** there are seven extracted water treatment plant, extracted water is treated and injected back into the stratum, the main process is: sedimentation tanks dewatering water - de-oil tanks de-oil - filtration - flocculation - sterilization - return to the stratum, the main process is: settling tanks dewatering - oil removal tanks de-oil - filtration - flocculation - sterilization - return to the stratum. flocculation - sterilization - reinjection.
The stratum where the wastewater is reinjected is the Zhi Luo Formation (depth below 1000m). There are several layers of thick muddy siltstone and mudstone and other weakly permeable or impermeable layers, through the upper and lower rock layers of the water-conducting structure is extremely undeveloped, it is unlikely to break through the impermeable layer of re-injection of water to the upper layers of transport and infiltration, and even less likely to enter the submerged layer and surface water. At the same time, the sandstone layer of Zhi Luo group has large porosity (19% to 22%), large water capacity, with the injection well as the base point, within the radius of 500m, only according to the average thickness of sandstone in the injection section of 30m (sandstone layer of Zhi Luo group is as thick as 200-340m), the calculation of the pore volume of about 5 million m3 hours. It can be seen that the choice of Zhi Luo group as a re-injection layer is reasonable and feasible, in the pressure-driven extracted water re-injection of Zhi Luo group strata, it is unlikely to break through the multi-layer water barrier and contamination of groundwater.
The extracted water must be processed before reinjection to reach the "groundwater quality standards" (GB/T14848-1993) Ⅲ standard value, so that there is no significant difference with the deep pressurized water quality, some components are also lower than the underground pressurized water quality, so it is unlikely to have an adverse effect on the deep pressurized water. In addition, the water injection of water is with the extraction of crude oil from the deep stratum, after the crude oil dewatering process, its volume is much smaller than the volume of water when mining crude oil, and then injected back into the deep stratum of the operating area, is conducive to the filling of the crude oil extraction zone, is unlikely to cause changes in the hydrogeological and engineering geological conditions.
However, the treated water generally contains high mineralization and hardness, and contains a certain amount of DO, H2S, CO2, sulfate-reducing bacteria and saprophytes. Therefore, it is easy to produce precipitation in the process of reinjection and block the wastewater treatment system and pore space of the formation, resulting in poor water injection, and in serious cases, it is easy to cause the flow back of extracted water to pollute surface water and underground diving.DO, H2S, CO2 and anaerobic bacteria may also cause corrosion of the wastewater treatment system and pipeline perforation, and it is also possible to make the extracted water leakage to the non-injection layer, resulting in groundwater contamination.
Through the field investigation, the Ordos Basin in the process of oil extraction, the amount of treated sewage as reinjection water is actually very small, most of the reinjection water is still the oil extraction department by buying local freshwater resources (TDS content less than 1.5mg/L) for reinjection, the basin needs a large amount of reinjection water, so that a large number of the local extremely valuable freshwater resources.
4. Impact on vegetation
Petroleum exploration and development is the process of continuous understanding and development of stratum oil reservoirs, not only expanding the scope of human activities, but also make the original no one to reach or difficult to enter the region into the reachable and easy to enter, especially in ecologically fragile areas, for the loess hills gully area, the Gobi sandy area, shrubs, artemisia grass in maintaining the balance of the ecosystem of the region has a very important role, and the surface of the ground is the most important part of the ecosystem. has a very important role, and the destruction of vegetation caused by surface stripping is difficult to recover in a short time. From the composition of the land, the well field, station (station) on the vegetation is a point impact, roads, gathering pipeline is a linear impact, linear impact is much greater than the point impact; from the way of the land, temporary land vegetation can be taken artificially and naturally restored, the permanent land is completely replaced by artificial ecosystems, although the artificial planting of trees and grasses, the vegetation coverage increased, but may cause genetic homogenization, ecosystem function is weakened.
Pollutants from the oil production process also have an impact on the vegetation resources growing on the soil, and pollutants exceeding the pollution tolerance threshold and adaptability of plants will lead to the deterioration of local fragile ecosystems. For the desert Gobi beach vegetation, the natural renewal is very slow, and not easy to recover. Generally speaking, the crude oil produced in the process of oil extraction and oil testing accumulates within 1m of the surface, and the oil content in the soil below 1m is very small, so it will not pollute the surface water layer, and basically does not have any impact on the regional groundwater. After the wastewater and alcohol-containing wastewater generated in the oilfield are specially collected and treated to meet the standards, all of them are reinjected into the Ordovician system without being discharged, except for part of the domestic wastewater used for greening.
Likewise, there is no impact on vegetation as the oil transportation is a closed underground pipeline transportation. When the crude oil leaks, under the action of the pipeline pressure, the crude oil erupts, coupled with the natural wind influence, the crude oil spattered on the surface of the surrounding plants, directly resulting in plant contamination, the situation is serious, resulting in the exhaustion of plants, death. The greater the pressure of oil transmission, the wider the spattering range, the more serious the pollution.
Three, the impact of geological and environmental issues on oil development
Oil exploitation damages the production environment, increases production costs, and triggers conflicts between the residents of the production area and the production units. The construction of oilfield roads and pipelines, the direction of the mountainous areas to the flood has a certain blocking effect, the water through the natural gullies flowing down, while the road and pipeline play a certain role in blocking and convergence, changing the direction of flood flow, the formation of local sections of larger floods, which will produce new water erosion. The polluted water with high mineralization will definitely accelerate this water erosion and shorten the service life of oil pipelines and so on.
Based on the characteristics of oil production and transportation (pipeline), it will not cause large and obvious geological problems (subsidence, landslides, mudslides, desertification) like coal mining, and it will not cause serious accidents (such as collapses) resulting in human and property losses. Its harm to the geological environment is relatively moderate (compared to coal resource extraction). However, its impact on water, soil, gas and crops will certainly jeopardize the originally harmonious ecological environment and cause strong dissatisfaction among local residents. In the absence of economic benefits for the local government and residents, the process of oil extraction and refining will be difficult, such as the acquisition of land for construction and the hiring of labor. These will directly slow down or even stop the smooth progress of production, thus increasing the cost of production; in addition, oil extraction and production caused by the loss of local land and water resources, seriously affecting the survival of local residents, in turn, the local people in order to regain their own land and water resources, hindering the oil sector of the extraction activities.