The Influence Evaluation Of Gas-field Exploitation On Groundwater Environment In South Sulige

Sulige gas field is the largest natural gas geological reserves of gas in China, which has great potential for development and has become another important energy base in the Ordos Basin. As a result of locating in the hinterland of Maowusu desert, groundwater in the south of Sulige gas field become the main water source used for gas production, agriculture and living water. However, during the normal operation of gas exploitation, waste water, mud and other pollutants are discharged into the mud pools. If the mud pools were not impermeable, pollutants will infiltrate into soil and pollute groundwater. Gas-field exploitation requires mining a large number of groundwater for drilling, fracturing, washing well and so on. Excessive exploitation leads to aquifer dewatering and cone depression, and these results in the tension of water supply and degradation of the ecological environment in gas field. In addition, the characteristic of gas-field exploitation is rolling exploitation and regional exploitation. Once some accidents occur during exploiting process, such as blowout, corrosion, leakage and blocking of pipelines, there will be tremendous influence on the whole regional environment especially the regional groundwater environment. Therefore, evaluating the influence of groundwater environment, which is generated by the exploitation, has great significance for reasonable construction of gas fields and production of groundwater environment in gas field. At the same time it also becomes key issues of construction and management of gas fields.This paper firstly carries out engineering analysis of gas-field construction and investigates geological and hydrogeological conditions. Secondly, the paper carries on groundwater quality present evaluation and analysis the main influencing factors of groundwater environment caused by gas-field exploitation. At last on the basis of referencing regional groundwater environmental impact assessment, this paper builds up the index system of groundwater environmental impact assessment, and uses the theory of sustainable development to compute and analyze. The main conclusions are as follows:1. Using existing data and engineering analysis materials, this study calculates and gathers the quantity of process water and production volume of sewage in all stages of gas-field exploitation.In exploration and construction periods, drilling wastewater, cuttings, waste mud, fracturing wastewater, living sewage and refuse are the main pollutants. In operation period, pollutants come from regular cleaning wells, waste generated in pipeline, living sewage and solid waste in gas gathering station. In retired period pollutants include a little living sewage and pipeline corrosion material. By calculation and analysis, the result is that the more influence on groundwater environment is drilling, constructing well sit and laying gas pipeline in exploration and construction periods, the next activities are gas gathering, transportation, equipment cleaning up, maintenance in operation period and well disposal in retired period.2. According to the indoor testing result of the collected groundwater samples in study area, groundwater chemical distribution characteristics are analyzed. In addition, groundwater quality is carried out present evaluation.According to the monitoring programs in quality standard for ground water (GB/T 14848-93) and 29 groups of groundwater samples in laboratory, we find that coliform, nitrite nitrogen, ammonia nitrogen, nitrate nitrogen, manganese, iron and other metal ions are in groundwater in study area. 13 indicators are selected, which include total hardness, total dissolved solids, dissolved oxygen (DO), Cr+6, Cl-, F-, ammonia nitrogen, nitrite nitrogen, nitrate nitrogen, volatile phenol, S2-, CODMn, and oil,. Fuzzy comprehensive evaluation method is used to evaluate groundwater quality. The evaluation result illustrates that groundwater quality is in the middle and southwestern area. In the center western study area bayanhurihu-zhulatu and northeastern region shaerligesumu groundwater quality reaches third standard. The worst area is accounted for 5% of the whole region, which is distributed in the middle western region ,such as Su-22 well (GW28), sunan-1(GW19), sunan-3(GW21), sunan-4(GW22) and these wells'mud pool (GW23, GW24, GW26, GW27). According to the result of comprehensive evaluation, groundwater quality of study area reaches the second standard, but there is poor groundwater quality in some areas. The main reason for this is that agricultural production activities use excessive chemical fertilizers and pesticides, which increases the content of triple-nitrogen in groudwater. At the same time, waste liquid and waste mud on the ground is not handled on time and infiltrates into the ground, which leads to increase the content of heavy metals and organic components and deteriorating groundwater quality.3. The evaluation index system of groundwater environment influence caused by gas-field exploitation is constituted by the methods of regional environmental impact assessment and the theory of sustainable utilization. In addition calculation and analysis is done. According to the evaluation method of sustainable use utilized in regional environmental impact assessment and combining with existing information, the level of groundwater sustainable use is determined as the target layer. Physical geography factors, gas field development project factors and environmental factors are the criteria layer. The precipitation infiltration coefficient and other eight indicators are the indicator layer. The method of multiple indicators and linear weighting function is used to calculate the composite index of groundwater sustainable use. AHP and entropy determine the weight of every indicator, and at last calculate the composite index of four stages is 0.2965, 0.1504, 0.2747, 0.2783 and 0.4644, 0.1517, 0.2417, 0.2422. The result is showing that groundwater environmental is graet influenced in construction period, which makes the environment for sustainable development of groundwater decline. Supervision and protective measure should be strenthen during the construction period.