Utilization And Water-salt Migration Characteristic In The Shallow Saline Water Under Horizontal Well Mining Conditions

The shortage of soil and water resources have become one of the important bottlenecks and restricted the economic development in the coastal area of North China.Meanwhile,there are a large idled resources with shallow saline groundwater and saline-alkaline soils.It has become an important issue of dealing with the local soil-water resources crisis and protecting the ecological environment to improve the saline-alkali land and to develop and utilize shallow saline water resources economically and effectively.The traditional method of salt reduction by drainage has many shortcomings such as large amount of work,low efficiency and so on.As the non-excavating horizontal directional drilling technique developed,horizontal well technology provides a new means and method for salt reduction by drainage in coastal areas.However,how to design reasonable engineering parameters and evaluate the feasibility to reduce salt by horizontal well drainage has become critical problem to popularize the method and improve the development and utilization efficiency of shallow salt water.Based on this,this paper selected the eastern coastal area of North China as the research area,and the soil salinity and shallow saline water as the research object.Meanwhile,on the basis of finding out distribution characteristics and genesis of the soil salinization and the shallow groundwater salinization in the research area.This paper studys movement of water and salt in the shallow saline aquifer by combing field horizontal well pumping test with numerical simulation method.Analyses dynamic characteristics of salt migration and desalination efficiency of brackish water under artificial controlling measures;Assesses the feasibility of salt water exploitation with the horizontal well technology;And put forward suitable development and utilization zoning of shallow salt water.Through this research,the main contents and results have been achieved:1.The soil salinization presented obviously zonation in the study area.The mild-salinization degree was found with the increase in distance from the coastline and the soil salinity increases remarkably with the depth increasing.In general,soil salt contents are significantly affected by depth and TDS of groundwater.Besides,the hydrochemical characteristic of shallow groundwater in the study area are consistent with the evolution law of soil salinization degree.The hydrochemical type of mild salinization area is primarily sulphate and halide,with a TDS less than 3g/L,and groundwater salinity originate from mineral dissolution in the local marine sedimentary.While hydrochemical type in moderate to severe salinization areas is mainly halide,with a TDS less than 3g/L,and the salt content of groundwater is directly affected by seawater intrusion.2.The recharge process can be divided into three stages through field horizontal well pumping test.The initial stage:the discharge of horizontal well are the storage capacity in the wellbore;the transitional stage:the discharge of horizontal well are mainly seepage from the upper phreatic aquifer;stabilization stage:the discharge of horizontal well are mainly from 70%of confined waters and 30%of phreatic waters.In addition,the horizontal well pumping formed the landing funnel with in shape of“tub”,The maximum drawdown was located in the middle of the filter pipe and related to the pumping point.The drawdown along the extension direction of horizontal well is obviously greater than that in the vertical direction.After pumping for a long time,the groundwater flow field in the test area changed apparently,the water table of each aquifer decreased obviously,and the salt content of each aquifer had a certain degree of decline.3.The numerical simulation results indicated that comprehensive recharge was influenced by the depth of water table and displayed a nonlinear relationship in the test area.The minimum depth of water table was 2-3m with the phreatic water recharge change from negative to positive on different hydrology-year.Based on this,combined with field surveys,it determined reasonable water table of 2.5m for drainage and desalination with horizontal well.The calculated results indicated that the groundwater level in the test area can be effectively reduced by continuous and intermittent operation condition,except for the case of rainy years or minimal pumping volume with the hydrus-modflow coupling model.When the well discharge is 1.0m~3/d·m,the control range of single horizontal well is 300-800m,and that of three horizontal wells is 800-1800m.Meanwhile,the control range of horizontal well is related to the pumping intensity,the depth of the horizontal section and the length of filter pipe.Therefore,except for extremely arid weather conditions,horizontal well drainage reduced the solute concentration in groundwater;the relative desalting efficiency is 4.25%-18.17%in the phreatic water,the degree of which depends on the infiltration capacity of leached water and the solute concentration of infiltrated water;While the relative desalting efficiency is3.93%-8.13%in the confined water,and the salt removal efficiency is related to the discharge of horizontal wells.4.Based on analysis of the hydrogeological and engineering technical requirements of exploitation of salt groundwater from horizontal well,it is showed that the horizontal well should be set in the circumstances where the depth of water table is from 3m to10m and the aquifer depth is from 5-30m,Meanwhile,it is suitable for aquifers with low permeability,the lithology of which could be silt,silty-fine with clay.Compared to the traditional methods of salt reduction by tube well drainage,the economic cost can be saved by about 19.2%.Meanwhile,through the analysis of mining technical conditions,saline water utilization mode,appropriate well type and other conditions in the study area,the development and utilization mode of shallow saline water are divided into three categories:(1)scattered extracting areas of agricultural irrigation;(2)small-scale utilization areas of agricultural irrigation and saline water desalination;(3)centralized extracting area for industrial water,urban greening and breeding.The research results are of great practical significance for enriching the percolation theory of horizontal well,improving the salt reduction technology by drainage and rational development and utilization of shallow saline water resources.