| Shallow natural gas as a natural resource is one of the hot issues in natural gas exploration at home and abroad,at the same time,in the process of engineering unloading and release,shallow gas will change the mechanical structure of geological bodies and induce engineering accidents.During the construction of the Nali Bridge project,it encountered shallow gas poor geology,which was the first time in Guangxi Province.Based on the project"Research Report on the Investigation and Countermeasures of Shallow Gas Geological Hazards of Nali Bridge on Gaoling to Shanzhai Secondary Highway in Guangxi Province",this paper analyzes the lithology and distribution characteristics of shallow gas stratigraphy and shallow gas in the bridge site area through geological exploration.At the same time,samples of gases and rocks in the exploration holes are collected,and the gas composition analysis and organic matter content tests are carried out on the collected samples,and the gas types and formation hydrocarbon generation capacity are studied.Evaluation of mudstone cover and reservoir characteristics by high-density electrometry and scanning electron microscopy.Finally,on the basis of exploring the geological storage characteristics of shallow gas in the study area,the influence of the flow rate of the engineering advanced exhaust wellhead on the water and gas movement pattern and flooding time of the project was studied by numerical calculation,and the layout of the engineering advanced exhaust well position was proposed.The main research contents and conclusions of this paper are as follows:（1）Through on-site geological drilling and gas concentration measurement,the lithology and shallow gas distribution characteristics of the formation are obtained.The deep mudstone layer is both a gas layer and a cover layer,the gas is mainly stored in the silty mudstone and sandstone thin layer,the shallow gas distribution is uneven and the size of each reservoir is not large,and the main reservoir section is 25 to 50m,which is a conventional pressure gas reservoir.（2）Through the analysis of gas components by sampling gas at the bridge site,the main component of shallow gas is methane,accounting for an average of 93.46%,and the heavy hydrocarbon content is less,accounting for 0.36%.The results of natural gas isotope analysis show that the natural gas type in the area is continental methane-type biogenetic gas.The organic matter type analysis of the mudstones in the study area was 1.24%on average,and the average hydrocarbon potential was 5.45mg/g,and the correlation between the two was good,and the kerogen was mainly based on type II.1-II.2,and the abundance of organic matter was high.The reservoir kerogen mirror plasmography Ro is less than0.6%,which is judged to be a low-maturity stage methane-type biogas.（3）Through the analysis of the cross-sectional graph of the high-density apparent resistivity inversion result,the high apparent resistivity value area was caused by the high silty content,and the low apparent resistivity area was caused by the high slurry content,and the inversion result was consistent with the known drilling data in the Ningming Basin.The mudstone of the east side of the bridge lacks a good cover layer,so there is less shallow gas in the depth range of the bridge pile.The strata on the west side of the bridge are covered with a fourth-series silty clay layer,and the shallow layer gas distribution range is wide.The measured porosity of the mudstone cover layer is 22.8%-34%,the permeability（2.4-22.2）×10-3μm2,and the sealing ability is average.The measured porosity of the silt mudstone reservoir is about 25.6%,and the permeability is about37.2～46.8μm2,which is poorly connected as a reservoir and it is difficult to form a large-scale gas reservoir.（4）The bridge pile of Nali Bridge is constructed in the form of drilled pouring piles,and the shallow gas on the pile foundation in the early stage of piling produces gas intrusion,destroying the integrity of the pile foundation,so that the pile quality of the borehole pouring pile does not meet the specification requirements of the first type of pile.Through numerical calculation,it is found that the flow rate of the wellhead of the exhaust well has a significant impact on the water and gas transport form and the flooding time of the gas storage reservoir,and the exhaust gas velocity of the wellhead must be controlled when the engineering is advanced. |
