Downhole geochemistry technique
Gas geochemical studies during engineering and survey works
Gas geochemical studies as part of engineering and environmental surveys are carried out in areas where bulk soils are spread with an admixture of construction, industrial waste and household waste (areas of unauthorized household dumps), swampy areas with a thickness of more than 2.0-2.5 m. Also, gas geochemical surveys are performed at unloading areas natural gases, wetlands.
The main danger of using bulk soils is associated with their ability to generate biogas, consisting of flammable and toxic components. The main ones are methane and carbon dioxide; as impurities are present: heavy hydrocarbon gases, nitrogen oxides, ammonia, carbon monoxide, hydrogen sulfide, molecular hydrogen, etc. Biogas is formed during the decomposition of “household” organic matter as a result of the life of anaerobic microflora in the soil at a depth of more than 2.0-2.5 m. In the upper aerated layers of soil strata, aerobic oxidation of organic matter and biogas products occurs.
Biogas is sorbed by the enclosing bulk soils and deposits of natural genesis, dissolves in groundwater and “verkhovodka” and dissipates into the surface atmosphere.
During construction on fill soils, there is a danger of biogas accumulation in technical undergrounds of buildings and utilities to fire and explosive concentrations of methane (5-15% at O2 12.1%) 1 or to toxic concentrations (above the MPC) of individual components.
To assess the degree of gas-geochemical hazard of bulk soils, to determine the possibility and conditions of using this territory for construction, as well as to develop a system of measures to protect buildings from biogas and ensure environmentally friendly living conditions for the population, the following are carried out:
- various types of surface gas surveys (blasthole, emission), accompanied by sampling of ground air and surface atmosphere;
- borehole gas geochemical studies (with layer-by-layer sampling of ground air, soil, groundwater);
- laboratory studies of the component composition of free ground air, gas phase of soils, dissolved gases and biogas dissipating into the surface atmosphere.
Based on the study of the surface and deep structure of the gas field, the gas-geochemical zoning of the territory is carried out – the allocation of zones of different degrees of danger in the soil massif.