| The isotope studies showed that natural gas (mainly CH4) can migrate from existing oil and gas fields and gas storage fields along abandoned oil and gas wells, fractured zones, faults (both pre-existing and those formed due to subsidence), and fractures formed due to hydraulic fracturing and acidizing. Waterflooding (secondary recovery) at pressures higher than fracturing pressures of the rocks can also result in fracturing (mostly vertical). Most petroleum engineers, however, consider faults as seals for oil and gas accumulations. True, but faults can act both as seals and avenues for gas migration.; The phenomenon of natural gas migration to the Earth's surface via faults and fractures from the oil and gas reservoirs is presented in this dissertation. This migrating gas becomes a potential health hazard and safety risk especially in the urban areas. Some background knowledge of the mechanics and paths of gas migration, land subsidence, and seismic activity is also presented. The preliminary study of gas migration to the Earth's surface and its impact on the environment was conducted since Spring of 2001 at the Rudolf W. Gunnerman Energy and Environment Laboratory at U.S.C. The gas fingerprinting (determination of gas origin) is also discussed.; The isotope ratios of gas samples analyzed (total of 42) indicate that gas has migrated (mainly along the faults and fractures) to the Earth's surface from the underlying oilfields or gas storage reservoirs. Thus, the gas is of thermogenic origin! Admixture of biogenic gases is also examined in four cases studied. |
