The Dynamical Evolution Of Natural Gas In Upper Paleozoic Of Ordos Basin

The paper analyzed the characteristics of sandstone and mudstone and the changes in the pore volume sizes during compression and rebound of Upper Paleozoic gas reservoirs in Ordos Basin, and carried out the experiment of water and gas under the differencer pressure migration, gas migration in the pores-cracks in dual-medium model, gas migration in sandy sediments and the experiment of gas migration in the reduced pressure condition and gas migration in the cracks mode. Studied the dynamical evolution of natural gas reservoir after drilled and the characteristics of gas migration. The conclusions are follows:(1) Generally, the axial compression coefficient of Sandstone in different pressure changes is from6.97×10-5to23.19×10-5mm/mm·MPa-1, the axial rebound coefficient is from6.34×10-5to15.05×10-5mm/mm·MPa-1, Corresponding, the pore volume compression coefficient is from2.67×10-4to96.11×10-4MPa-1, the pore volume rebound coefficient is from2.67×10-4to56.01×10-4MPa-1.The axial compression coefficient of mudstone in different pressure changes is from2.79×10-5to2.90×10-5mm/mm·MPa-1, the axial rebound coefficient is from1.86×10-5to2.95×10-5mm/mm·MPa-1.(2) The pressure decreased, as the gas reservoirs are drilled, then the rebound and expansion of the sandstone, mudstone and fluid in gas reservoir become the main driving forces which help gases move uphill. In differential pressure-driven, the natural gas migrated and accumulated at the same time, which made plenty residual gas remain in the formation. At buoyancy-driven, Gas migration is limited in sandstone, gas can only partially floating though cracks developed. Therefore, the differential pressure-driven and buoyancy-driven play weak roles in natural gas exploitation, the main forces come from the elastic energy of the rebound and expansion of skeleton and fluid in gas reservoirs.