Underpressured and overpressured tight gas sands: I. Flow, buoyancy and limiting conditions

This paper explores the steady-state flow situation through a uniform aquifer of gas being supplied from the lower end of the aquifer, with the gas subject to buoyancy and also a pressure drive. Two groups of situations arise. Either the ratio of the buoyancy pressure to the permeable flow pressure is greater or less than unity. For low permeability ("tight" gas sands) it is shown that both underpressured and overpressured situations can occur, depending on the precise values of updip and downdip pressures. For highly permeable flows such is not the case. Limitations at the moment are the simplifications of constant cross-section for the aquifer, as well as constant permeability and constant aquifer temperature, together with the steady-state assumption. These restrictions will be lifted in the next paper in the series. Perhaps the salient point is that one can account for underpressured tight gas sands, but the extreme sensitivity of the system to the ratio of buoyancy pressure to permeable flow pressure means that it is currently extremely difficult to predict, ahead of the drill, which situation will prevail in a given basin.