Flow tests on small core plugs have indicated that a large amount of gas istrapped and not recovered by water flooding a gas sand. Instead of 1 to 15 percent pore space, as is usually assumed, the residual gas saturation is 15 to 50per cent pore space, and is thus of the same magnitude as residual oil afterwater flooding oil sands.
A thorough investigation was made to ascertain that large amounts ofresidual gas actually remain in reservoirs after a water flood and that thiscondition is not merely a laboratory phenomenon. In field experiments, theamount of gas left in a watered-out gas sand was measured by use of a pressurecore barrel and the residual gas saturation of two watered-out gas sands wasdetermined by electric log evaluation. In the laboratory, an investigation wasmade of factors that could possibly cause the value of residual gas saturationas measured on small core plugs to differ from that in the reservoir, and theeffect of these factors on the amount of residual gas saturation was studied.The factors studied include flooding rate, static pressure; temperature, samplesize and saturation conditions before flooding. All evidence established that arelatively high gas saturation is trapped in water flooded gas sands and thatthis residual gas saturation can be measured in the laboratory by tests onsmall core plugs.
There has been general agreement among engineers that very high recovery ofgas could be obtained from natural reservoirs by water displacement. Gasrecoveries of 80 to 95 per cent of the original gas in place have become thenormal expectation in water drive fields. The assumption of high recovery hasbeen based on:
low density and viscosity of gas compared with water;
the erroneous assumption that the flow relationships in a gas-liquid systemwhere gas is the displaced phase will be the same as when it is the displacingphase.
It has long been recognized that gas can flow at very low gas saturations(in the range of 1 to 15 per cent pore space) in systems where liquid is beingdisplaced by gas. By assuming the reversibility of this process, the conclusionwas reached that the residual gas saturation following water flooding of a gasreservoir would be the same (l to 15 per cent) as that at which gas firstflowed continuously as a displacing phase.