Natural Gas Technology - Efficiency of Gas Displacement from Porous Media by Liquid Flooding

Flow tests on small core plugs have indicated that a large amount of gas is trapped and not recovered by water flooding a gas sand. Instead of I to 15 per cent pore space, as is usually assumed, the residual gas saturation is 15 to 50 per cent pore space, and is thus of the same magnitude as residual oil after water flooding oil sands. A thorough investigation was made to ascertain that large amounts of residual gas actually remain in reservoirs after a water flood and that this condition is not merely a laboratory phenomenon. In field experiments, the amount of gas left in a watered-out gas sand was measured by use of a pressure core barrel and the residual gas saturation of two watered-out gas sands was determined by electric log evaluation. In the laboratory, an investigation was made of factors that could possibly cause the value of residual gas saturation as measured on small core plugs to differ from that in the reservoir, and the effect of these factors on the amount of residual gas saturation was studied. The factors studied include flooding rate, static pressure, temperature, sample size and saturation conditions before flooding All evidence established that a relatively high gas saturation is trapped in water flooded gas sands and that this residual gas saturation can be measured in the laboratory by tests on small core plugs. INTRODUCTION There. has been general agreement among engineers that very high recovery of gas could be obtained from natural reservoirs By water displacement. Gas recoveries of 80 to 95 per cent of the original gas in place have become the normal expectation in water drive fields. The assumption of high recovery has been based on: 1. low density and viscosity of gas compared with water; 2. the erroneous assumption that the flow relationship in a gas-liquid system where gas is the displaced phase will be the same as when it is the displacing phase. 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 being displaced by gas. By assuming the reversibility of this process, the conclusion was reached that the residual gas saturation following water flooding of a gas reservoir would be the qame (1 to 15 per cent) as that at which gas first flowed continuously as a displacing phase. Recent laboratory relative permeability Studies have demonstrated that the flow characteristics are very different in gas-liquid systems, depending on whether gas is displacing or being displaced by, a liquid. Also, it has been shown that there is no difference between the flow characteristics of oil and water or gas and water in water wet porous rocks. The residual gas saturation that can be expected following water flooding of a gas reservoir then would be in the same range as the residual oil saturation normally expected after water flooding an oil reservoir! i.e., in the range of 15 to 50 per cent pore space, depending on the rock characteristics. Obviously, such a difference in residual gas saturation means very important differences in recoverable gas reserves from water drive reservoirs. For example, if the original gas saturation in a field were 70 per cent, and the residual gas following flooding were 35 per cent, only half of the gas in place could be recovered by complete water drive, compared to the previously expected 80 to 95 per cent. This is a situation in which complete pressure maintenance could result in very greatly reduced recovery, since straight pressure depletion recoveries from gas reservoirs can approach 80 to 90 per cent. Such a change in thinking must be based on more complete information than a series of small core tests. Hence the work reported herein was undertaken with the objective of determining whether or not residual gas saturations indicated from small core relative permeability tests at atmospheric pressure and room temperature are representative of the residual gas saturations which could be expected after water flood of natural reservoirs. A study was made both through laboratory and fields tests to determine any differences in residual saturation which might be occasioned by differences in pressure, temperature, rate of flooding, and original saturation conditions. Four separate types of laboratory experiments and two field mesurements were made in this investigation. The apparatus and testing methods of each will he discussed individually. LABORATORY EXPERIMENTS, APPARATUS AND PROCEDURE Relative Permeability Tests Steady state flow experiments Here conducted using the Penn State type apparatus. The equipment used has beer (described in a previous publication? "Irreducible" water satu-ration was established in the core' by imposing ,a capillary pressure of 45 psi before simultaneous flow of air and water