Material balance method and classification of non-uniform water invasion mode for gas reservoirs with water considering the effect of water sealed gas

In order to clarify the characteristics of non-uniform water invasion in water-bearing gas reservoirs, it is necessary to introduce the non-uniformity coefficient (A) and water invasion constant (B) to characterize the non-uniformity degree of reservoir physical properties and the activity degree of peripheral water, respectively, based on the dual mechanism of water invasion to recharge the formation energy and seal off the gas in the reservoir. Then, the material balance method considering the phenomenon of water sealed gas was established. On this basis, the water invasion characteristic curve chart of water-bearing gas reservoirs was plotted, and the non-uniform water invasion mode was classified based on the example gas reservoir. And the following research results were obtained. First, in the water invasion characteristic curve chart of water-bearing gas reservoirs which is plotted based on the material balance method considering the influence of water sealed gas, the upper right area and the lower left area are defined as recharge area and seal area, respectively. By taking A=0 and B=2 as the boundary, the recharge area is divided into strong recharge area and weak recharge area. By taking A=2 and B=2 as the boundary, the seal area is divided into strong seal area and weak seal area. And correspondingly there are four water invasion modes, i.e., strong recharge, weak recharge, weak seal and strong seal. Second, for fractured gas reservoirs, the non-uniformity degree of reservoir physical properties is high, and water sealed gas can be formed easily after water invasion. The dimensionless relative pseudo-pressure data of this type of gas reservoir is located in the seal area of the water invasion characteristic curve chart. Third, for the gas reservoirs whose reservoir physical properties are relatively uniform, the dimensionless relative pseudo-pressure data is located in the recharge area of the water invasion characteristic curve chart, and the recharge effect of water invasion on formation energy is greater than the weakening effect of water sealed gas on formation energy. Fourth, with the increase of A, the non-uniformity degree of reservoir physical properties increases, the water invasion characteristic curve shifts from the upper right to the lower left, and the recovery factor of gas reservoir decreases continuously. With the increase of B, the recharge effect of water invasion on formation energy and the weakening effect of water sealed gas on formation energy are both weakened, the distribution range of water invasion characteristic curve narrows to the recharge/seal boundary, and the corresponding range of gas reservoir recovery factor also narrows. © 2021, Natural Gas Industry Journal Agency. All right reserved.