Heterogeneity effect on non-wetting phase trapping in strong water drive gas reservoirs

摘要

In strong water drive gas reservoirs (WDGR), the presence of entrance water in the gas zone has negative effects on the relative permeability; therefore, gas is trapped behind the water front as a non-wetting phase. Understanding WDGR could be complicated and depends on both the petrophysical and operational parameters, such as, reservoir heterogeneity, permeability, production rate and so on. In order to quantify the uncertainty associated with reservoirs, it is critical to create porous media models that incorporate stratigraphic details. In the present study, experimental models were used to simulate WDGR and describe the heterogeneity effect on residual gas saturation and the recovery factor. In models, distinct gas and water (aquifer portion) zones were designed, wherein the ratio of the permeability of the aquifer to the gas zone was varied over three ranges. All tests were conducted in the presence of connate water, and the main WDGR set-up was constructed for high pressure operational conditions. All porous media were characterized by Dykstra—Parsons coefficient as heterogeneity index. The results demonstrate that the residual gas saturation depends on both heterogeneity index and permeability ratio. Results reveal that heterogeneity is not always detrimental to gas recovery. In addition, when the ratio of the aquifer to gas zone permeability is less than one, the amount of trapped gas reduces as the heterogeneity of the porous media increases and consequently, the recovery factor may be improved.