Analytical model for gravity segregation in WAG displacement recovery of inclined stratified reservoirs

摘要

The present study deals with the extension of Stone's model to develop the analytical model of gravity segregated length (distance travelled by injected water and gas together before the complete segregation) and gravity segregated water zone height in water-alternative-gas (WAG) displacement for inclined stratified heterogeneous reservoirs. However, as per our knowledge, until now, all available analytical models of gravity segregation in the WAG displacement process consider the assumption that the reservoir is homogeneous i.e. one-layer system with constant porosity and permeability. In reality, most of the reservoirs are heterogeneous with random variation in permeability due to the variation in the depositional environments, which have a major impact on a vertical sweep in WAG displacement. The analytical models of gravity segregation for dipping stratified heterogeneous reservoir are developed by solving the fractional flow equations of displacement of fluids through MOC (method of characteristics). Then the results of these analytical models are compared with 3D finite-difference compositional reservoir simulation. The developed analytical models give appropriately similar results as 3D finite-difference compositional model. Also, analytical model results show that the complete gravity segregated length (vertical sweep efficiency) increases with an increase in reservoir dip angle and mobility ratio, and a decrease in gravity number, vertical permeability, and horizontal permeability heterogeneity. On the other hand, the gravity segregated water zone height decreases with an increase in reservoir dip angle and gravity number.