Models for gravity segregation in gas enhanced oil recovery (EOR) indicate that the distance injected gas and water travel together before complete segregation scales with the injection rate Q. Therefore, in cases where injection pressure is limiting, increasing injectivity can improve sweep efficiency. We examine several strategies. Reducing skin resulting from damage at the wellbore face directly increases volumetric sweep of gas. Even in the absence of damage at the wellbore face, most of the injection pressure is dissipated near the injection well, but most of segregation of gas and water occurs much further from the well. Therefore, if injection pressure is limited, increasing mobility near the injection well has a large impact on Q, with a direct benefit in delaying gravity segregation. There is also a relatively small increase in gravity segregation in the near-well region. An analytical model for gravity segregation in homogeneous reservoirs can be extended to a case where permeability is stimulated within a cylindrical region inside a larger cylindrical reservoir. The effect of this stimulation in increasing Q at fixed injection pressure can be estimated as well. One can increase the volume swept by gas before segregation by as much as 170%, though a large volume must be stimulated to reach this optimum.The model represents schematically a number of ways proposed in gas EOR for delaying segregation beyond the possibilities with uniform, steady coinjection of Newtonian fluids: alternate injection of gas and liquid [water alternating gas (WAG) or surfactant alternating gas (SAG) with foam]; injection of gas above water; and injection of shear-thinning foam. In all these cases, the process gives higher mobility near the well, allowing an increase in injection rate, and thereby increases the distance to the point of segregation.The model can be extended directly to the case of shear-thinning (power-law) foam. One obtains a differential equation for the segregation process, in place of the algebraic equation that results for Newtonian fluids. In the limit of extremely shear-thinning behavior, it is possible to double the distance to the point of segregation with no increase in injection pressure. The model can also be applied to foams that are shear-thinning only at high superficial velocity (i.e., near the well). Simulations fit the theoretical prediction well.

Models for gravity segregation in gas improved oil recovery (IOR) indicate that the distance injected gas and water travel together before complete segregation scales with the injection rate Q. Therefore, in cases where injection pressure is limiting, reducing skin resulting from damage at the wellbore face directly increases volumetric sweep of gas in IOR. Even in the absence of damage at the wellbore face, most of the injection pressure is dissipated near the well, but most of the segregation occurs much further from the well. Therefore, if injection pressure is limited, increasing mobility near the injection well has a large impact on Q, with a direct benefit in delaying gravity segregation. There is also a relatively small increase in gravity segregation in the near-well region. An analytical model for gravity segregation in homogeneous reservoirs can be extended to a case where permeability is stimulated within a cylindrical region inside a larger cylindrical reservoir. The effect of this stimulation in increasing Q at fixed injection pressure can be estimated as well. One can increase the volume swept by gas before segregation by as much as 170%, though a large volume must be stimulated to reach this optimum.The model represents schematically a number of ways proposed in gas IOR for delaying segregation beyond the possibilities with uniform, steady co-injection of Newtonian fluids: alternate injection of gas and liquid (WAG, or SAG with foam); injection of gas above water; and injection of shear-thinning foam. In all these cases the process gives higher mobility near the well, allowing an increase in injection rate, and thereby increases the distance to the point of segregation.The model can be extended directly to the case of shear-thinning (power-law) foam. One obtains a differential equation for the segregation process, in place of the algebraic equation that results for Newtonian fluids. In the limit of extremely shearthinning behavior, it is possible to double the distance to the point of segregation with no increase in injection pressure. Simulations fit the theoretical prediction well. SPE 112375where Q is total volumetric injection rate of gas and water, kv vertical permeability, ρw and ρg densities of water and gas, respectively, g gravitational acceleration, and λrt m the total relative mobility in the mixed zone.Using the method of characteristics, Rossen and van Duijn (2004) show that this equation is rigorously correct making only the usual assumptions of fractional-flow theory:1. The reservoir is homogeneous, although possibly anisotropic (kv ≠ kh).2. The reservoir is cylindrical with an open outer boundary, and the reservoir is confined by no-flow barriers above and below. 3. The injection well is completed over the entire height of the reservoir. 4. The region of interest is at steady state, with steady injection of fluids at volumetric rate Q and injected fractional flow of water fw=fw J . This implies that any remaining oil in the region of interest is at its residual saturation and immobi...