Mass Transfer From Bypassed Zones During Gas Injection
- J.E. Burger (University of Houston) | K.K. Mohanty (University of Houston)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- May 1997
- Document Type
- Journal Paper
- 124 - 130
- 1997. Society of Petroleum Engineers
- 1.6.9 Coring, Fishing, 5.4.1 Waterflooding, 4.6 Natural Gas, 5.4.9 Miscible Methods, 5.4 Enhanced Recovery, 2.4.3 Sand/Solids Control, 5.3.2 Multiphase Flow, 4.3.4 Scale, 5.3.1 Flow in Porous Media, 5.8.6 Naturally Fractured Reservoir, 5.2.1 Phase Behavior and PVT Measurements, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 5.4.2 Gas Injection Methods
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Gasflooding in oil reservoirs leads to bypassing of the oil due to gravitational, viscous and/or heterogeneity effects. The bypassed oil can be recovered by the flowing solvent by pressure-driven, gravity-driven, dispersion/diffusion-driven and capillarity-driven crossflow/mass transfer. It is difficult to represent all of these mechanisms explicitly in large-scale simulations. They need to be quantified at the laboratory scale, scaled-up appropriately and represented in large-scale simulations as added empirical terms e.g., cross-flow terms in dual-porosity models. In this work, we have studied the effect of the orientation of the bypassed region and the enrichment of the solvent on the mass transfer. Laboratory-scale mass transfer and coreflood experiments were conducted. Numerical simulation was used to identify the role of the different mechanisms. Results indicate that the mass transfer is the least for the vertical orientation, intermediate for the inverted orientation and the highest for the horizontal orientation. The mass transfer increases with enrichment for all orientations. Liquid phase diffusion controls vertical orientation mass transfer for the fluids studied. Phase behavior determines the liquid phase saturation. Capillary pumping does not contribute to the mass transfer of oil because the interfacial tension decreases towards the flowing region. Gravity-driven flow contributes the most to the mass transfer in the horizontal and the inverted orientations. The gravity-driven flow, however, is impeded by the capillarity whose magnitude decreases with solvent enrichment. Oil recovery in the horizontal gasfloods is nonmonotonic with enrichment for this fluid system in an almost homogeneous Berea core. Multiphase flow in the near- miscible floods leads to less gravity override compared to the FCM floods. In the heterogeneous core studied, the heterogeneity is very strong and the capillary forces do not prevent bypassing. The capillary forces, in fact, reduce oil recovery by diminishing mass transfer from the bypassed regions.
The bypassing of oil is common in gasflooding due to rock heterogeneity, gravity override and viscous fingering. The extent of bypassing depends on the solvent enrichment as well as the injection strategies such as WAG ratio. Recent laboratory corefloods have shown that near-miscible solvents can be as or more effective than the first contact miscible solvents because of lower bypassing and favorable microscopic displacement efficiency. A significant fraction of oil initially bypassed by a solvent can be recovered subsequently by crossflow/mass transfer from the bypassed regions to the flowing region. Since near-miscible solvents appear attractive for gasflooding, it is important to identify and quantify the mechanisms of mass transfer when the solvent is not first contact miscible.
Many reservoirs are naturally fractured. Waterflooding has traditionally been considered for the fractured reservoirs if the reservoir is water-wet. During waterflooding, the water displaces the oil from the fractures first and then imbibes into the matrix blocks due to the capillarity, if water-wet. Large quantities of water is typically recycled for significant oil production. Gasflooding of fractured reservoir is getting consideration only recently. Hara and Christman found CO2 flooding to be an attractive option for a diatomite reservoir; diffusion was the main mechanism for oil recovery from the matrix blocks. Firoozabadi et al. found miscible gas injection to be efficient in a fractured reservoir; gravity drainage was the key mechanism of oil recovery. Beliveau and Payne found the oil recovery to be 27% of the oil in place in a tertiary CO2 flood in a fractured reservoir. If near-miscible solvent floods are considered for fractured reservoirs, one needs to quantify the mass transfer between the matrix blocks and the fractures.
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