Optimization of Miscible Water-Alternate-CO2 Injection (Based on analytical model)
- P. Bedrikovetsky (Moscow State Oil & Gas Academy) | G.M. Andrade (CENPES Research Centre, PETROBRAS) | L.E.A. Ferreira (E&P-BA, PETROBRAS) | G.L. Menezes (E&P-BA, PETROBRAS)
- Document ID
- Society of Petroleum Engineers
- SPE/DOE Improved Oil Recovery Symposium, 21-24 April, Tulsa, Oklahoma
- Publication Date
- Document Type
- Conference Paper
- 1996. Society of Petroleum Engineers
- 5.2 Reservoir Fluid Dynamics, 5.5 Reservoir Simulation, 5.2.1 Phase Behavior and PVT Measurements, 1.8 Formation Damage, 5.4 Enhanced Recovery, 5.3.2 Multiphase Flow, 6.5.2 Water use, produced water discharge and disposal, 5.4.9 Miscible Methods, 5.7.2 Recovery Factors, 5.5.3 Scaling Methods, 5.4.1 Waterflooding, 5.3.4 Reduction of Residual Oil Saturation, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 5.4.2 Gas Injection Methods, 4.3.4 Scale
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A new analytical model for the miscible CO2-WAG is developed. The model is based on analytical solutions of non-self-similar and self-similar problems for system of hyperbolic equations of mass conservation laws. Explicit formulae allow to analyse propagation of displacement and phase transition fronts, mechanisms of trapping of oil with the sequential injection of water and gas slugs, mobility ratios on shock fronts and the dynamics of water and gas slugs.
Five different regimes for gas-water injection have been distinguished depending on the water-gas ratio. They differ from each other by different structure of the mixture zone and displacement mechanisms caused by phenomena of two-phase displacement and phase transitions.
The analytical model presented shows that the higher the WGR the lower the recovery, but the more favourable is the mobility ratio on the displacement front. These suggest the existence of an optimal water-gas ratio (WGR).
As it follows from the analytical model there does exist a minimum slug size which prevents gas breakthrough via all the water slugs. With the injection of thinner slugs a connected gas network appears in the reservoir which will catch up the front of water and will create an unstable gas-oil front at the presence of the connate water only. So, simultaneous injection of gas and water, which accords to the reduction of slug size to the zero limit, is not an optimal WAG regime, as it was suggested in the literature. On the other hand the thinner the slugs the higher the displacement efficiency. These speculations suggest the existence of an optimal slug size with the miscible WAG.
Disadvantages of the traditional waterflooding of oil reservoirs are: - unfavourable mobility ratio on the displacement front for highly viscous oil leading to low sweep efficiency
- strong capillary forces resulting in high residual oil saturation. However, capillary forces with waterflooding cause capillary imbibition of low permeable patterns allowing some incremental oil recovery during the late stage of production.
Miscible gas injection when compared with waterflood causes a significant reduction of the residual oil due to phase transitions and inter phase mass transfer between oil and gas. Nevertheless, the mobility ratio on the gas-oil displacement front is even less favourable than with the waterflood resulting in unstable displacement and viscous fingering. Diffusive mechanism of the oil sweep from low permeable patterns by gas is less intensive than imbibition with the waterflooding.
Sequential injection of water and miscible gas slugs in an oil reservoir (Water-Alternate-Gas Injection, WAG) seems to gain advantages of both, waterflood and miscible gas flooding: mobility of the displacing water-gas system is even lower than water mobility at the same total saturation, so sweep due to heterogeneity and well placing geometry increases with WAG viscous fingering and the early breakthrough are also prevented; miscible gas reduces saturation of oil trapped by water and capillary imbibition of low permeable parts by water works with WAG also.
The importance of all the above mentioned mechanisms was shown by analysis of a number of pilot tests.
The physical mechanisms of the incremental recovery using WAG, mentioned above, can be captured by 1-D model which takes into account interaction between water and gas slugs during the sequential injection, phase transitions and effects of phase compositions on relative permeabilities and phase viscosities. This model, however, does not take into account viscous fingering.
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