Scaled Model Studies of CO2 Floods
- G.A. Rojas (U. de Oriente) | Tao Zhu (U. of Oklahoma) | S.B. Dyer (Saskatchewan Research Council) | Sara Thomas (U. of Alberta) | S.M Farouq Ali (U. of Alberta)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- May 1991
- Document Type
- Journal Paper
- 169 - 178
- 1991. Society of Petroleum Engineers
- 5.8.5 Oil Sand, Oil Shale, Bitumen, 5.4.1 Waterflooding, 4.1.9 Tanks and storage systems, 2.4.3 Sand/Solids Control, 5.6.4 Drillstem/Well Testing, 4.6 Natural Gas, 5.3.4 Reduction of Residual Oil Saturation, 5.4.9 Miscible Methods, 4.1.5 Processing Equipment, 6.5.2 Water use, produced water discharge and disposal, 5.3.2 Multiphase Flow, 5.4 Enhanced Recovery, 5.4.2 Gas Injection Methods, 5.5 Reservoir Simulation, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 5.4.10 Microbial Methods, 5.2.1 Phase Behavior and PVT Measurements, 4.1.2 Separation and Treating, 5.4.6 Thermal Methods
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Immiscible CO2 floods With alternating water injection are promising forheavy-oil formations ill-suited for thermal promising for heavy-oil formationsill-suited for thermal treatment. This investigation uses scaled models to showthat a 20% PV slug Of CO2 can recover 5 to 50% of the oil in place (OIP),depending on the operating conditions. The CO2 requirement is small, less than100 std m/stock-tank M.
Many heavy-oil reserves in Alberta, Saskatchewan, and other areas areunsuitable for the application of thermal recovery methods, such as steaminjection and in-situ combustion because of small thicknesses (less than 6 m),large depths ( >1000 m), or other rock, fluid, and geological conditions.Immiscible CO2 flooding may be a suita-ble recovery process for some of thesereservoirs. Previous studi by Klins and Farouq Ali and Rojas and Farouq Alithat used numerical simulation and scaled physical model experiments showedthat this process can recover up to 30% OOIP incremental oil over thatrecovered by waterflood. The process efficiency depends on oil viscosity, whichshould be at most around 1000 mPa-s. Simulation studies have shown that thelower limit of viscosity is about 70 mPa-s. Considering that a recovery methodfor moderately viscous oils, which are not suitable for thermal methods, isstill lacking, the immiscible CO2 flooding process merits a closeexamination.
The miscible CO2 flooding process consists of the injection of a limitedvolume of CO2 (gas) with water in the water-alternating gas (WAG) mode. Thelast Slug Of CO2 is driven continuously with water. The mechanistic features ofthe process have been discussed in previous papers. Briefly, the CO2 reducesoil viscosity by about a factor of 10 and leads to an increase in the oilvolume on the order of 10%. Furthermore, the carbonic acid formed tends toreact with the OEP to form narrow emulsion banks that help stabilize thedisplacement front. The WAG scheme similarly mitigates the effects of anunfavorable mobility ratio, as in some miscible displacement processes. At theend of the flood, CO2 blowdown contributes a few percentiles of additionalrecovery. On the whole, the CO2 requirement is much smaller, by a factor of 10to 30, than for a typical miscible process. Previous work has shown that theimmiscible CO2 flooding process efficiency depends on the injection rates ofthe CO2 and water slugs, the total volume of CO2, and other factors related torock and fluid properties. Recent work showed that the operating pressure, thenumber of slugs, and the extent of phase equilibrium are also importantfactors.
This paper gives the scaling criteria for the process and examines theeffects of several variables, including pressure, gas saturation, and formationheterogeneity.
The scaling criteria for the immiscible CO2 process were derived by bothdimensional and inspectional analyses. An outstanding example of this type ofdevelopment is the work of Geertsma et al. Dimensional analysis is a well-knownmethod based on the premise that the variable of interest can be expressed as aproduct premise that the variable of interest can be expressed as a product ofpowers of the remaining variables, the equation being dimensionally consistent.Thus, it is important to identify all relevant variables.
Inspectional analysis has been discussed by a number of authors. In thisprocedure, the relevant differential equations are combined to form a singleequation, and the coefficients are made dimensionless to derive the scalinggroups. For the same variables, dimensional analysis may yield a larger numberof groups than does inspectional analysis. Even without that, the number ofgroups is too large to satisfy in an experiment. A careful examination of thegroups is necessary to determine which are more important to satisfy and whatis the likely error caused by relaxing the remaining groups.
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