Linear Model Studies of the Immiscible CO2 WAG Process for Heavy-Oil Recovery
- Steve B. Dyer (PanCanadian Petroleum Ltd.) | S.M. Farouq Ali (U. of Alberta)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- May 1994
- Document Type
- Journal Paper
- 107 - 111
- 1994. Society of Petroleum Engineers
- 5.2.1 Phase Behavior and PVT Measurements, 4.3.4 Scale, 5.4.10 Microbial Methods, 5.4.6 Thermal Methods, 5.2 Reservoir Fluid Dynamics, 5.4 Enhanced Recovery, 5.5 Reservoir Simulation, 2.4.3 Sand/Solids Control, 4.6 Natural Gas, 5.4.1 Waterflooding, 5.8.5 Oil Sand, Oil Shale, Bitumen, 5.3.2 Multiphase Flow
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The immiscible carbon dioxide flooding process, in the water-alternating-gas(WAG) mode, holds considerable promise for the recovery of moderately viscousoils, where promise for the recovery of moderately viscous oils, wherereservoir conditions are unsuitable for the application of thermal recoverymethods.
This paper discusses the results of an experimental study, employing apartially scaled linear model. The principal objective is to quantify themechanisms involved principal objective is to quantify the mechanisms involvedin the immiscible displacement of heavy oil by the carbon dioxide WAG processat a low pressure. The variables studied include: slug size, WAG ratio, numberof WAG cycles, and secondary versus tertiary recovery modes. Two oils withviscosities of 150 and 1055 mPa s were tested. A notable feature of this workis the analysis of the relative efficiency of each WAG slug.
The experimental results can be summarized as follows. Optimal total carbondioxide slug size is approximately 30 to 40% HCPV. The optimum number of WAGcycles, based on runs with large carbon dioxide slugs, appears to be ten. Lowvelocity displacements tended to increase gravity override, with a reduction inoil recovery. A high WAG ratio resulted in early water breakthrough and highcumulative water-oil ratios. Oil recovery in the case of a low viscosity oil(150 mPa s), by the carbon dioxide WAG process, was inferior relative to awaterflood, suggesting that process, was inferior relative to a waterflood,suggesting that the process is not appropriate for light oil reservoirs.Tertiary displacement of heavy oils does not appear to be an effective use ofthe immiscible carbon dioxide flooding process. process
Many heavy oil reservoirs in Alberta and Saskatchewan (Canada) are notsuitable for the application of thermal recovery methods, such as steaminjection and in situ combustion. This is primarily because of thin paysections (less than 10 m) and considerable depths >1000 m). As well, manyheavy oil formations often have an underlying water sand. Under theseconditions, thermal methods are often inefficient and uneconomical as a resultof excessive vertical heat loss and heat scavenging by the bottom water. Thisprovides the motivation for searching an alternative for such marginal heavyoil formations.
Moderately viscous heavy oils lack the necessary extractable hydrocarbons(C5-C30) for miscibility with carbon dioxide to be economically attained. Insome cases (25-35 deg. API the miscibility pressure exceeds the fracturepressure, and the displacement is carried out in the pressure, and thedisplacement is carried out in the immiscible mode.
Laboratory studies were conducted to determine the effectiveness of theimmiscible carbon dioxide process for the recovery of a moderately viscousheavy oil. In this study, a "moderately viscous heavy oil" is definedas an oil in the 20 deg. API gravity range, having a viscosity of 1000--2000mPa s at reservoir conditions. In Saskatchewan alone, the in-place oil in thiscategory is about 10 billion cubic meters. These studies were used to determineand optimize the recovery process mechanisms. The main mechanisms associatedwith the immiscible carbon dioxide flooding process are oil viscosityreduction, oil phase swelling and process are oil viscosity reduction, oilphase swelling and solution gas drive during pressure blowdown. Detailsregarding the transport of carbon dioxide in heavy oil, as well as the effectsof carbon dioxide on reservoir fluids were described, in detail, in a previouspaper.
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