Modeling Steam-Assisted Gravity Drainage With a Black-Oil Proxy
- Mohammad Ghasemi (NTNU) | Curtis H. Whitson (NTNU/PERA)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- May 2013
- Document Type
- Journal Paper
- 155 - 171
- 2013. Society of Petroleum Engineers
- 5.4.10 Microbial Methods, 5.1.1 Exploration, Development, Structural Geology, 5.5 Reservoir Simulation, 5.3.9 Steam Assisted Gravity Drainage, 5.5.8 History Matching, 5.1.5 Geologic Modeling, 4.6 Natural Gas
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This paper describes an alternative approach to model steam-assisted gravity drainage (SAGD) with an isothermal black-oil (BO) reservoir simulator. The oil-viscosity reduction caused by heating in the actual SAGD process is emulated by a tuned saturated pseudo-oil viscosity relation in which solution gas/oil ratio (Rs) is used as a "proxy for temperature." In the BO formulation, fully saturated oil viscosity (µ*o) at reservoir pressure equals lo that would be attained at steam-chamber temperature (T*) in the actual SAGD process; initial oil viscosity (µoi ) with initial Rs=0 represents initial oil viscosity at reservoir temperature; and BO gas properties represent steam at T*.
After careful analysis of the SAGD process, one finds that oil flows only along a narrow zone along the outer edge of the steam chamber--the "edge oil-flow zone." The temperature gradient within this narrow zone is perpendicular to the oil-flow direction and is practically impossible to model with any precision because of the large temperature variation and dynamic steam-chamber shape over time. The BO-model solubility gradient also varies, analogous to temperature in a thermal model, from zero to fully saturated (Rs*) with an associated drop in oil viscosity from µoi to µo.
SAGD design requires many hundreds of runs to find operational conditions that maximize economic value (e.g., injector and producer location, rates, pattern spacing, and steam-chamber temperature T*). The proposed BO proxy model runs several times faster than a thermal model while maintaining similar performance behavior.
The proxy-model saturated pseudo-oil viscosity µo(p) relation used is found by history matching a full-physics thermal-model performance prediction of oil rate, bottomhole flowing pressure, and cumulative oil for a 2D homogeneous model. We have found a single-constant µo(p) equation that yields a good match to thermal SAGD performance. The tuned pseudo-oil viscosity relation honors the measured initial reservoir and fully heated (at T*) oil viscosities. Its dependence on Rs is not physical, but reflects the use of Rs as a transform variable for temperature, capturing the strong spatial variation of temperature and oil viscosity within the localized steam/oil boundary region in which oil has been mobilized. The pseudo-oil viscosity relation, defined by a single empirical best-fit constant n--for a given T* and a set of thermal properties--appears to be applicable for a wide range of reservoir heterogeneity, injection and production rates, and well placement. Consequently, it should be possible to use the BO proxy model for SAGD optimization of T*, control rates, and injector/producer vertical-depth difference. We also see the potential of using the BO proxy model for solvent-based SAGD, with the pseudo-oil viscosity model depending on both T* and solvent; thermal compositional modeling is yet even slower and less suitable for optimization.
|File Size||6 MB||Number of Pages||17|
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