Thoughts on Simulating the Vapex Process
- David Layton Cuthiell (Self-employed) | Neil Roger Edmunds (Laricina Energy Ltd)
- Document ID
- Society of Petroleum Engineers
- SPE Heavy Oil Conference Canada, 12-14 June, Calgary, Alberta, Canada
- Publication Date
- Document Type
- Conference Paper
- 2012. Society of Petroleum Engineers
- 1.2.3 Rock properties, 3 Production and Well Operations, 4.1.9 Heavy Oil Upgrading, 2.4.3 Sand/Solids Control, 1.8 Formation Damage, 5.3.9 Steam Assisted Gravity Drainage, 5.2.1 Phase Behavior and PVT Measurements, 5.5 Reservoir Simulation, 5.3.2 Multiphase Flow, 5.5.8 History Matching, 5.4.6 Thermal Methods, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 4.3.4 Scale, 4.3.3 Aspaltenes
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The Vapor Extraction (Vapex) process and its many hybrid variants have attracted a great deal of attention as potentially less energy intensive alternatives for exploiting heavy oil and bitumen resources. However, despite much work over the past two decades, uncertainty remains about the basic mechanisms, the scaling aspects and the most appropriate methods of numerically simulating these processes. This paper offers some insights into several of these outstanding questions. The questions are examined in the context of an extensive and well-documented set of Vapex experiments carried out by Maini and his colleagues over the past 10 years.
We have experimented with different methods of simulating these experiments using a physics-based reservoir simulator. Despite the high permeability (greater than 200 Darcys in all of the experiments), we find that capillary pressure plays a significant role in the drainage. The simulations suggest that most of the drainage takes place in the capillary transition zone along the edge of the vapor chamber, rather than in the single-phase zone ahead of it which has not yet been contacted by vapor.
It has been emphasized in the literature that the near-linear scaling of oil rate with height observed in the experiments is dramatically different from the square root of height dependence predicted by the original analytic model of Vapex. However, the experiments also show an increasing solvent fraction in the produced oil phase as height increases. When this "solvent mixing?? effect is separated out of the rates, the remaining height dependence is less than linear, though still greater than square root of height.
The relative roles of molecular diffusion and mechanical dispersion in Vapex have been widely discussed in the literature. Generally, mechanical dispersion is expected to play a larger role in these high permeability experiments (vis-à-vis the field), due to larger fluid velocities. We present a method of inferring the diffusion/dispersion present in the simulations, despite a hidden component of numerical dispersion caused by the numerical method itself. We find that the experiments are well matched with values of diffusion and dispersion in line with literature correlations, and that the contribution of mechanical dispersion is perhaps not as large relative to that of molecular diffusion as might be expected.
The paper also provides some thoughts on questions we believe are still unanswered, including mechanisms behind the height dependent mixing phenomenon and the scaling of the experimental results to the much greater heights and lower permeabilities characteristic of the field.
Shortly after the original development of the Steam-Assisted Gravity Drainage (SAGD) process, work began on an analogous solvent-mediated process, in which solvent dilution of the reservoir oil played a similar viscosity reducing role to the thermal energy of SAGD (Butler and Mokrys, 1989; Dunn et al, 1989). Experiments to test the concept of solvent-assisted gravity drainage began in the late 1980s, utilizing either a vapor phase solvent (ethane and CO2) or a miscible liquid solvent (toluene). The name "Vapor Extraction?? or "Vapex?? was actually first used to describe a combined steam-propane process (Butler, 1991). Over time, emphasis has moved from liquid solvents to gaseous solvents, motivated by the desire to minimize mass inventory of solvent in the reservoir at any particular time. The term "Vapex?? has come to be used primarily to describe a vaporized solvent-only gravity drive process. We will use it in this sense.
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