This paper describes the application of a genetic algorithm to the development of a solvent-additive SAGD process. A review of related field projects and key simulation studies is provided, together with a discussion of the pros and cons of potential alkane solvents. Economics and the impact of dynamic and ultimate retention are discussed.
A general conclusion drawn from literature is that optimal solvent application to SAGD will likely involve time variations in both rate and composition of the solvent. This results in an optimization problem that has a large number of dimensions, and is very nonlinear. Genetic algorithms, which mimic biological evolution, have been found by us to be extremely effective in addressing such problems. The general methodology of application to solvent additives by Laricina Energy.is described.
A key product of this effort, optimized for a simple clastic reservoir, is presented. The genetic algorithm produced an operable process, which could be described as a new combination of pre-existing concepts. The process offers material improvements in thermal bitumen supply costs, as well as recovery factor. Major reductions in the physical steam/oil ratio (SOR), (and therefore) capital intensity and carbon emissions, are indicated.
The addition of light hydrocarbon solvents to steam has long been regarded as the simplest and most important potential increase in SAGD performance. Recently, at least two commercial implementations of such processes have begun operation. In the current economic environment, advances with in situ technology are all the more important.
The perceived benefits of solvent addition to SAGD include:
reduced SOR
increased well productivity
reduced capital intensity to startup
increased recovery via reduced Sor
increased recovery via higher (economic)
volumetric sweep
In addition to the above, EnCana1 has recently indicated that their solvent-assisted process (SAP) allows for greater well spacing than with steam alone.
There are as yet no published analyses of the detailed transport mechanisms for the increased oil rates observed with solvents. In general, solvent vapor accumulates ahead of the steam front, where it mobilizes and drains oil from regions that may be considerably cooler than the steam zone. Thus, the average temperature of the drained volume is much less than for the same recovery by steam, accounting for the SOR improvement. The oil rate increase is qualitatively explained by lower oil phase viscosities in the drainage zone.
