A simulation study of polymer flooding and surfactant flooding using horizontal wells.

摘要

The apparent economic attractiveness of polymer flooding and surfactant flooding has always been lessened by the fact that expensive polymer or surfactant is injected and then many years are required to produce the incremental oil. This is especially true in shallow, low-permeability oil reservoirs with low injectivity, but is also a factor even for reservoirs with high injectivity in expensive environments such as offshore. The potential for horizontal wells to accelerate oil production and thus improve the discounted cash flow may seem obvious, but the precise improvement depends on many complex factors such as vertical permeability and horizontal well location and can only be assessed using realistic simulations. In this study, the potential of improving polymer flooding and surfactant flooding by using horizontal wells has been systematically investigated with a compositional chemical flooding simulator (UTCHEM) that can model both three-dimensional heterogeneous reservoirs and realistic process behavior. For the purpose of this study, the capability of modeling horizontal wells was added to the simulator and the code modifications were successfully validated through a comparison of the numerical results to published analytical solutions. The reservoir description played a central role in this study. Stochastic reservoir descriptions were generated and multiple realizations of these were made to see the impact of heterogeneity on polymer flooding and surfactant flooding with both vertical and horizontal wells. While horizontal wells were found to improve injectivity, especially of the high-viscosity polymer, the sweep efficiency of the floods sometimes increased and sometimes decreased when horizontal wells were substituted for vertical wells. The location of the horizontal well was very important with respect to both the sweep efficiency and the injectivity and needed to be in a relatively high-permeability location to accelerate oil production. Oil recovery was found to be very sensitive to the correlation length, indicating that the stochastic approach for reservoir description has advantages over the layered approach that has traditionally been used in this type of study. The vertical permeability was the most sensitive parameter for horizontal wells and needed to be moderate to high (on the order of 0.1) to give adequate injectivity, although using vertical drainholes off the horizontal injector was found to shorten the project life in cases of low vertical permeability. Surfactant flooding was found to be more sensitive to these factors than waterflooding and even polymer flooding. Many other factors were investigated to evaluate the potential of improving the cost effectiveness of polymer flooding and surfactant flooding and these results are reported in this study. Under some circumstances, the use of horizontal injectors with vertical producers appeared to have economic merit, but these circumstances are clear only after a careful study with realistic reservoir and process descriptions and detailed economic analysis.