A Framework for the Production Performance and Thermal Efficiency of Steam Injection Process in Layered Heavy Oil Reservoirs

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Abstract Thermal efficiency is an important evaluation parameter for the recovery performance of steam injection operation in heavy oil reservoirs. Especially, for the layered heavy oil reservoirs (LHOR), considering the simultaneous calculations on steam injection profile and heat loss rate, an accurate description on the thermal efficiency is more difficult. The purpose of this work is to propose a framework for the production performance and thermal efficiency assessment of steam injection process in LHOR. Regarding the heat loss rate of interlayer effect, considering the limitation of Marx-Langenheim equation, we propose a pore-volume-injected (PVI) related correlation by using the method of numerical simulation. Meanwhile, a time-variable three-zone heating model (i.e., steam zone, hot-liquid zone and cold oil zone) for steam injection process is proposed. Then, using the mass and energy conservation formulas, the radii of steam zone and hot-liquid zone can be characterized. In order to accurately simulate the difference between different fluid flow zones, a steam overlap behavior in steam zone is considered. On the other hand, an exponential temperature distribution equation in hot-liquid zone is also applied to fully characterize the non-linear temperature distribution in formation. Therefore, the productivity of cyclic steam stimulation (CSS) process in the LHOR can be characterized. Thereafter, the obtained data of liquid production and formation temperature can be used to assess the thermal efficiency of steam injection operation. For the steam injection process in heavy oil reservoir, the thermal energy mainly distributes in oil reservoir, flow barrier and production fluid. Our calculation results indicate that their fractions are time-variable and space-variable. During the initial stage of CSS process, a sharp decrease on the thermal efficiency can be observed. For a typical 3-layered heavy oil reservoir, due to the productivity variations among different oil layers, the thermal efficiency of each oil layer varies from 0.75 to 0.79 at the end of the first CSS cycle. Besides, the average thermal efficiency of the LHOR increases about 0.03 from the end of cycle 1 to the end of cycle 3, resulting from the gradually reduced liquid production. As the reservoir Net-to-Gross Ratio (NTG) increases, the liquid productivity is enhanced and more heat energy is extracted by the produced liquid. Furthermore, a greater permeability difference can result in a significant discrepancy for the thermal efficiency of the different layers. As the permeability difference of the three layers increases from 1 to 4, the difference of thermal efficiencies for the 3 layers is enhanced from 0.04 to 0.09 at the end of the first CSS cycle. Additionally, it is also observed that the average thermal efficiency is gradually reduced with the bottom-hole pressure (BHP) decreases during the production stage. The framework proposed in this work provides an accurate evaluation tool for the production performance and thermal efficiency of steam injection process in heavy oil reservoirs.

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