Optimization of Conversion Timing from Cold Production to CSS in Extra-Heavy Oil Reservoir

Extra-heavy oil reservoirs with foamy oil flow characteristics are commonly exploited using cold production method in the early stage of development. However, during cold production period, production rate decreases with the declining reservoir pressure. CSS has been proved to be an applicable follow-up approach to enhance the production rate, and the conversion timing from cold production to CSS is closely relates to economic return in such reservoirs. 1D core displacement and 2D micro flow experiments were conducted with typical foamy oil samples under different temperatures to investigate the effect of temperature on the displacement efficiency and flow characteristic. Furthermore, numerical simulations were carried out to analysis the effect of the formation pressure during conversion on the overall performance of cold production and CSS process. Experiment results illustrate that the pseudo-bubble point pressure increases and critical gas saturation decreases as temperature increases. The foamy oil displacement efficiency dramatically decreases when temperature rises to a certain value, which indicates that an optimum temperature range exists for foamy oil displacement to function fully. And thus the follow-up thermal recovery process should be postponed appropriately. Numerical simulation results show that the best conversion timing from cold production to CSS is when formation pressure approaching the pseudo-bubble point pressure. And that will lead to the highest overall recovery degree during cold production and CSS stages, because, in this way, foamy oil displacement mechanism can be fully utilized, and CSS is still effective for production enhancement although the formation pressure has declined to some extent. This study proposed optimized conversion timing from cold production to CSS for extra-heavy foamy oil reservoirs.