A calculation model about reservoir thermal efficiency of in-situ upgrading for oil shale via steam injection

Abstract

The reasonable determination of the reservoir thermal efficiency is essential for the successful application of thermal recovery technology in oilfield. However, in terms of the in-situ upgrading for oil shale via steam injection, there is currently no calculation model to evaluate the reservoir thermal efficiency. In this work, a calculation model was established based on the theory of steam flooding front prediction in thermal recovery, and it was validated mainly through comparing with numerical simulation results. One-quarter of a five-spot well group was applied for this simulation, the response surface analysis and sensitivity studies were performed to study the effect of various parameters on reservoir thermal efficiency. Injection rate, reservoir thickness, kerogen concentration and steam temperature all have significant effect on reservoir thermal efficiency, and there is a strong interaction between injection rate and reservoir thickness. High thermal efficiency mainly depends on increasing heat reserved in reservoir and reducing heat loss to upper and lower formations. Meanwhile, the existence of gravity override has an adverse effect on thermal efficiency. Therefore, the sensitivity studies are to explore the comprehensive influence of different parameters on above three aspects. The results show that appropriately increasing the injection rate of steam in a certain range can effective improve and stabilize the efficiency. Under the premise of well thermal insulation of the upper and lower formations, the reservoir thickness is supposed to large enough, and the reservoir with high kerogen concentration can be selected. Increasing steam temperature and reducing initial pyrolysis temperature not only can ensure the high conversion ratio of kerogen pyrolysis, but also has beneficial effect on maintaining the higher reservoir thermal efficiency. Reducing the mobility ratio of steam flooding can avoid the correction shape factor being too small, thus improving the thermal efficiency.