Discussion on the sweep efficiency of hybrid steam−chemical process in heavy oil reservoirs: An experimental study

Abstract

Non-condensable gas (NCG), foam and surfactant are the three commonly-used additives in hybrid steam−chemical processes for heavy oil reservoirs. Their application can effectively control the steam injection profile and increase the sweep efficiency. In this paper, the methods of microscale visualized experiment and macroscale 3D experiment are applied to systematically evaluate the areal and vertical sweep efficiencies of different hybrid steam−chemical processes. First, a series of static tests are performed to evaluate the effect of different additives on heavy oil properties. Then, by a series of tests on the microscale visualized model, the areal sweep efficiencies of a baseline steam flooding process and different follow-up hybrid EOR processes are obtained from the collected 2D images. Specifically, they include the hybrid steam–N2 process, hybrid steam–N2/foam process, hybrid steam−surfactant process and hybrid steam–N2/foam/surfactant process (N2/foam slug first and steam−surfactant co-injection then). From the results of static tests and visualized micromodels, the pore scale EOR mechanisms and the difference between them can be discussed. For the vertical sweep efficiencies, a macroscale 3D experiment of steam flooding process and a follow-up hybrid EOR process is conducted. Thereafter, combing the macroscale 3D experiment and laboratory-scaled numerical simulation, the vertical and overall sweep efficiencies of different hybrid steam−chemical processes are evaluated. Results indicate that compared with a steam flooding process, the areal sweep efficiency of a hybrid steam–N2 process is lower. It is caused by the high mobility ratio in a steam–N2–heavy oil system. By contrast, the enhancement of sweep efficiency by a hybrid steam–N2/foam/surfactant process is the highest. It is because of the high resistance capacity of NCG foam system and the performance of surfactant. Specifically, a surfactant can interact with the oil film in chief zone and reduce the interfacial energy, and thus the oil droplets/films formed during steam injection stage are unlocked. For NCG foam, it can plug the chief steam flow zone and thus the subsequent injected steam is re-directed. Simultaneously, from the collected 2D images, it is also observed that the reservoir microscopic heterogeneity can have an important effect on their sweep efficiencies. From the 3D experiment and laboratory-scaled numerical simulation, it is found that a N2/foam slug can increase the thermal front angle by about 15° and increase the vertical sweep efficiency by about 26%. Among the four processes, a multiple hybrid EOR process (steam–N2/foam/surfactant process) is recommended than the other ones. This paper provides a novel method to systematically evaluate the sweep efficiency of hybrid steam–chemical process and some new insights on the mechanisms of sweep efficiency enhancement are also addressed. It can benefit the expansion of hybrid steam−chemical processes in the post steamed heavy oil reservoirs.