A group of engineers from the Tianjin branch of CNOOC jointly with researchers from the China University of Petroleum (East China) have tested the effectiveness of the steam flooding technology at offshore heavy oil fields. They injected steam into the oil wells after conventional flooding, which significantly increased the volume of oil recovered on the shelf.
The conventional flooding technology, which is widely used at offshore heavy oil fields, entails injecting water into the formation to displace oil. Although this method is relatively inexpensive, its effectiveness is limited: the rock can retain up to 95% of its oil reserves as a result. In order to increase oil recovery, the Chinese specialists have proposed using steam flooding, i.e., the injection of hot steam, which heats the oil, reduces its viscosity and facilitates its displacement. Instead of abandoning flooding, this approach aims to supplement it by introducing a steam phase after the water one.
As part of the study, the scientists conducted 11 laboratory experiments under conditions as close to real ones as possible. To that end, they used tubular and volumetric tanks (1D and 3D models) simulating the geometry of oil reservoirs. Oil and formation water from a field in Bohai Bay were used as the working media. Every experiment began with flooding, which continued until a specified degree of water cut (60% or 90%) was reached, after which the steam injection phase would begin. In the course of the experiments, the scientists varied the interval between the stages, the steam injection rate and the presence of special chemical additives. The additives included nitrogen foam, which blocks channels with high permeability and slows down premature steam breakthrough, and a displacing agent, which reduces the surface tension between oil and water and makes it easier to release oil from the pores. Under the influence of steam, the formation was effectively heated: oil viscosity, totaling about 9,100 mPa·s at 25°C, dropped significantly, after which the oil became mobile, the pressure increased and oil began to get actively displaced.
The results showed that steam flooding after water flooding makes oil recovery much more efficient. In some cases, the efficiency increase exceeded 55% compared to water injection alone. In 3D models, which approached real conditions in terms of geometry, a high steam feed rate led to a more uniform heating of significant rock volumes and increased oil recovery. Supplementary use of chemical additives gave an increase in oil recovery of up to 5.47% compared to additive-free steam flooding.
Nevertheless, despite the high efficiency of this technology, its practical application in offshore conditions is associated with a number of difficulties. The need to use large-sized equipment (steam generators, heat-insulated pipelines and power supply systems) entails considerable costs. Plus, high fuel consumption during steam generation causes greenhouse gas emissions. Therefore, the scientists are faced with the task of finding the optimal balance between financial and environmental costs and maximum efficiency of oil production.
