Saudi Aramco, the world's leading petroleum energy provider, has a long history in deploying reservoir sweep monitoring technologies to ensure maximum oil recovery efficiency at the desired production rates. Determination of the sweep efficiency and Remaining Oil Saturation (ROS) is important and sets the economical basis for future work-over and EOR projects.
Accurate determination of ROS, particularly in water flooded areas, is extremely difficult due to notable differences in salinity between the original (connate) water and injected water. To overcome this challenge, Saudi Aramco has adopted data acquisition programs that include in-situ measurements insensitive to water salinity and fluid displacement processes. These technologies include Nuclear Magnetic Resonance in Log-Inject-Log mode, Carbon/Oxygen, and Dielectric logs. Results from well logs are routinely calibrated with data from Wire-line formation testers and special core analysis. ROS results from the different methods are quality checked and integrated to produce reliable figures.
This presentation highlights Saudi Aramco's ROS acquisition programs, including limitations of these technologies. The presentation also discusses ongoing efforts to further reduce the uncertainties in future ROS measurements.
Oil producing companies usually estimate the original volume of oil in place at time zero before they initiate production. After they start producing the oil, they routinely monitor the percentage of oil remaining. This remaining percentage at a given time is generally termed remaining oil saturation (ROS). This process continues until all the oil that can be produced through conventional methods, like waterflooding, has been achieved. At this time, the remaining oil is referred to as residual oil saturation (Sor). Notionally, the value of ROS can be as low as Sor; but usually it is greater.
At the microscopic scale, viscous and capillary forces are the main opposing forces, governing any immiscible displacement process. Displacement processes are carefully engineered to maximize viscous forces while minimizing capillary forces to enhance displacement efficiency and maximize oil recovery. These forces are largely dictated by interrelated reservoir attributes: wettability, pore structure and fluid dynamics. Much research has been done to investigate the fluid distributions and displacement mechanisms and the interplay of these attributes and forces at the microscopic level.
At the macroscopic level, reservoir structure and features, e.g., fault, fractures, barriers, strata forms, heterogeneities, make reservoirs geologically complex. Coupled with production/ injection fluid flow dynamics and effects of gravity, these multidimensional complexities dominate fluid movement within the reservoir and, therefore, result in complex fluid distributions, both vertically and laterally. A diagrammatic demonstration of the displacement process at both scales: microscopic and macroscopic, is presented in Fig. 1.