Visualizing Low-Salinity Waterflooding
Low-salinity waterflooding has recently attracted considerable interest from governments, researchers, and oil companies, who are evaluating the method’s potential to increase oil recovery. While laboratory work and field tests have indicated that low-salinity waterflooding can have positive results, and implementation of the technology is planned, there are still a number of challenges and uncertainties related to the technique. It is generally accepted that the effects of low-salinity waterflooding are caused by wettability alteration, but the processes involved are still not fully understood.
Heriot-Watt University has been conducting research to investigate the mechanisms of interactions between low-salinity water, crude oil, and porous media at reservoir conditions. Industry support for the work was secured through Industry Technology Facilitator (ITF), and professor Mehran Sohrabi, director of the Center for Enhanced Oil Recovery (EOR) and CO2 Solutions at the university, is leading the project team.
Low-Salinity Water Injection
Waterflooding improves oil recovery by displacing oil, and injection water is usually taken from the nearest available source (which is seawater for most offshore fields). The first effects of low-salinity water injection were published in the 1990s, when work at the University of Wyoming indicated that injecting water with a total dissolved-solids content of less than 5,000 ppm resulted in a higher oil recovery than that attained by injecting high-salinity water such as seawater. Since then, a number of studies with varying results have been published, and the recovery benefits described are likely to be field dependent.
Low-salinity waterflooding has a number of potential benefits. It has relatively low costs compared with other techniques. It can be implemented in ongoing and new waterflood projects onshore and offshore and has shown potential for sandstone and carbonate reservoirs. In addition, it can be used alone or in conjunction with other EOR techniques, e.g., polymer injection, as a technology enabler. It also can alleviate problems associated with conventional waterflooding, such as scale formation and souring.
However, the chemical mechanisms that take place at pore scale when low-salinity water is injected in an oil reservoir are not adequately understood. Thus, reliable prediction of which reservoirs will respond positively to low-salinity waterflooding and the size of the recovery response is very challenging.
Although several mechanisms have been proposed based on laboratory coreflood experiments, many conflicting results have been reported in the technical literature. Interpretation of these results can be difficult because of the complex nature of rocks and the influence of experimental artifacts, such as sample preparation and the capillary end effects.
The Low Salinity EOR research initiative, a joint industry project (JIP), has been making use of Heriot-Watt University’s facilities, experience, and expertise in advanced multiphase flow visualization and testing to obtain direct experimental evidence that will reveal the fundamental mechanisms involved in low-salinity water injection.
The work program pays special attention to investigating the role of fluid/fluid (brine/crude oil) interactions taking place during low-salinity water injection and the direct visualization of microscopic processes involved in the injection and mixing of low-salinity water with crude oil (Fig. 1 above). This enabled the researchers to directly see, for the first time, that a physical reaction takes place in the oil when it comes in contact with low-salinity water. Fluid characterization tests and measurements indicate that certain compounds of crude oil that are known to be surface-active are activated when low-salinity water is injected (Fig. 2).
The JIP results provide a consistent explanation for wettability alteration and the subsequent improved oil recovery based on fluid/fluid and rock/fluid interactions taking place during low-salinity water injection.
This finding is significant because, in addition to providing a novel insight into the actual mechanisms of oil recovery by low-salinity water, and in particular, the poorly understood role of crude oil/brine interactions, it may also lead to the development of a robust and simple method for evaluating the response of a reservoir to low-salinity water injection. Live crude oil samples of interest to the project sponsors have been used in this study, which further ensures the value of the results.
Next Phase of JIP
Building on the success of Phase 1 of the JIP, researchers are planning Phase 2, which will be kicked off later this year. State-of-the-art analytical techniques will be used to detect minute but crucial changes in the composition of crude oil and brine resulting from contact with low-salinity water.
A number of crude oil samples with different characteristics will be selected and used in the investigation to identify samples that respond positively to low-salinity water, as well as those that do not. The role of the rock, including rock type (sandstone and carbonates) and the initial wettability of the rock, will be systematically and thoroughly investigated.
Compared with sandstone reservoirs, there is a significant gap in the understanding of low-salinity waterflooding in carbonates. As carbonate reservoirs hold more than 70% of conventional oil reserves, production from these formations is critical for the petroleum industry, and thus it is an area requiring more research and development (R&D).
There are a number of challenges to be tackled. For example, carbonate formations are typically moderately to strongly oil-wet, but waterflooding techniques generally work better in water-wet formations. In addition, the temperature requirements of the reservoir for effective low-salinity waterflooding are not clearly understood in carbonate reservoirs.
ITF is planning a workshop later this year on EOR in carbonate reservoirs, focusing on low-salinity waterflooding, polymer flooding, gas flooding, and other emerging technologies. The workshop will bring together subject matter experts from ITF member companies and key industry stakeholders to discuss the challenges in more depth and identify areas in which collaborative R&D could advance industry efforts to increase recovery rates.
Visualizing Low-Salinity Waterflooding
Colin Sanderson, ITF, and Mehran Sohrabi, SPE, Heriot-Watt University
01 November 2014
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