(362v) Experimental Evaluation of the Level Detection in Separators Using Electrical Tomography

1. Objectives/Scope

In this work, the performance of an electrical tomography sensor is evaluated for the use of monitoring phase interface levels in oil-water separators. Oil-water separators are commonly used in the energy industry to separate oil and water from emulsion. Faulty liquid level detection is a major cause of separation failures in the oil and gas industry. Environmental conditions of level detection are challenging and many instruments relying on old technology are underperforming or failing completely. Float level sensors get stuck due to deposit formation, standpipes cannot measure emulsions and nucleonic devices may cause environmental hazards. Erroneous level detection can lead to faulty control of processes and can cause costly environmental problems, decreased product quality or suboptimal output rates. Electrical tomography has been proposed as a method of level detection and has many advantages compared to old techniques. Electrical tomography does not contain moving parts and is safe to use since it does not use radiation. The measurement rate of electrical tomography is fast, enabling real time monitoring of the processes. This paper presents experimental verification of the level detection accuracy using an electrical tomography probe sensor in laboratory conditions simulating an oil-water separator. Real interface levels are viewed and measured through a transparent wall on the test vessel.

2. Methods, Procedures, Process

In electrical tomography, excitation voltages are sequentially applied to electrodes on the surface of the probe and the current responses are measured from all the other electrodes. The electrical properties of the medium are estimated based on the measurements of the excitation voltages and current measurements. A vertical conductivity profile of the contents of the production vessel is estimated based on the measurements. The measurement sequence is fast and a measurement rate of several measurements per second can often be achieved. Different phase levels can be detected from the estimated conductivity profile due to the conductivity difference of oil and water. The water content changes in emulsion can also be seen in the conductivity profile. In this study the gas/oil, oil/emulsion and emulsion/water interfaces are estimated. Note that it is also possible to estimate the water/sand interface in sand detection applications.

3. Results and observations

The probe sensor was installed in a rectangular iron vessel where one face is transparent. A photograph of the vessel can be seen in the left side of Figure 1. The height, width and depth of the vessel are 150 cm, 50 cm and 50 cm, respectively. Nine different cases were measured with different heights of the water/emulsion, emulsion/oil and oil/air interfaces. Test cases where all phases were not present were also conducted, for example, a case with only oil in the vessel. Two different saline solutions were created by adding either 50 g/liter or 250 g/liter of sea salt to tap water. Emulsions with two different water cuts were created (the water cut of the emulsions were 20 % and 40 %). The water and oil were mixed thoroughly resulting in very stable emulsions and significant separation was not observed during the tests. Teboil E Automatic Transmission Fluid (ATF) was used as the oil in the tests.

The phase interfaces were measured with a measuring tape and the measured interface levels were compared to the estimated levels. When measuring the interface positions, it was observed that the oil emulsion interface was not sharp, and a gradient of the water cut was clearly visible in the interface. When a gradient is present in an interface accurate visual identification of the interface is difficult. The gradient was clearly visible also in the estimated conductivity profile. The emulsion/water interface was clearly visible through the transparent glass.

One example of the estimated conductivity profile (for Case 8) can be seen in the attached right Figure 1. The left of Figure 1 shows a photograph of the vessel and different phases (air, oil, emulsion and water). The right of Figure 1 shows the estimated conductivity profile in logarithmic scale and the measured and estimated interface levels (units in cm) are shown in the legend. The estimated profile shows all interfaces accurately and the gradually changing water cut in the oil/emulsion interface is visible in the conductivity profile. There is a 2.2 cm difference in the measured and the estimated oil/emulsion interface due to the difficulty to measure the location of the interface with measuring tape since there is no clear interface (only a gradient in the water cut). The air/oil and emulsion/water interfaces are estimated very accurately and the errors between the measured and estimated interface heights are 0.6 cm and 0.3 cm. Similar estimation accuracy was observed in all other studied cases and the difference between measured and estimated interface levels were below 1 cm.

The estimated mean conductivity values for the 20 % and 40 % water cut emulsions were 3.81e-7 S/cm and 6.2e-7 S/cm, respectively. Based on the estimated conductivity values the homogeneity of the emulsion can be assessed. For example, we can see if there is a conductivity gradient in the emulsion or is the conductivity of the emulsion even.

4. Conclusions

In this study, it is shown that electrical tomography can be used for the monitoring of the phase interface levels of air, oil, emulsion and water. Furthermore, the estimated conductivity profile reveals any gradient in the conductivity when no clear interface level is present. The accuracy of the level estimation was less than 1 cm in all studied cases. The accurate level estimation makes the reliable control of separation processes possible.

Figure 1: The photograph of the test vessel and different phases (air, oil, emulsion and water) are shown on the left. The estimated logarithmic conductivity profile as function of height is shown in right figure. The estimated and measured phase interfaces are shown in the legend. Note the gradient in the oil/emulsion interface that can be seen both in the image and the estimated conductivity profile.