Introduction
Separation processes are usually monitored by capacitive measuring systems. (1-3) The single electrode capacitance probes (SeCaP ™)measuring principle is based on the fact that the dielectric property of fluid exposed to a free electrode of an oscillator circuit affects the frequency of the oscillator. (4,5) It is therefore possible to determine the composition of the oil/water dispersion adjacent to each individual electrode.
A vertical distribution in the measuring cell can be generated, and the dynamics of the mixing separation process is clearly visualised in the time domain. This has previously been shown to work for profile gauges fitted to separators, but now we see that the system in the refined form is able to detect small differences in fluid composition such as the difference between gas and foam. The SeCaP system can also be used to determine water content in oil/water dispersions. (6)
Materials and Methods
Description of Dispersion Characterisation Probe
A 500 bar test cell with a volume of 500ml has been used to investigate the separation of North Sea oil and saline water. The test cell is made of titanium and is fitted with 92 internal SeCaP electrodes monitoring the content in the vessel. The high pressure rating of the cell is required to perform separation tests on actual down hole well samples. The cell is fitted with a channel through the titanium body and is placed in a temperature-controlled environment, thus controlling the temperature during the test runs. Pressurised bottom-hole hydrocarbon sampled can be transferred to the test cell without any contamination with oxygen or other unwanted compounds. The dispersion characterisation probe (DCP) is shown in Figures 1a and 1b.
Figure 1: Photograph Showing Part of DCR with DCP in Centre (a)
and Detail of Design of DCP with Wall Cut-off (b)
The test cell has a square cross-section with a side measuring 45mm and a height of 247.8mm, making a total volume of 500ml. Each SeCaP electrode has a diameter exposed to the fluid of 15mm. To distribute all 92 electrodes over the cell height, the electrodes overlap, making an electrode resolution of 5mm. The cell also contains a variable speed mixer (centrifugal lifter) that mixes the water and oil phase smoothly within the cell (see Figure 2a). An external motor drives the shaft. The fluid at the bottom of the DCP enters the hollow shaft and exits through the vanes at the top. The mixing unit is driven by a frequency controlled motor, through a magnetic coupling, and both the torque and the rotational speed are measured, thus establishing the mixing power input to the test volume. This is an important parameter to control since it is desired to match the mixing power per unit volume in the cell to the normal power dissipation per unit volume in real processes such as pressure drop due to friction in pipes, pressure drop across valves and so forth.
Description of Dispersion
Characterisation Rig
The layout of the dispersion characterisation rig (DCR) is shown in Figure 2b. The DCR is a flexible test facility that makes it possible to undertake a number of different types of tests. The system can be used for bottom hole samples at high pressure.
Figure 2a: The Mixer or Stirrer
Figure 2b: The Layout of the DCR
Category:
Integrated Operations
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References
