Locating Oil-Water Interfaces in Process Vessels

Description:

Technical Abstract:
Crude oil is always produced with connate water, and the water-to-oil ratio is often greater than 10 to 1. In the field, the well stream is first separated into its three phases: natural gas, crude oil, and produced water. Unfortunately, crude oil and produced water often form stable emulsions that severely impede the oil-water separation. Development of a thick emulsion or rag layer between the oil and water phases is the precursor to an upset. Such upsets reduce production and increase the emissions to land, water and air. The current method of using floats to sense the oil-water interface in separators, treaters, and desalters fails when the emulsion layer thickens.

Goal:
The current goal is to assess the capability of a new technique for locating the oil/water inferface and measuring the thickness of any rag layer inside production vessels. The technique consists of locating any oil/water or oil/emulsion or emulsion/water interfaces using fiber-optic pressure sensors. The relative amounts of oil and water present at any location can be obtained by calculating the bulk average density form the veritcal pressure profile. In pure water, the pressure increases by 0.433 psi per foot of vertical depth compared to 0.38 psi for a clean 38° API crude oil. Therefore the pressure gradient must be measured to 0.01 psi/ft. Recent work by LoPresti [1, 2] indicates that this can be achieved using fiber pressure sensors.

This work aims at improving the current fiber optic sensors to achieve real-time measurement of the pressure profile in a test column. Differential measurements of pressure will be obtained using the repsonse of fiber Bragg gratings to applied strain. A shift in the wavelength reflected by the grating can be translated into a physical shift in a focused spot of light using a diffraction graing, CCD camera, appropriate optics, and computer processing. Using the appropriate transducers, pressure from the weight of the oil or water can be translated to a fiber strain larger than the rexolution limit of the system.