This column is moderated by Béla Lipták, automation and safety consultant and editor of the Instrument and Automation Engineers' Handbook (IAEH). If you have an automation-related question for this column, write to [email protected].
Q: We need to measure an oil/water/gas multiphase flow using an orifice plate. The gas-oil ratio (GOR) is 80% and the water cut is 15%. The flowmeter is to be in the production flowline near the wellhead, upstream of the production separator. I know that this location of an orifice meter is not ideal, but our client insists. Further, the line size can't be changed because of process design criteria.
We have a separate flow control loop at this location, so highly accurate measurement isn't our concern. It can be inaccurate. Rather, I need to find a simple way to size the orifice plate as accurately as possible. In your opinion, can I calculate the size of three orifices for the three phases, and add up their areas to get the size proper size for the three-phase flow?
Considering all this, please give me your best advice.
Pooyan Ebrahimi
[email protected]
A1: In your process description, you indicated that measurement accuracy isn't important, but you didn't say what is important, what the purpose of the measurement is? I assume your purpose is just to alarm if drastic changes occur in the total flow or if the oil-gas ratio (OGR) suddenly changes. In other words, the purpose of your installation is like that of a flow alarm, and therefore it's sufficient for this sensor to act like a flow switch. In any case, I'll first make a few general comments about measuring multiphase (oil, gas, water, solids) flows from oil wells, and after that I'll answer your specific question.
Naturally, the most accurate method of measuring multiphase flow is to first separate the components of the total stream, and only then measure the flows of the individual components leaving the separator, using high precision flowmeters like Coriolis (Figure 1). I understand your client doesn't want to do this because they want to know about any flow changes as soon as they occur. If they desire to place the flowmeter before the separator, or if the purpose of the installation is to eliminate the individual separators, then using multiphase flowmeters is a possible option.
There are several multiphase meters on the market (Figure 2). They usually consist of pressure- and temperature-compensated venturi elements combined with high- or low-energy nuclear densitometers or magnetic resonance sensors. These meters can distinguish the flow rates of oil, gas and water, but suffer serious limitations. They can also reduce the amount of hardware on the topside of the well (onshore) or can be placed on the platform in offshore applications. In offshore applications, if they’re designed for high-pressure and low-temperature operation, they can also be placed in the ocean. In both cases (onshore or offshore), they can eliminate well-testing lines, which can be long and expensive, while measuring OGR or flow in real time eliminates the separation delay.
