Separating oil, water, gas and sand is a crucial process in oil production, needing careful control. Density profiling offers the most accurate information on the oil/water emulsion layer, resulting in a number of cost-saving and efficiency benefits.
The picture many people have of oil production where a hole is drilled in the ground and crude oil flows or is pumped is a great oversimplification. In reality what is produced from the well is a mixture of crude oil, water, emulsion, gas, sand and finer solids. One of the first stages of the oil production process is to separate out these various elements; this is achieved in a separator where gravity is used with the heavier sand and water dropping to the lower part of the vessel and the lighter crude oil and gas above. If a hydrocarbon–water emulsion forms there is no clear interface and the separation process requires careful control and possibly the addition of demulsifier chemicals.
Information about the thickness and position of the emulsion layer can be provided even with a relatively simple nucleonic system, giving a number of benefits:
• Cost savings from reduced emulsifier use as the emulsifier dose rate can be controlled automatically based on the emulsion layer thickness.
• Reduced oil in the water cut as the lower edge of the emulsion can be monitored and kept well above the outlet.
• Downtime will no longer be caused by a high emulsion layer overloading electrostatic coalescing plates.
In addition, monitoring the solids level within the vessel will ensure that reductions in available internal volume – which reduce residence times and hence separation efficiency – can be monitored and action taken as required.
Emulsion position and thickness For a typical separator, we can provide a simple solution to give both the level and thickness of the emulsion layer. The mounting position and span is selected to cover the full emulsion/interface range in normal operation. Having long measuring path lengths through the liquid increases the accuracy of the individual density measurements and makes the system less prone to the effects of build-up on the vessel wall that would affect the accuracy.
All of the required calculations can easily be carried out in the control system by taking the three inputs from the nucleonic density transmitter. The simplest control philosophy would use the upper emulsion oil boundary to control the water cut and the emulsion thickness to control the rate of demulsifier addition.
A measurement of the solids level in the vessel can also be carried out using a nucleonic transmitter; this can either be part of the emulsion measuring system, using the same single radioactive source, or a separate system mounted at a different location on the vessel, usually near the inlet.
Profiling The most accurate visualisation of the separation efficiency and of the emulsion distribution within the separator is given by using a full density profiling solution which will give more information and allow better optimisation of the separation process.
With our multi detector density profile solution, several detectors are mounted on the tank wall and a single source is inserted into the vessel at the midpoint of the measuring range in a drywell. Multiple systems can be used to give measurement along the length of a separator:
• A first stage mounted near the inlet to measure solids and the initial state of separation.
• A second stage to measure the development of the emulsion layer.
• A third stage near the weir to control the interface level and ensure separation.
The benefits of radiation-based emulsion measurement Both the systems detailed have the advantage that the only internal parts are the radioactive source and the drywell used to locate it in the correct location inside the vessel. The advantages of this approach are:
• No parts with failure potential are installed internally so there is no possibility of having to isolate the vessel for maintenance or repair.
• The external detectors can easily be exchanged in case of damage or failure.
• The detectors used will all be of the same type, reducing the need to hold individual spares.
• Process connections as small as 1½” can be used for the insertion drywell connection, allowing the possibility of retrofitting to existing vessels.
Increasing the amount of information available for accurate process control will decrease costs and increase process uptime when compared to having a simple interface level measurement. By monitoring both the position and thickness of the emulsion layer, accurate control of the water cut and demulsifier addition is achieved. This improves separation performance and decreases demulsifier use.
Measurement based on gamma radiation has a reputation for being difficult to design, build, install and operate. Yet, nearly 60 years after the technique was first introduced, there are still measurement challenges in oil and gas production and refining that can only be reliably solved by the use of a radiation-based solution. With over five decades of involvement with nucleonic devices coupled with extensive application experience, Endress+Hauser can make the implementation of radiation-based measurements as simple as possible.
Read more about the use of the Gammapilot for density profile measurement in the oil & gas industry, in our brochure.