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The importance of sand monitoring in separator tanks - part one

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Oilfield Technology,

Excessive build-up of sand in a separator tank can clog the drain pot, resulting in damage to devices further downstream and requiring an unscheduled shutdown for cleaning. Reliable sand detection is vital and, as Emerson’s Marianne Williams explains, the latest vibrating fork level detectors provide an effective solution.

The segregation of oil, gas and water in separator tanks is an important part of the oil and gas production process, as it enables hydrocarbon recovery to be maximised. It also allows the flow or production rate of the individual component streams to be measured, so that essential data about the quantity of fluids produced from each well can be captured. Sand from the well stream will inevitably accumulate in a separator over time, but when this build-up becomes excessive, it can become tightly packed and clog the sand drain pot. This is highly undesirable, as it not only takes up valuable space within the tank but can also result in a costly unscheduled shutdown to enable the sand to be cleaned out manually. In addition, if the sand is not properly drained, it can be pumped out with the water, which can cause damage to devices further downstream.

Figure 1. Separator tanks such as these rely on gravitational settling to segregate the various components of the well stream.

It is therefore vital for operators to install technology that can continuously monitor sand build-up and issue an alarm before it becomes excessive, to enable proactive maintenance to be scheduled. Traditionally, nucleonic technology has been used to perform continuous monitoring, but the latest vibrating fork level detectors offer several advantages. New functionality in these devices provides a liquid-to-sand interface option for continuous monitoring of sand build-up, enabling clean-out cycles to be optimised or automated, and leading to increased production efficiency.

Separating well stream components

Separators rely on gravitational settling to segregate the various components of the well stream. After the flow enters the tank, the gas quickly separates from the liquid because it weighs much less than either oil or water. The gas is then routed into a separate chamber and leaves the separator through an outflow pipe to a gas processing system. Meanwhile, the oil forms a layer on top of the water because it is less dense. This oil layer then spills over a weir into the tank’s oil chamber, and the oil and water each leave the separator to their respective processing systems through different outflow pipes.

Sand control measures

If the well stream contains sand, it will accumulate over time in a lower chamber of the separator. The quantity of sand reaching the separator can be minimised through protective control measures, such as expandable sand screens and gravel packing. However, implementing such measures is far from a straightforward process. Even wells that are close to each other can be geologically different, with some producing lots of sand and others very little, and this makes it difficult to predict the level of sand control that will be required. Each well pad may need a different sand control technology, which adds complexity to operations. Also, installing these protective measures is time-consuming and requires a lot of skilled resources.

Build-up of deposits

Even when appropriate sand control measures are installed and performing correctly, it is inevitable that some material will still gradually accumulate in the separator over a long period of time. Should these control measures not be performing properly, the rate of build-up will be accelerated, which can eventually lead to the sand drain pot becoming clogged. The usual method of sand disposal is to open the drain pot, enabling high pressure water jets to flush away the material in a controlled fashion, but clogging prevents this from happening.

Figure 2. Sand build-up at the bottom of a separator tank.

There are various reasons why excessive sand in the separator is undesirable. It encourages the formation of unwanted emulsions between the oil and water, takes up valuable space in the tank, and leads to a reduction in oil flow rate. In addition, when the sand build-up reaches a certain level, it can get pumped out with the water. This can result in devices downstream of the separator – such as pumps, valves and flowmeters – suffering costly damage through blocking, abrasion or erosion. Furthermore, excessive build-up can lead to an unscheduled shutdown to enable removal of the sand blocking the outlet. This would be time-consuming and extremely costly in terms of lost production.

Nucleonic technology

To maximise separator and production efficiency and prevent devices from suffering costly damage, it is crucial to implement technology that provides continuous monitoring and detection of sand build-up. This can be achieved with nucleonic technology, in which signals from an array of gamma emitters held within dip pipes are received by a detector at the other side of the vessel. Oil, water and sand have different densities and therefore attenuate these gamma signals by varying amounts. This enables their respective levels to be calculated from the intensity of the signals reaching the detector. However, this method has several drawbacks, including its levels of risk and complexity, the need for yearly validation requirements, the necessity to comply with local laws, and its high cost of ownership. Consequently, operators are increasingly seeking alternative solutions that are more reliable, safer, less complex and not as costly.

This is part one of a two-part article. Part two can be read here.

Author: Marianne Williams, Emerson

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