Slug Detection

Prepare for slugs before they hit the process

Even if process facilities are designed to avoid or handle liquid slugs, it is also possible to provide early warning of incoming slugs before they reach the process facilities.

Slug detection gives operators an opportunity to prepare and adjust their process and minimize the impact on equipment and process, including reducing water and oil levels in the separator to prevent water or oil/foam overflow.

Common causes for slugs include – usually in combination:
•Long distance or tie-back from producing wells to processing plant
•High GVFs – allows gas to travel at a higher velocity and gradually separate from the liquid
•Low flow rates – turbulence is reduced and liquid separates and floats along bottom of pipe
•Low points ("U" or "V" shapes) on the pipeline from the wells to the process facilities
•Slugging wells ("severe slugging") – liquids flow back until they "plug" the well

When the accumulated liquid along the bottom of the pipe or in a well reaches a certain level and "plugs" the pipe, the higher velocity gas gets blocked. Pressure behind the liquid "plug" increases, until there is enough energy to push the liquid away from the low point and ultimately into the process.

Sentech slug detection

Slug detection may be achieved by installing two Sentech Water Cut Meters on the same pipe, with a short and well defined distance d between them. The signals from the SeCaP sensors on each meter is then processed by the software through well proven cross-correlation methods.

Although the Sentech Water Cut Meters is designed to measure water in oil, it is also able to detect gas slugs (permittivity lower than that of dry oil).

Depending on process conditions, Sentech's solution for Slug Detection may be used to detect:
•Liquid slugs (when "normal" flow is mostly gas, i.e. GVF > 50%)
•Water cut of liquid slugs (for above conditions)
•"Single phase" water slugs (regardless of normal process conditions)
•Gas slugs (when "normal" flow is mostly liquid, i.e. GVF < 50%)
•Intermittent gas/liquid slugging flow

By recognising the flow profile, measuring the duration of slugs and the travel time from one meter to the next, it is also possible to detect and calculate the volume and velocity of slugs. Even with a very fast slug, propagating through the piping at say 20 m/s, Sentech’s technology will produce minimum ten to 20 readings of measurement data.

d = distance from outlet of WCM #1 to WCM #2

td = time for slug to travel d

ts = duration of slug

velocity of slug v = d / td

length of slug ls = v · ts

ID = pipe inner diameter

slug volume V = ls · p · (½ID)²

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