Trip Tank is a small metal tank with small capacity about 20-40 bbls with 1 bbl divisions inside and it is used to monitor the well. There are several operations that we can use the trip tank to monitor the well as follows;
1. Trip Out Of Hole (TOOH): While tripping out of hole, the trip tank is used to track volume of mud replacing volume of drill string. The volume of mud should be equal to displacement volume of any kind of tubular tripped out of hole.
2. Trip In Hole (TIH): While tripping in hole, the drilling string (bit, BHA and drill pipe) is ran back in the hole, the trip tank must be use to keep track volume gain. The expected volume gain should be equal to the displacement volume of whole string.
Learn more about pipe displacement
3. Flow check: The trip tank is utilized to determine well condition in order to see if the well is still under static condition.
The importance of trip tank is as follows:
1. Provide sufficient hydrostatic pressure to prevent influx from reservoir. When TOH, mud hydrostatic will be lost because mud volume must substitute drill pipe volume pulled out of hole. If hydrostatic pressure decreases too much, influx from reservoir can come into the hole and make a trouble in well control. For this reason, mud in trip tank must be filled into hole to maintain hydrostatic pressure.
2. Kick Indicator: Volume of mud from the trip tank is pumped in the hole can be an indicator that relates to a situation occurring in wellbore as kick. If the volume of mud measured by trip tank is less than the expected volume of drill pipe volume tripped out of hole, the suspect problem is kick because volume of kick substitutes volume of mud.
The circulation system while tripping
I would like to show the circulation system while tripping out of hole therefore you will be more understanding about how trip tank works.
While Tripping Out of Hole (TOH), a trip tank pump will circulate mud into a bell nipple in order to keep the hole full all the time and the over-flow mud will return back to the trip tank. Once every stand is pulled, the mud volume in the well will decrease because the drill pipe is pulled out of hole. Since the trip tank pump is always run while tripping, the annulus will be full all the time (see figure below).
Reference book: Well Control Books