In an upcoming abandonment campaign, we are considering an option to set a barite suspension plug. This will allow for the removal of a subsea vertical tree in order to remove the tubing and set an abandonment cement plug.
However, we have identified a few technical challenges in deploying the barite plug though our fluid handling set up.
Equipment set up:
The Light Well Intervention Subsea Intervention Device (dual riser equivalent but without riser to surface) is installed on top of the subsea vertical tree, which allows for pumping through the tubing (via a crossover from the tree to the Subsea Intervention Device) and annulus (via the annulus access line) with hoses connected from the rig to the tree (see drawing attached). No fluid is actually pumped through the Subsea Intervention Device’s BOP or lubricator system.
The production access hose is a 400m long 2” (ID) BlackEagle hose on a reeler deployed to ~150m water depth from a rig.
The flow path is from the cement unit -> through the hose on the reeler -> through two 2” isolation valves subsea -> through the VXT/Subsea Intervention Device x-over outlet -> through the vertical x-tree and into the tubing.
Return fluid from the well is taken through a similar hose connected to the annulus access port on the subsea tree to the rig.
Following challenges envisaged for barite plug deployment:
While our set-up may be unusual, I can draw the analogy to a coiled tubing operation, where fluid is pumped through the hose (coiled-tubing) stored on a reeler. Pumping cement or frac-fluid (high solid content fluid) is a common operation.
We would like to ask the forum about experience of deploying barite plugs through similar set ups or through coiled tubing.
Has it been done? If yes, what are the lessons learned? Are there any points we need to consider to make the operation successful?
Barite plugs are utilized in well control operations primarily for sealing formations that are flowing into the wellbore. They are not abandonment plugs. A Barite plug is considered a temporary barrier and is generally not inflow tested to be considered as a barrier.
I would not recommend removing the tree with only a barite plug in the well.
Barite plugs are normally pumped down to the required depth through the drillstring or coil and then allowed to settle. Once the flow is stopped and the plug is settled, the next step is to place a cement plug on top of the barite plug to create a permanent plug.
Now pumping down the tubing and the allowing the plug to settle will not allow you to pull back above the plug and then place a cement plug, so the plug would have to be long enough to ensure that a barrier is formed both in the annulus and in the tubing. If the pill needs to settle both in the annulus and in the tubing string it will be a challenge to get that balanced correctly.
The objective of a barite plug is to have the weighting material settle rapidly and evenly once it gets to its required location. This is normally done by displacing the barite plug out of the string and into the annulus and then allow it to settle below the bit.
Once that is confirmed and the flow from the well has stopped, a cement plug is then pumped to form a permanent barrier.
Factors that affect settling rate of Barite include density; gel strengths and viscosity of the plug. The heavier the plug the slower the settling rate will be because of a higher solids content resulting in higher gel strengths and viscosities to allow the plug to be pumped. The higher the concentration of barite, the higher the viscosity resulting in slower settling times.
You will need to run some pilot tests with the fluids and the barite to determine the settling time. Measure the settled volume in a graduated cylinder at several different time intervals. Measure the settled volume of the barite when it has all fallen out. This can be useful in determining the actual settled volume vs. the calculated settled volume.
The pilot tests will also allow you to see what settles in the surface pipework
Do not pump spacers before or behind the plug as that may cause the plug to settle out.
Pump and displace the plug with the cement unit.
Ensure that there is a backup pump available and lined up and ready to pump in case of a failure of the first pump.
Once the plug is being pumped and flowing the barite will not settle out so there are generally no issues with settling in surface lines, but do not stop pumping with any part of the plug in the lines or in the surface equipment.
The tanks where the plug is mixed will need to have agitation systems to ensure that the pill does not settle at surface
For a barite plug to be effective it is recommended that once spotted to wait on the plug setting, which in a well can take approximately 8 hours.
If you are looking at abandonment why not consider pumping sandaband as that can give you an abandonment plug without the need to set a cement plug and it will probably be simpler to displace that rather than barite.