Balanced cement plug is the method to set cement plug by applying concept of balanced hydrostatic. Generally speaking, when we say “balance”, it means that hydrostatic pressure at the end of the drilling string is the same as hydrostatic pressure in the annulus (see the figure below).
(Balance hydrostatic pressure both side)
This cement plug is not a routine operation and it is used for several situations as set kick off plug, plug and abandon, etc.
When the calculation is done, you will know how much you need to pump until you balance hydrostatic for both sides. However, practically, you may need to under displace few barrels therefore, you will be able to pull dry and let cement to fall down while pulling out of hole. It also prevents cement to be lifted due to swabbing effect.
(Under displacement few barrels)
Cement stinger sized can vary from drill pipe (5” drill pipe) to small tubing (2-7/8”). I personally prefer small pipe as tubing because I can minimize change of swabbing cement while pulling out with the smaller pipe. For example, you can have 500 ft of 2-7/8” tubing as your cement string and the rest of them are string of drill pipe.
Length of cement is based on each operator’s experience and it can vary from few hundred feet to several hundred feed. Typically, the length of cement is the length of cement after the cement string is pulled out. But if you cement your BHA, the length of cement will based of height required to meet the specification. Moreover, the length of cement is depended on the job as well. If you just do normal cement kick off plug, you may need 500 ft of cement. However, if you want to cement your stuck BHA, you may need to set long cement plug across your BHA plus extra feet.