Spotting Balanced Pills|
A balanced pill is a volume of viscous material (pill or plug) placed in the bottom of the working string and up into the casing or liner, such that the fluid column of the viscous pill in the working string is the same height as the column of pill fluid in the annulus. The pill is balanced to the degree that the total fluid column in the working string and the annulus provide the same hydrostatic head at the bottom of the working string. When placing the balanced pill to slow fluid loss, pill volume should include both the length of area of fluid loss plus some volume for anticipated loss.
If you know the length of the interval that you want the pill to occupy (hbal), calculate the pill volume (vpill) using Equation 16. Values for the tubing and annular capacities are found in Table 19, “API Tubing — Weight, Dimensions, and Capacities,” and Table 20, “Annular Capacity.”
EXAMPLE K. Balanced Pill in a Known Interval
Another approach to spotting a pill or plug is to find out how much footage a pill of a given volume will cover if it is balanced in the tubing and annulus. In this case, you will already know the starting volume of the pill in barrels.
EXAMPLE L. Balanced Pill of Known Volume
Spotting the pill at the correct depth means that you need to chase it with a volume of working fluid that will leave it at the same height in the tubing and outside in the annulus. To find the chase volume, determine the total volume of the tubing and subtract the interval height (hbal) that you want left in the tubing. Also, make sure to add in the volume of the lines from the pit to the drill floor (vsurf).
EXAMPLE M. Chase Volume to Place a Balanced Pill