A cementing head (plug container) is attached to the top of the casing string. Within this cementing head will be the bottom plug and the top plug. The diameter of these plugs wil be slightly less than the casing ID. The mud is conditioned prior to cementing, by reciprocating the casing vertically with the hoisting system; ideally a distance at least equal to the spacing between centralizes and scratchers, as mud is circulated down th inside of the casing, thereby opening the check valve in the float collar and returning up the annulus. This procedure will break the gel in the annulus. Any rock particles present in the annulus will be carried to surface because of the reciprocation and circulation as the well is prepared for cementing. The bottom plug is now dropped on top of the mud column in the casing, and the calculated volume of cement is pumped into the casing behind this plug. This bottom plug separates the drilling mud from the cement, thereby minimizing contamination of the cement by the mud as the wipers remove the mud from the wall of the casing in front of the cement.
When the calculated volume of cement has been pumped into the casing, the top plug is dropped on the cement column. Once the bottom plug reaches the internal shoulder at the top of the float collar, flow will be stopped when that plug seats on the shoulder, since flow is blocked by the plug. The result is a pressure increase , which will rupture a diaphragm within the bottom plug. When the diaphragm ruptures, it will be known at the surface, since the pressure will drop and flow will resume. Pumping will continue until top plug seats on the remains of the bottom plug. When this occurs, flow again will stop and pressure will rise, indicating that the top plug has reached a position on top of the remains of the bottom plug.
The casing is reciprocated throughout the entire pumping process in order to break the gel in the annulus and permit the cement to distribute itself around the casing. When the top plug seats the system is shut down to provide the pre-determined time for the cement to set. Once the cement has set, further drilling can now proceed. The bit size for the next hole is attached to the bottom of the drill string and tripped into the hole and drilling is resumed. The top and bottom drillable plugs are then drilled out along with the float collar. The bottom joint of casing filled with set cement along with the guide shoe are now drilled out. Drilling now proceeds to the next casing point.
