Assume that the well is shut in (annular preventer & choke are closed) and a small amount of mud is pumped into the well using the cementing unit. The pressure will begin to increase immediately. This pressure is an imposed pressure, and is felt uniformly throughout the well bore.
As an example: Pumping is stopped and 900 psi is held on the pump. This pressure (900 psi) is felt inside the BOP stack, inside the drill string, at the bottom of the hole, at the casing shoe, and everywhere else in the circulating system.
Such procedures are usually done after each casing string. It is referred to as testing the casing shoe and is done in order to determine the amount of pressure the formation at the shoe can withstand. Under normal conditions,the formation fracture pressure will increase with depth. This means that formations normally get stronger, and therefore harder to fracture, as depth increases.
Note: Under normal conditions, the weakest point in the annulus will be at the casing shoe.
It is possible to conduct three different types of casing shoe tests:
• Leak-Off Test: Pumping into the shut-in well continues until mud is lost to the formation. It is noted by a non-linear relationship between volume pumped and pressure increase.
• Pressure Integrity Test: Pumping proceeds until a predetermined imposed (pump) pressure is obtained ¾ without any loss of mud into the formation.
• Fracture Test: Pumping proceeds until the formation is fractured. Although this type of test is occasionally done, it is not a normal way of conducting a shoe test.
