Formation fracture pressure, or formation breakdown pressure is the pressure required to rupture a formation, so that whole mud can flow into it. The symbol PFB is usually used to denote this pressure; see picture below. Commonly this is expressed as a pressure gradient, GFB, with the units of psi/foot. The formation breakdown pressure is usually determined for formations just below a casing shoe by means of a leak-off test. This test of the formation strength, also known as a formation integrity test or FIT, is effected after the casing has been run and cemented in place. This allows formations to be tested after the minimum of disturbance and damage due to drilling, and allows a clear indication of strength to be determined for one isolated zone.
The general procedure for a leak-off test is as follows:
- Casing is run and cemented in place. The cement is allowed to harden before testing takes place, to prevent the formation of ‘micro-annulus’ between cement and casing after the casing expands under pressure.
- The shoe and cement is drilled out, and five to ten feet of new formation is drilled. Some companies will drill as much as 20 ft of new formation.
- The bit is pulled to the shoe and the hole circulated clean, with balanced mud weight in and out.
- The well is closed in using the blowout preventer, and a chiksan line to the cement unit made up on the drill pipe. The cement unit pump is used because it is a high pressure, low volume pump and small volume can be accurately measured using the cement unit. With the well closed in, the cement pump is used to pump a small volume at a time into the hole (typically ¼ or ½ a barrel each time). Since this is being pumped into a closed well, the pressure in the well rises. So long as the system remains closed and nothing breaks, the pressure increase for each volume pumped will be the same. A graph of well pressure versus volume pumped will show a near straight line until a break occurs. At this point fluid is being injected into the formation and the pressure rise will be smaller. Further pumping will not necessarily show a pressure rise but more commonly a pressure drop.
Picture bellow will be explain about leek of test to calculate formation fracture pressure
We apply pressure in our well bore and increase the pumping rate and pumping pressure and after the pumping pressure increased they will be find meeting point where the pressure will drop and the point is leak off test pressure. Total pressure applied to our formation = maximum pumping pressure when leak of test + hydrostatic pressure = formation fracture pressure .
Gradient formation fracture is formation fracture pressure : Casing shoe depth in TVD
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