Pore Pressure and Fracture Pressure

 





Pore pressure gradient and Fracture gradient curve

  

Pore pressure is the pressure exerted by fluids in the pore spaces of rock. It can be classified into 3 types: normal, over-pressured or subnormal; depending upon whether the pore pressure is equal to, greater than or less than the hydrostatic pressure at a given depth. If the pore pressure exceeds the hydrostatic pressure exerted by the mud inside the well, the fluid in the pore spaces can flow into the well, called a kick.

The fracture pressure is the upper limit at which a rock can withstand pressure from the mud column. It is also the minimum lateral stress that must be overcome to fracture the rock. When rock fracture, drilling mud can flow out of the well into the formation and the mud return are lost instead of circulating back to the surface.

Both pore and fracture pressures vary with depth. The pore pressure gradient is a curve that shows how the pore pressure in the well changes with depth. The fracture gradient is a curve that shows how the fracture pressure in the well changes with depth.

The pore pressure and fracture gradient defines the boundaries of the safe drilling mud weight to be used. Drillers strive to keep mud weight between these two curves.

Sent in by Ekwe Umahi Makolia, TalkOil9ja.