Fluids and Rocks - Introduction to Carbon Capture and Sequestration - page 416

Figure 9.6.2 Permeability of different kinds of rocks
General ranges of permeability ( k ) in two different units for different kinds of rocks. Data
from Freeze and Cherry [9.22].
The intermediate curve between hydrostatic and lithostatic profi les is
the more interesting one, and is referred to as the fracture gradient , or
frac gradient . At pressures greater than the frac gradient pressure at any
depth, rock can fracture, creating high-permeability vertical pathways.
The frac gradient can be determined in a well by a well-testing procedure
known as a leak-off test, whereby the bottom of the well is insulated from
the rest of the well and high fl uid pressure is applied through tubing. The
frac pressure is the pressure at the point when a sharp increase in fl ow
rate occurs as fl uid pressure increases, at this point the formation begins
to fracture. When this test is conducted at multiple depths during the
drilling of the well, we obtain an average fracture gradient.