Page 74 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 50

[ ] Well Design
1.4.4

to increase. If these trends change then it is likely that the pore pres-
sure trend is also deviating from normal. These indications may be
hard or impossible to interpret in streaky, nonhomogeneous forma-
tions. It is clearly very important to take all available offset data and
interpret it intelligently to arrive at a picture that is likely to be close
to reality.
Relationship between pore pressure and fracture gradient. When
a casing is drilled out, it is normal to perform a leakoff test. This test
applies pressure to the wellbore and should detect the point at which
formation fracture is initiated without actually causing deep fractures.
This pressure then dictates how far the well can be safely drilled ahead.
The fracture gradient is the pressure applied to the formation to
initiate failure divided by the true vertical depth (TVD) of the forma-
tion being tested.
The resistance of the formation to fracturing comes from the addi-
tion of the formation fluid pore pressure and the tensile strength of the
rock. The tensile strength of the rock at depth comes from the natural
(unconfined) rock strength plus the supporting stresses imposed on the
formation by field stresses. Therefore, by taking a formation test to the
start of leakoff, information can be gained on the minimum field stress.
In an increasing gradient transition, pore pressures start to increase
above normal. As pore pressure increases, so does the fracture gradi-
ent. If the lithology and the field stress gradient do not change during
the transition zone, the pore pressures and fracture gradients will plot
similar but diverging lines on a depth-pressure plot. So it is desirable
to set a casing shoe as far into the transition zone as possible. This
gives increased shoe strength for drilling the next section.
The following formula was proposed by Eaton to calculate forma-
tion fracture gradient:

where
Fg = Fracture gradient, psi/ft
S = Overburden load, lbs
D = Depth in feet (therefore S/D = overburden gradient)
Pp = Pore pressure (therefore Pp/D = pore pressure gradient)
v = Poissons Ratio

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